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This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.
functionfunc(){};newfunc(x,y,z);
(OR)
// Create a new instance using function prototype.varnewInstance=Object.create(func.prototype)// Call the functionvarresult=func.call(newInstance,x,y,z),// If the result is a non-null object then use it otherwise just use the new instance.console.log(result&&typeofresult=== 'object' ? result : newInstance);
ES6 Class syntax:
ES6 introduces class feature to create the objects
A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.
Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.
The prototype on object instance is available through Object.getPrototypeOf(object) or __proto__ property whereas prototype on constructors function is available through Object.prototype.
What is the difference between Call, Apply and Bind
The difference between Call, Apply and Bind can be explained with below examples,
Call: The call() method invokes a function with a given this value and arguments provided one by one
varemployee1={firstName: "John",lastName: "Rodson"};varemployee2={firstName: "Jimmy",lastName: "Baily"};functioninvite(greeting1,greeting2){console.log(greeting1+" "+this.firstName+" "+this.lastName+", "+greeting2);}invite.call(employee1,"Hello","How are you?");// Hello John Rodson, How are you?invite.call(employee2,"Hello","How are you?");// Hello Jimmy Baily, How are you?
Apply: Invokes the function with a given this value and allows you to pass in arguments as an array
varemployee1={firstName: "John",lastName: "Rodson"};varemployee2={firstName: "Jimmy",lastName: "Baily"};functioninvite(greeting1,greeting2){console.log(greeting1+" "+this.firstName+" "+this.lastName+", "+greeting2);}invite.apply(employee1,["Hello","How are you?"]);// Hello John Rodson, How are you?invite.apply(employee2,["Hello","How are you?"]);// Hello Jimmy Baily, How are you?
bind: returns a new function, allowing you to pass any number of arguments
varemployee1={firstName: "John",lastName: "Rodson"};varemployee2={firstName: "Jimmy",lastName: "Baily"};functioninvite(greeting1,greeting2){console.log(greeting1+" "+this.firstName+" "+this.lastName+", "+greeting2);}varinviteEmployee1=invite.bind(employee1);varinviteEmployee2=invite.bind(employee2);inviteEmployee1("Hello","How are you?");// Hello John Rodson, How are you?inviteEmployee2("Hello","How are you?");// Hello Jimmy Baily, How are you?
Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array.
Whereas Bind creates a new function that will have this set to the first parameter passed to bind().
JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json
Parsing: Converting a string to a native object
JSON.parse(text);
Stringification: converting a native object to a string so it can be transmitted across the network
The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.
The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.
Some of the examples of this method are,
letarrayIntegersOriginal1=[1,2,3,4,5];letarrayIntegersOriginal2=[1,2,3,4,5];letarrayIntegersOriginal3=[1,2,3,4,5];letarrayIntegers1=arrayIntegersOriginal1.splice(0,2);// returns [1, 2]; original array: [3, 4, 5]letarrayIntegers2=arrayIntegersOriginal2.splice(3);// returns [4, 5]; original array: [1, 2, 3]letarrayIntegers3=arrayIntegersOriginal3.splice(3,1,"a","b","c");//returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]
Note: Splice method modifies the original array and returns the deleted array.
Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.
The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
A Map may perform better in scenarios involving frequent addition and removal of key pairs.
What is the difference between == and === operators
JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,
Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value.
There are two special cases in this,
NaN is not equal to anything, including NaN.
Positive and negative zeros are equal to one another.
Two Boolean operands are strictly equal if both are true or both are false.
Two objects are strictly equal if they refer to the same Object.
Null and Undefined types are not equal with ===, but equal with ==. i.e,
null===undefined --> false but null==undefined --> true
Some of the example which covers the above cases,
0==false// true0===false// false1=="1"// true1==="1"// falsenull==undefined// truenull===undefined// false'0'==false// true'0'===false// false[]==[]or[]===[]//false, refer different objects in memory{}=={}or{}==={}//false, refer different objects in memory
An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.
In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.
For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener
consthandler=()=>console.log("This is a click handler function");document.addEventListener("click",handler);
Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.
constfirstOrderFunc=()=>console.log("Hello, I am a First order function");consthigherOrder=(ReturnFirstOrderFunc)=>ReturnFirstOrderFunc();higherOrder(firstOrderFunc);
Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function.
Let's take an example of n-ary function and how it turns into a currying function,
constmultiArgFunction=(a,b,c)=>a+b+c;console.log(multiArgFunction(1,2,3));// 6constcurryUnaryFunction=(a)=>(b)=>(c)=>a+b+c;curryUnaryFunction(1);// returns a function: b => c => 1 + b + ccurryUnaryFunction(1)(2);// returns a function: c => 3 + ccurryUnaryFunction(1)(2)(3);// returns the number 6
Curried functions are great to improve code reusability and functional composition.
A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.
Let's take an example to see the difference between pure and impure functions,
As per the above code snippets, the Push function is impure itself by altering the array and returning a push number index independent of the parameter value. . Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array.
Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.
The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope.
Let's take an example to demonstrate the usage,
letcounter=30;if(counter===30){letcounter=31;console.log(counter);// 31}console.log(counter);// 30 (because the variable in if block won't exist here)
You can list out the differences in a tabular format
var
let
It is been available from the beginning of JavaScript
Introduced as part of ES6
It has function scope
It has block scope
Variables will be hoisted
Hoisted but not initialized
Let's take an example to see the difference,
functionuserDetails(username){if(username){console.log(salary);// undefined due to hoistingconsole.log(age);// ReferenceError: Cannot access 'age' before initializationletage=30;varsalary=10000;}console.log(salary);//10000 (accessible to due function scope)console.log(age);//error: age is not defined(due to block scope)}userDetails("John");
What is the reason to choose the name let as a keyword
let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.
How do you redeclare variables in switch block without an error
If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,
letcounter=1;switch(x){case0:
letname;break;case1:
letname;// SyntaxError for redeclaration.break;}
To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.
letcounter=1;switch(x){case0: {letname;break;}case1: {letname;// No SyntaxError for redeclaration.break;}}
The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a let or const variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone.
What is IIFE(Immediately Invoked Function Expression)
IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,
(function(){// logic here})();
The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,
(function(){varmessage="IIFE";console.log(message);})();console.log(message);//Error: message is not defined
encodeURI() function is used to encode an URL. This function requires a URL string as a parameter and return that encoded string.
decodeURI() function is used to decode an URL. This function requires an encoded URL string as parameter and return that decoded string.
Note: If you want to encode characters such as / ? : @ & = + $ # then you need to use encodeURIComponent().
Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache.
Let's take an example of adding function with memoization,
constmemoizAddition=()=>{letcache={};return(value)=>{if(valueincache){console.log("Fetching from cache");returncache[value];// Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript identifier. Hence, can only be accessed using the square bracket notation.}else{console.log("Calculating result");letresult=value+20;cache[value]=result;returnresult;}};};// returned function from memoizAdditionconstaddition=memoizAddition();console.log(addition(20));//output: 40 calculatedconsole.log(addition(20));//output: 40 cached
Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation.
Let's take a simple example of variable hoisting,
console.log(message);//output : undefinedvarmessage="The variable Has been hoisted";
The above code looks like as below to the interpreter,
varmessage;console.log(message);message="The variable Has been hoisted";
In the same fashion, function declarations are hoisted too
In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance.
For example, the prototype based inheritance written in function expression as below,
functionBike(model,color){this.model=model;this.color=color;}Bike.prototype.getDetails=function(){returnthis.model+" bike has"+this.color+" color";};
Whereas ES6 classes can be defined as an alternative
classBike{constructor(color,model){this.color=color;this.model=model;}getDetails(){returnthis.model+" bike has"+this.color+" color";}}
A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains
Own scope where variables defined between its curly brackets
As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.
Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor
Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.
A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.
Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the postMessage interface, and those pages can manipulate the DOM.
How do you reuse information across service worker restarts
The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's onfetch and onmessage handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.
IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.
Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.
Local storage: It stores data for current origin with no expiration date.
Session storage: It stores data for one session and the data is lost when the browser tab is closed.
Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).
A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs.
For example, you can create a cookie named username as below,
You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case.
For example, you can delete a username cookie in the current page as below.
document.cookie="username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";
Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.
What is the main difference between localStorage and sessionStorage
LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.
The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage(window.localStorage) and sessionStorage(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local).
For example, you can read and write on local storage objects as below
The session storage provided methods for reading, writing and clearing the session data
// Save data to sessionStoragesessionStorage.setItem("key","value");// Get saved data from sessionStorageletdata=sessionStorage.getItem("key");// Remove saved data from sessionStoragesessionStorage.removeItem("key");// Remove all saved data from sessionStoragesessionStorage.clear();
The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events.
The syntax would be as below
window.onstorage=functionRef;
Let's take the example usage of onstorage event handler which logs the storage key and it's values
window.onstorage=function(e){console.log("The "+e.key+" key has been changed from "+e.oldValue+" to "+e.newValue+".");};
Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.
Terminate a Web Worker:
Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.
w.terminate();
Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code
A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.
Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.
A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action.
Let's take a simple example of how to use callback function
functioncallbackFunction(name){console.log("Hello "+name);}functionouterFunction(callback){letname=prompt("Please enter your name.");callback(name);}outerFunction(callbackFunction);
The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events.
Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.
functionfirstFunction(){// Simulate a code delaysetTimeout(function(){console.log("First function called");},1000);}functionsecondFunction(){console.log("Second function called");}firstFunction();secondFunction();Output;// Second function called// First function called
As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.
Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.
You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.
loadScript("/script1.js",function(script){console.log("first script is loaded");loadScript("/script2.js",function(script){console.log("second script is loaded");loadScript("/script3.js",function(script){console.log("third script is loaded");// after all scripts are loaded});});});
The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,
Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,
Promise.all([Promise1,Promise2,Promise3]).then(result)=>{console.log(result)}).catch(error=>console.log(`Error in promises ${error}`))
Note: Remember that the order of the promises(output the result) is maintained as per input order.
Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first
varpromise1=newPromise(function(resolve,reject){setTimeout(resolve,500,"one");});varpromise2=newPromise(function(resolve,reject){setTimeout(resolve,100,"two");});Promise.race([promise1,promise2]).then(function(value){console.log(value);// "two" // Both promises will resolve, but promise2 is faster});
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression "use strict"; instructs the browser to use the javascript code in the Strict mode.
Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.
The strict mode is declared by adding "use strict"; to the beginning of a script or a function.
If declared at the beginning of a script, it has global scope.
"use strict";x=3.14;// This will cause an error because x is not declared
and if you declare inside a function, it has local scope
x=3.14;// This will not cause an error.myFunction();functionmyFunction(){"use strict";y=3.14;// This will cause an error}
The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, it will be true.
For example, you can test IE version using this expression as below,
letisIE8=false;isIE8=!!navigator.userAgent.match(/MSIE 8.0/);console.log(isIE8);// returns true or false
If you don't use this expression then it returns the original value.
console.log(navigator.userAgent.match(/MSIE 8.0/));// returns either an Array or null
Note: The expression !! is not an operator, but it is just twice of ! operator.
The undefined property indicates that a variable has not been assigned a value, or declared but not initialized at all. The type of undefined value is undefined too.
varuser;// Value is undefined, type is undefinedconsole.log(typeofuser);//undefined
Any variable can be emptied by setting the value to undefined.
The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object.
You can empty the variable by setting the value to null.
The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.
The mouseEvent getModifierState() is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.
Let's take an input element to detect the CapsLock on/off behavior with an example,
<inputtype="password" onmousedown="enterInput(event)" />
<pid="feedback"></p><script>functionenterInput(e){varflag=e.getModifierState("CapsLock");if(flag){document.getElementById("feedback").innerHTML="CapsLock activated";}else{document.getElementById("feedback").innerHTML="CapsLock not activated";}}</script>
The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.
Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable
msg="Hello";// var is missing, it becomes global variable
The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.
The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases
The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.
Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object.
There are two ways of event flow
Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.
Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.
The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,
What is the difference between document load and DOMContentLoaded events
The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).
What is the difference between native, host and user objects
Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec.
Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects.
User objects are objects defined in the javascript code. For example, User objects created for profile information.
What is the difference between an attribute and a property
Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,
<inputtype="text"value="Name:">
You can retrieve the attribute value as below,
constinput=document.querySelector("input");console.log(input.getAttribute("value"));// Good morningconsole.log(input.value);// Good morning
And after you change the value of the text field to "Good evening", it becomes like
console.log(input.getAttribute("value"));// Good eveningconsole.log(input.value);// Good evening
The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).
Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an <a> element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression.
For example, the below link notify the message without reloading the page
JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.
Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.
No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).
Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are,
Web page has finished loading
Input field was changed
Button was clicked
Let's describe the behavior of click event for button element,
<!doctypehtml><html><head><script>
function greeting() {alert('Hello! Good morning');}</script></head><body><buttontype="button"onclick="greeting()">Click me</button></body></html>
JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name Mocha, but later the language was officially called LiveScript when it first shipped in beta releases of Netscape.
The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.
The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)
<p>Click DIV1 Element</p><divonclick="secondFunc()">DIV2<divonclick="firstFunc(event)">DIV1</div></div><script>
function firstFunc(event) {alert("DIV 1");event.stopPropagation();}
function secondFunc() {alert("DIV 2");}</script>
The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.
The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,
The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,
JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.
Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.
For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,
varform=document.querySelector("#registration-form");// Listen for changes to fields inside the formform.addEventListener("input",function(event){// Log the field that was changedconsole.log(event.target);},false);
ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.
JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.
When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.
When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.
When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.
Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.
The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.
For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.
<script>
var msg;
function greeting() {alert('Good morning');}
function start() {msg=setTimeout(greeting,3000);}
function stop() {clearTimeout(msg);}</script>
The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.
For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.
<script>
var msg;
function greeting() {alert('Good morning');}
function start() {msg=setInterval(greeting,3000);}
function stop() {clearInterval(msg);}</script>
Using indexOf: In an ES5 or older environment, you can use String.prototype.indexOf which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.
You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.
The above regular expression accepts unicode characters.
How do you get the current url with javascript
You can use window.location.href expression to get the current url path and you can use the same expression for updating the URL too. You can also use document.URL for read-only purposes but this solution has issues in FF.
console.log("location.href",window.location.href);// Returns full URL
You can check whether a key exists in an object or not using three approaches,
Using in operator: You can use the in operator whether a key exists in an object or not
"key"inobj;
and If you want to check if a key doesn't exist, remember to use parenthesis,
!("key"inobj);
Using hasOwnProperty method: You can use hasOwnProperty to particularly test for properties of the object instance (and not inherited properties)
obj.hasOwnProperty("key");// true
Using undefined comparison: If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.
How do you loop through or enumerate javascript object
You can use the for-in loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using hasOwnProperty method.
The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,
How do you make first letter of the string in an uppercase
You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.
You can use new Date() to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy
vartoday=newDate();vardd=String(today.getDate()).padStart(2,"0");varmm=String(today.getMonth()+1).padStart(2,"0");//January is 0!varyyyy=today.getFullYear();today=mm+"/"+dd+"/"+yyyy;document.write(today);
How do you check if a string starts with another string
You can use ECMAScript 6's String.prototype.startsWith() method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,
JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.
" Hello World ".trim();//Hello World
If your browser(<IE9) doesn't support this method then you can use below polyfill.
if(!String.prototype.trim){(function(){// Make sure we trim BOM and NBSPvarrtrim=/^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;String.prototype.trim=function(){returnthis.replace(rtrim,"");};})();}
You can use the logical or operator || in an assignment expression to provide a default value. The syntax looks like as below,
vara=b||c;
As per the above expression, variable 'a 'will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.
An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users' screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.
What is the way to find the number of parameters expected by a function
You can use function.length syntax to find the number of parameters expected by a function. Let's take an example of sum function to calculate the sum of numbers,
functionsum(num1,num2,num3,num4){returnnum1+num2+num3+num4;}sum.length;// 4 is the number of parameters expected.
A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.
The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,
You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,
Math.floor(Math.random()*10)+1;// returns a random integer from 1 to 10Math.floor(Math.random()*100)+1;// returns a random integer from 1 to 100
Note: Math.random() returns a random number between 0 (inclusive), and 1 (exclusive)
Can you write a random integers function to print integers with in a range
Yes, you can create a proper random function to return a random number between min and max (both included)
functionrandomInteger(min,max){returnMath.floor(Math.random()*(max-min+1))+min;}randomInteger(1,100);// returns a random integer from 1 to 100randomInteger(1,1000);// returns a random integer from 1 to 1000
Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler rollup.
Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.
No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.
A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,
/pattern/modifiers;
For example, the regular expression or search pattern with case-insensitive username would be,
What are the string methods available in Regular expression
Regular Expressions has two string methods: search() and replace().
The search() method uses an expression to search for a match, and returns the position of the match.
varmsg="Hello John";varn=msg.search(/John/i);// 6
The replace() method is used to return a modified string where the pattern is replaced.
The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.
varpattern=/you/;console.log(pattern.exec("How are you?"));//["you", index: 8, input: "How are you?", groups: undefined]
The output is going to be 33. Since 1 and 2 are numeric values, the result of the first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.
The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect.
For example, in the below function a debugger statement has been inserted. So
execution is paused at the debugger statement just like a breakpoint in the script source.
You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.
You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,
Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript
functionhttpGet(theUrl){varxmlHttpReq=newXMLHttpRequest();xmlHttpReq.open("GET",theUrl,false);// false for synchronous requestxmlHttpReq.send(null);returnxmlHttpReq.responseText;}
Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.
functionhttpGetAsync(theUrl,callback){varxmlHttpReq=newXMLHttpRequest();xmlHttpReq.onreadystatechange=function(){if(xmlHttpReq.readyState==4&&xmlHttpReq.status==200)callback(xmlHttpReq.responseText);};xmlHttp.open("GET",theUrl,true);// true for asynchronousxmlHttp.send(null);}
How do you convert date to another timezone in javascript
You can use the toLocaleString() method to convert dates in one timezone to another. For example, let's convert current date to British English timezone as below,
What are the properties used to get size of window
You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let's use them combination of these properties to calculate the size of a window or document,
What is the difference between proto and prototype
The __proto__ object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas prototype is the object that is used to build __proto__ when you create an object with new.
Give an example where do you really need semicolon
It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error ".. is not a function" at runtime due to missing semicolon.
// define a functionvarfn=(function(){//...})(// semicolon missing at this line// then execute some code inside a closurefunction(){//...})();
and it will be interpreted as
varfn=(function(){//...})(function(){//...})();
In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.
The freeze() method is used to freeze an object. Freezing an object does not allow adding new properties to an object,prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.
constobj={prop: 100,};Object.freeze(obj);obj.prop=200;// Throws an error in strict modeconsole.log(obj.prop);//100
Remember freezing is only applied to the top-level properties in objects but not for nested objects.
For example, let's try to freeze user object which has employment details as nested object and observe that details have been changed.
In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the final keyword which is used in various languages.
How to convert string to title case with javascript
Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,
functiontoTitleCase(str){returnstr.replace(/\w\S*/g,function(txt){returntxt.charAt(0).toUpperCase()+txt.substring(1).toLowerCase();});}toTitleCase("good morning john");// Good Morning John
You can use the <noscript> tag to detect javascript disabled or not. The code block inside <noscript> gets executed when JavaScript is disabled, and is typically used to display alternative content when the page generated in JavaScript.
<scripttype="javascript">
// JS related code goes here
</script><noscript><ahref="next_page.html?noJS=true">JavaScript is disabled in the page. Please click Next Page</a></noscript>
What are various operators supported by javascript
An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,
Arithmetic Operators: Includes + (Addition),– (Subtraction), * (Multiplication), / (Division), % (Modulus), + + (Increment) and – – (Decrement)
Comparison Operators: Includes = =(Equal),!= (Not Equal), ===(Equal with type), > (Greater than),> = (Greater than or Equal to),< (Less than),<= (Less than or Equal to)
Logical Operators: Includes &&(Logical AND),||(Logical OR),!(Logical NOT)
Assignment Operators: Includes = (Assignment Operator), += (Add and Assignment Operator), – = (Subtract and Assignment Operator), *= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
Ternary Operators: It includes conditional(: ?) Operator
typeof Operator: It uses to find type of variable. The syntax looks like typeof variable
Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,
functionf(a,b, ...theArgs){// ...}
For example, let's take a sum example to calculate on dynamic number of parameters,
What happens if you do not use rest parameter as a last argument
The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.
Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior,
How do you copy properties from one object to other
You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects. The syntax would be as below,
Object.assign(target, ...sources);
Let's take example with one source and one target object,
The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method
constobject={property: "Welcome JS world",};Object.seal(object);object.property="Welcome to object world";console.log(Object.isSealed(object));// truedeleteobject.property;// You cannot delete when sealedconsole.log(object.property);//Welcome to object world
What are the differences between freeze and seal methods
If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.
How do you determine if an object is sealed or not
The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true
If it is not extensible.
If all of its properties are non-configurable.
If it is not removable (but not necessarily non-writable).
Let's see it in the action
constobject={property: "Hello, Good morning",};Object.seal(object);// Using seal() method to seal the objectconsole.log(Object.isSealed(object));// checking whether the object is sealed or not
The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example,
You can use the Object.keys() method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,
The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.
constuser={name: "John",printInfo: function(){console.log(`My name is ${this.name}.`);},};constadmin=Object.create(user);admin.name="Nick";// Remember that "name" is a property set on "admin" but not on "user" objectadmin.printInfo();// My name is Nick
WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,
newWeakSet([iterable]);
Let's see the below example to explain it's behavior,
varws=newWeakSet();varuser={};ws.add(user);ws.has(user);// truews.delete(user);// removes user from the setws.has(user);// false, user has been removed
The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it.
Other differences are,
Sets can store any value Whereas WeakSets can store only collections of objects
WeakSet does not have size property unlike Set
WeakSet does not have methods such as clear, keys, values, entries, forEach.
The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
newWeakMap([iterable]);
Let's see the below example to explain it's behavior,
varws=newWeakMap();varuser={};ws.set(user);ws.has(user);// truews.delete(user);// removes user from the mapws.has(user);// false, user has been removed
The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it.
Other differences are,
Maps can store any key type Whereas WeakMaps can store only collections of key objects
WeakMap does not have size property unlike Map
WeakMap does not have methods such as clear, keys, values, entries, forEach.
The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,
vara=1;uneval(a);// returns a String containing 1uneval(functionuser(){});// returns "(function user(){})"
The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,
The uneval function returns the source of a given object; whereas the eval function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,
varmsg=uneval(functiongreeting(){return"Hello, Good morning";});vargreeting=eval(msg);greeting();// returns "Hello, Good morning"
An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,
function(optionalParameters){//do something}constmyFunction=function(){//Anonymous function assigned to a variable//do something};[1,2,3].map(function(element){//Anonymous function used as a callback function//do something});
Let's see the above anonymous function in an example,
ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the get keyword whereas Setters uses the set keyword.
varuser={firstName: "John",lastName : "Abraham",language : "en",getlang(){returnthis.language;},setlang(lang){this.language=lang;}};console.log(user.lang);// getter access lang as enuser.lang='fr';console.log(user.lang);// setter used to set lang as fr
The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,
constnewObject={};Object.defineProperty(newObject,"newProperty",{value: 100,writable: false,});console.log(newObject.newProperty);// 100newObject.newProperty=200;// It throws an error in strict mode due to writable setting
What is the difference between get and defineProperty
Both have similar results until unless you use classes. If you use get the property will be defined on the prototype of the object whereas using Object.defineProperty() the property will be defined on the instance it is applied to.
Can I add getters and setters using defineProperty method
Yes, You can use the Object.defineProperty() method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,
The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
What are the conventions to be followed for the usage of switch case
Below are the list of conventions should be taken care,
The expression can be of type either number or string.
Duplicate values are not allowed for the expression.
The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
The break statement is used inside the switch to terminate a statement sequence.
The break statement is optional. But if it is omitted, the execution will continue on into the next case.
An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
try{eval("greeting('welcome)");// Missing ' will produce an error}catch(err){console.log(err.name);}
Entry Controlled loops: In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
Exit Controlled Loops: In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.
Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.
The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.
How do you perform language specific date and time formatting
You can use the Intl.DateTimeFormat object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,
An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a next() method which returns an object with two properties: value (the next value in the sequence) and done (which is true if the last value in the sequence has been consumed).
Synchronous iteration was introduced in ES6 and it works with below set of components,
Iterable: It is an object which can be iterated over via a method whose key is Symbol.iterator.
Iterator: It is an object returned by invoking [Symbol.iterator]() on an iterable. This iterator object wraps each iterated element in an object and returns it via next() method one by one.
IteratorResult: It is an object returned by next() method. The object contains two properties; the value property contains an iterated element and the done property determines whether the element is the last element or not.
Let's demonstrate synchronous iteration with an array as below,
The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn't start processing the event loop until the async function has finished executing the code.
Note: It allows Node.js to perform non-blocking I/O operations even though JavaScript is single-threaded.
Call Stack is a data structure for javascript interpreters to keep track of function calls(creates execution context) in the program. It has two major actions,
Whenever you call a function for its execution, you are pushing it to the stack.
Whenever the execution is completed, the function is popped out of the stack.
Let's take an example and it's state representation in a diagram format
functionhungry(){eatFruits();}functioneatFruits(){return"I'm eating fruits";}// Invoke the `hungry` functionhungry();
The above code processed in a call stack as below,
Add the hungry() function to the call stack list and execute the code.
Add the eatFruits() function to the call stack list and execute the code.
Delete the eatFruits() function from our call stack list.
Delete the hungry() function from the call stack list since there are no items anymore.
The event queue follows the queue data structure. It stores async callbacks to be added to the call stack. It is also known as the Callback Queue or Macrotask Queue.
Whenever the call stack receives an async function, it is moved into the Web API. Based on the function, Web API executes it and awaits the result. Once it is finished, it moves the callback into the event queue (the callback of the promise is moved into the microtask queue).
The event queue constantly checks whether or not the call stack is empty. Once the call stack is empty and there is a callback in the event queue, the event queue moves the callback into the call stack. If there is a callback in the microtask queue as well, it is moved first. The microtask queue has a higher priority than the event queue.
A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,
The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action.
varx="100";vary=+x;console.log(typeofx,typeofy);// string, numbervara="Hello";varb=+a;console.log(typeofa,typeofb,b);// string, number, NaN
What is the purpose of compareFunction while sorting arrays
The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value. Let's take an example to see the usage of compareFunction,
You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of reverse() method in an example,
You can use Math.min and Math.max methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array,
How do you find min and max values without Math functions
You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values,
The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it's usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,
// Initialize an array afor(leti=0;i<a.length;a[i++]=0);
You can use the import.meta object which is a meta-property exposing context-specific meta data to a JavaScript module. It contains information about the current module, such as the module's URL. In browsers, you might get different meta data than NodeJS.
The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,
It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a for loop. For example, the below for loop uses multiple expressions in a single location using comma operator,
for(vara=0,b=10;a<=10;a++,b--)
You can also use the comma operator in a return statement where it processes before returning.
TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as
What are the advantages of typescript over javascript
Below are some of the advantages of typescript over javascript,
TypeScript is able to find compile time errors at the development time only and it makes sures less runtime errors. Whereas javascript is an interpreted language.
TypeScript is strongly-typed or supports static typing which allows for checking type correctness at compile time. This is not available in javascript.
TypeScript compiler can compile the .ts files into ES3,ES4 and ES5 unlike ES6 features of javascript which may not be supported in some browsers.
An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.
varinitObject={a: "John",b: 50,c: {}};console.log(initObject.a);// John
The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,
classEmployee{constructor(){this.name="John";}}varemployeeObject=newEmployee();console.log(employeeObject.name);// John
What happens if you write constructor more than once in a class
The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a SyntaxError error.
classEmployee{constructor(){this.name="John";}constructor(){// Uncaught SyntaxError: A class may only have one constructorthis.age=30;}}varemployeeObject=newEmployee();console.log(employeeObject.name);
You can use the super keyword to call the constructor of a parent class. Remember that super() must be called before using 'this' reference. Otherwise it will cause a reference error. Let's the usage of it,
You can use the Object.getPrototypeOf(obj) method to return the prototype of the specified object. i.e. The value of the internal prototype property. If there are no inherited properties then null value is returned.
You can use the Object.setPrototypeOf() method that sets the prototype (i.e., the internal Prototype property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,
The Object.preventExtensions() method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let's see the usage of this property,
constnewObject={};Object.preventExtensions(newObject);// NOT extendabletry{Object.defineProperty(newObject,"newProperty",{// Adding new propertyvalue: 100,});}catch(e){console.log(e);// TypeError: Cannot define property newProperty, object is not extensible}
How do you define multiple properties on an object
The Object.defineProperties() method is used to define new or modify existing properties directly on an object and returning the object. Let's define multiple properties on an empty object,
The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.
Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it.
Let's see the below function before Obfuscation,
functiongreeting(){console.log("Hello, welcome to JS world");}
And after the code Obfuscation, it would be appeared as below,
Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation .
How do you perform form validation using javascript
JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted.
Lets' perform user login in an html form,
How do you perform form validation without javascript
You can perform HTML form validation automatically without using javascript. The validation enabled by applying the required attribute to prevent form submission when the input is empty.
What are the DOM methods available for constraint validation
The below DOM methods are available for constraint validation on an invalid input,
checkValidity(): It returns true if an input element contains valid data.
setCustomValidity(): It is used to set the validationMessage property of an input element.
Let's take an user login form with DOM validations
functionmyFunction(){varuserName=document.getElementById("uname");if(!userName.checkValidity()){document.getElementById("message").innerHTML=userName.validationMessage;}else{document.getElementById("message").innerHTML="Entered a valid username";}}
If an element's value is greater than its max attribute then rangeOverflow property returns true. For example, the below form submission throws an error if the value is more than 100,
No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,
An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.
You can use the Object.getOwnPropertyNames() method which returns an array of all properties found directly in a given object. Let's the usage of it in an example,
You can use the Object.getOwnPropertyDescriptors() method which returns all own property descriptors of a given object. The example usage of this method is below,
The extends keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,
classChildClassextendsParentClass{ ... }
Let's take an example of Square subclass from Polygon parent class,
How do I modify the url without reloading the page
The window.location.href property will be helpful to modify the url but it reloads the page. HTML5 introduced the history.pushState() and history.replaceState() methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,
How do you check whether an array includes a particular value or not
The Array#includes() method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let's see an example to find an element(numeric and string) within an array.
You can use length and every method of arrays to compare two scalar(compared directly using ===) arrays. The combination of these expressions can give the expected result,
The new URL() object accepts the url string and searchParams property of this object can be used to access the get parameters. Remember that you may need to use polyfill or window.location to access the URL in older browsers(including IE).
How do you print numbers with commas as thousand separators
You can use the Number.prototype.toLocaleString() method which returns a string with a language-sensitive representation such as thousand separator,currency etc of this number.
What is the difference between java and javascript
Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas Javascript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let's see the major differences in a tabular format,
Feature
Java
JavaScript
Typed
It's a strongly typed language
It's a dynamic typed language
Paradigm
Object oriented programming
Prototype based programming
Scoping
Block scoped
Function-scoped
Concurrency
Thread based
event based
Memory
Uses more memory
Uses less memory. Hence it will be used for web pages
JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace,
functionfunc1(){console.log("This is a first definition");}functionfunc1(){console.log("This is a second definition");}func1();// This is a second definition
It always calls the second function definition. In this case, namespace will solve the name collision problem.
Even though JavaScript lacks namespaces, we can use Objects , IIFE to create namespaces.
Using Object Literal Notation: Let's wrap variables and functions inside an Object literal which acts as a namespace. After that you can access them using object notation
varnamespaceOne={functionfunc1(){console.log("This is a first definition");}}varnamespaceTwo={functionfunc1(){console.log("This is a second definition");}}namespaceOne.func1();// This is a first definitionnamespaceTwo.func1();// This is a second definition
Using IIFE (Immediately invoked function expression): The outer pair of parentheses of IIFE creates a local scope for all the code inside of it and makes the anonymous function a function expression. Due to that, you can create the same function in two different function expressions to act as a namespace.
(function(){functionfun1(){console.log("This is a first definition");}fun1();})();(function(){functionfun1(){console.log("This is a second definition");}fun1();})();
Using a block and a let/const declaration: In ECMAScript 6, you can simply use a block and a let declaration to restrict the scope of a variable to a block.
{letmyFunction=functionfun1(){console.log("This is a first definition");};myFunction();}//myFunction(): ReferenceError: myFunction is not defined.{letmyFunction=functionfun1(){console.log("This is a second definition");};myFunction();}//myFunction(): ReferenceError: myFunction is not defined.
How do you invoke javascript code in an iframe from parent page
Initially iFrame needs to be accessed using either document.getElementBy or window.frames. After that contentWindow property of iFrame gives the access for targetFunction
document.getElementById("targetFrame").contentWindow.targetFunction();window.frames[0].frameElement.contentWindow.targetFunction();// Accessing iframe this way may not work in latest versions chrome and firefox
You can use the getTimezoneOffset method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTC
You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below,
What are the different methods to find HTML elements in DOM
If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,
document.getElementById(id): It finds an element by Id
document.getElementsByTagName(name): It finds an element by tag name
document.getElementsByClassName(name): It finds an element by class name
jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,
$(document).ready(function(){// It selects the document and apply the function on page loadalert("Welcome to jQuery world");});
Note: You can download it from jquery's official site or install it from CDNs, like google.
V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors.
Note: It can run standalone, or can be embedded into any C++ application.
JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.
letage=50;// age is a number nowage="old";// age is a string nowage=true;// age is a boolean
You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,
JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let's take an example where it is used to avoid redundancy when accessing an object several times.
a.b.c.greeting="welcome";a.b.c.age=32;
Using with it turns this into:
with(a.b.c){greeting="welcome";age=32;}
But this with statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.
for(vari=0;i<4;i++){// global scopesetTimeout(()=>console.log(i));}for(leti=0;i<4;i++){// block scopesetTimeout(()=>console.log(i));}
The output of the above for loops is 4 4 4 4 and 0 1 2 3
Explanation: Due to the event queue/loop of javascript, the setTimeout callback function is called after the loop has been executed. Since the variable i is declared with the var keyword it became a global variable and the value was equal to 4 using iteration when the time setTimeout function is invoked. Hence, the output of the first loop is 4 4 4 4.
Whereas in the second loop, the variable i is declared as the let keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is 0 1 2 3.
ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.
No, you cannot redeclare let and const variables. If you do, it throws below error
Uncaught SyntaxError: Identifier 'someVariable' has already been declared
Explanation: The variable declaration with var keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.
varname="John";functionmyFunc(){varname="Nick";varname="Abraham";// Re-assigned in the same function blockalert(name);// Abraham}myFunc();alert(name);// John
The block-scoped multi-declaration throws syntax error,
letname="John";functionmyFunc(){letname="Nick";letname="Abraham";// Uncaught SyntaxError: Identifier 'name' has already been declaredalert(name);}myFunc();alert(name);
No, the const variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)
constuserList=[];userList.push("John");// Can mutate even though it can't re-assignconsole.log(userList);// ['John']
In E5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let's compare the behavior with an examples,
Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes.
In E6, this feature enables using dynamic expressions as below,
vargreeting=`Welcome to JS World, Mr. ${firstName}${lastName}.`;
In ES5, you need break string like below,
vargreeting='Welcome to JS World, Mr. '+firstName+' '+lastName.`
Note: You can use multi-line strings and string interpolation features with template literals.
The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,
Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let's see the usage of this tagged template behavior of an IT professional skill set in an organization,
varuser1="John";varskill1="JavaScript";varexperience1=15;varuser2="Kane";varskill2="JavaScript";varexperience2=5;functionmyInfoTag(strings,userExp,experienceExp,skillExp){varstr0=strings[0];// "Mr/Ms. "varstr1=strings[1];// " is a/an "varstr2=strings[2];// "in"varexpertiseStr;if(experienceExp>10){expertiseStr="expert developer";}elseif(skillExp>5&&skillExp<=10){expertiseStr="senior developer";}else{expertiseStr="junior developer";}return`${str0}${userExp}${str1}${expertiseStr}${str2}${skillExp}`;}varoutput1=myInfoTag`Mr/Ms. ${user1} is a/an ${experience1} in ${skill1}`;varoutput2=myInfoTag`Mr/Ms. ${user2} is a/an ${experience2} in ${skill2}`;console.log(output1);// Mr/Ms. John is a/an expert developer in JavaScriptconsole.log(output2);// Mr/Ms. Kane is a/an junior developer in JavaScript
ES6 provides a raw strings feature using the String.raw() method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,
varcalculationString=String.raw`The sum of numbers is \n${1+2+3+4}!`;console.log(calculationString);// The sum of numbers is 10
If you don't use raw strings, the newline character sequence will be processed by displaying the output in multiple lines
varcalculationString=`The sum of numbers is \n${1+2+3+4}!`;console.log(calculationString);// The sum of numbers is// 10
Also, the raw property is available on the first argument to the tag function
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables.
Let's get the month values from an array using destructuring assignment
What are default values in destructuring assignment
A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let's take an example for both arrays and object use cases,
How do you swap variables in destructuring assignment
If you don't use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let's swap two number variables in array destructuring assignment,
Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.
The dynamic imports using import() function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in stage4 proposal. The main advantage of dynamic imports is reduction of our bundle's sizes, the size/payload response of our requests and overall improvements in the user experience.
The syntax of dynamic imports would be as below,
Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let's take comparison and sorting features,
Comparison:
varlist=["ä","a","z"];// In German, "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"varl10nDE=newIntl.Collator("de");varl10nSV=newIntl.Collator("sv");console.log(l10nDE.compare("ä","z")===-1);// trueconsole.log(l10nSV.compare("ä","z")===+1);// true
Sorting:
varlist=["ä","a","z"];// In German, "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"varl10nDE=newIntl.Collator("de");varl10nSV=newIntl.Collator("sv");console.log(list.sort(l10nDE.compare));// [ "a", "ä", "z" ]console.log(list.sort(l10nSV.compare));// [ "a", "z", "ä" ]
The for...of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for...of statement on arrays would be as below,
The output of the array is ['J', 'o', 'h', 'n', '', 'R', 'e', 's', 'i', 'g']
Explanation: The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an Array.
Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.
What are the problems with postmessage target origin as wildcard
The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “*” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities.
How do you avoid receiving postMessages from attackers
Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver's end using the “message.origin” attribute. For examples, let's check the sender's origin http://www.some-sender.com on receiver side www.some-receiver.com,
//Listener on http://www.some-receiver.com/window.addEventListener("message",function(message){if(/^http://www\.some-sender\.com$/.test(message.origin)){console.log('You received the data from valid sender',message.data);}});
You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.
The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.
JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.
What is the difference between internal and external javascript
Internal JavaScript: It is the source code within the script tag.
External JavaScript: The source code is stored in an external file(stored with .js extension) and referred with in the tag.
Yes, JavaScript is faster than server side script. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.
You can apply the checked property on the selected checkbox in the DOM. If the value is true means the checkbox is checked otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below,
<inputtype="checkbox" id="checkboxname" value="Agree" /> Agree the
conditions<br />
console.log(document.getElementById(‘checkboxname’).checked);// true or false
You can use the String.prototype.charCodeAt() method to convert string characters to ASCII numbers. For example, let's find ASCII code for the first letter of 'ABC' string,
"ABC".charCodeAt(0);// returns 65
Whereas String.fromCharCode() method converts numbers to equal ASCII characters.
The output of the above expression is "W".
Explanation: The bracket notation with specific index on a string returns the character at a specific location. Hence, it returns the character "W" of the string. Since this is not supported in IE7 and below versions, you may need to use the .charAt() method to get the desired result.
The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,
newError([message[,fileName[,lineNumber]]])
You can throw user defined exceptions or errors using Error object in try...catch block as below,
try{if(withdraw>balance)thrownewError("Oops! You don't have enough balance");}catch(e){console.log(e.name+": "+e.message);}
The EvalError object indicates an error regarding the global eval() function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,
newEvalError([message[,fileName[,lineNumber]]])
You can throw EvalError with in try...catch block as below,
try{thrownewEvalError('Eval function error','someFile.js',100);}catch(e){console.log(e.message,e.name,e.fileName);// "Eval function error", "EvalError", "someFile.js"
What is the difference between a parameter and an argument
Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function
The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let's take an example to test for any odd elements,
vararray=[1,2,3,4,5,6,7,8,9,10];varodd=(element)=>element%2!==0;console.log(array.some(odd));// true (the odd element exists)
What is the difference between Shallow and Deep copy
There are two ways to copy an object,
Shallow Copy:
Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
if we change some property value in the duplicate one like this:
empDetailsShallowCopy.name="Johnson";
The above statement will also change the name of empDetails, since we have a shallow copy. That means we're losing the original data as well.
Deep copy:
A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.
How do you create specific number of copies of a string
The repeat() method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification.
Let's take an example of Hello string to repeat it 4 times,
"Hello".repeat(4);// 'HelloHelloHelloHello'
How do you return all matching strings against a regular expression
The matchAll() method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,
How do you trim a string at the beginning or ending
The trim method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can use trimStart/trimLeft and trimEnd/trimRight methods. Let's see an example of these methods on a greeting message,
sudheerj/javascript-interview-questions
JavaScript Interview Questions & Answers
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Table of Contents
What are the possible ways to create objects in JavaScript
There are many ways to create objects in javascript as below
Object constructor:
The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.
Object's create method:
The create method of Object creates a new object by passing the prototype object as a parameter
Object literal syntax:
The object literal syntax (or object initializer), is a comma-separated set of name-value pairs wrapped in curly braces.
Note: This is an easiest way to create an object
Function constructor:
Create any function and apply the new operator to create object instances,
Function constructor with prototype:
This is similar to function constructor but it uses prototype for their properties and methods,
This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.
(OR)
ES6 Class syntax:
ES6 introduces class feature to create the objects
Singleton pattern:
A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.
What is a prototype chain
Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.
The prototype on object instance is available through Object.getPrototypeOf(object) or __proto__ property whereas prototype on constructors function is available through Object.prototype.
What is the difference between Call, Apply and Bind
The difference between Call, Apply and Bind can be explained with below examples,
Call: The call() method invokes a function with a given
this
value and arguments provided one by oneApply: Invokes the function with a given
this
value and allows you to pass in arguments as an arraybind: returns a new function, allowing you to pass any number of arguments
Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array.
Whereas Bind creates a new function that will have
this
set to the first parameter passed to bind().What is JSON and its common operations
JSON is a text-based data format following JavaScript object syntax, which was popularized by
Douglas Crockford
. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/jsonParsing: Converting a string to a native object
Stringification: converting a native object to a string so it can be transmitted across the network
What is the purpose of the array slice method
The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.
Some of the examples of this method are,
Note: Slice method won't mutate the original array but it returns the subset as a new array.
What is the purpose of the array splice method
The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.
Some of the examples of this method are,
Note: Splice method modifies the original array and returns the deleted array.
What is the difference between slice and splice
Some of the major difference in a tabular form
How do you compare Object and Map
Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.
What is the difference between == and === operators
JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,
Some of the example which covers the above cases,
What are lambda or arrow functions
An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.
What is a first class function
In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.
For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener
What is a first order function
First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.
What is a higher order function
Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.
What is a unary function
Unary function (i.e. monadic) is a function that accepts exactly one argument. It stands for a single argument accepted by a function.
Let us take an example of unary function,
What is the currying function
Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function.
Let's take an example of n-ary function and how it turns into a currying function,
Curried functions are great to improve code reusability and functional composition.
What is a pure function
A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.
Let's take an example to see the difference between pure and impure functions,
As per the above code snippets, the Push function is impure itself by altering the array and returning a push number index independent of the parameter value. . Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array.
Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.
What is the purpose of the let keyword
The
let
statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with thevar
keyword used to define a variable globally, or locally to an entire function regardless of block scope.Let's take an example to demonstrate the usage,
What is the difference between let and var
You can list out the differences in a tabular format
Let's take an example to see the difference,
What is the reason to choose the name let as a keyword
let
is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that uselet
already as a traditional keyword as close tovar
as possible.How do you redeclare variables in switch block without an error
If you try to redeclare variables in a
switch block
then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.
What is the Temporal Dead Zone
The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a
let
orconst
variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone.Let's see this behavior with an example,
What is IIFE(Immediately Invoked Function Expression)
IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,
The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,
How do you decode or encode a URL in JavaScript?
encodeURI()
function is used to encode an URL. This function requires a URL string as a parameter and return that encoded string.decodeURI()
function is used to decode an URL. This function requires an encoded URL string as parameter and return that decoded string.Note: If you want to encode characters such as
/ ? : @ & = + $ #
then you need to useencodeURIComponent()
.What is memoization
Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization,
What is Hoisting
Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting,
The above code looks like as below to the interpreter,
In the same fashion, function declarations are hoisted too
This hoisting makes functions to be safely used in code before they are declared.
What are classes in ES6
In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below,
Whereas ES6 classes can be defined as an alternative
What are closures
A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains
Let's take an example of closure concept,
As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.
What are modules
Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor
Why do you need modules
Below are the list of benefits using modules in javascript ecosystem
What is scope in javascript
Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.
What is a service worker
A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.
How do you manipulate DOM using a service worker
Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the
postMessage
interface, and those pages can manipulate the DOM.How do you reuse information across service worker restarts
The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's
onfetch
andonmessage
handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.What is IndexedDB
IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.
What is web storage
Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.
What is a post message
Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).
What is a Cookie
A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below,
Why do you need a Cookie
Cookies are used to remember information about the user profile(such as username). It basically involves two steps,
What are the options in a cookie
There are few below options available for a cookie,
How do you delete a cookie
You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below.
Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.
What are the differences between cookie, local storage and session storage
Below are some of the differences between cookie, local storage and session storage,
What is the main difference between localStorage and sessionStorage
LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.
How do you access web storage
The Window object implements the
WindowLocalStorage
andWindowSessionStorage
objects which haslocalStorage
(window.localStorage) andsessionStorage
(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as belowWhat are the methods available on session storage
The session storage provided methods for reading, writing and clearing the session data
What is a storage event and its event handler
The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below
Let's take the example usage of onstorage event handler which logs the storage key and it's values
Why do you need web storage
Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.
How do you check web storage browser support
You need to check browser support for localStorage and sessionStorage before using web storage,
How do you check web workers browser support
You need to check browser support for web workers before using it
Give an example of a web worker
You need to follow below steps to start using web workers for counting example
Here postMessage() method is used to post a message back to the HTML page
and we can receive messages from web worker
What are the restrictions of web workers on DOM
WebWorkers don't have access to below javascript objects since they are defined in an external files
What is a promise
A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.
The syntax of Promise creation looks like below,
The usage of a promise would be as below,
The action flow of a promise will be as below,
Why do you need a promise
Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.
What are the three states of promise
Promises have three states:
What is a callback function
A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let's take a simple example of how to use callback function
Why do we need callbacks
The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events. Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.
As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.
What is a callback hell
Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
What are server-sent events
Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.
How do you receive server-sent event notifications
The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,
How do you check browser support for server-sent events
You can perform browser support for server-sent events before using it as below,
What are the events available for server sent events
Below are the list of events available for server sent events
What are the main rules of promise
A promise must follow a specific set of rules:
.then()
methodWhat is callback in callback
You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.
What is promise chaining
The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,
In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,
.then
handler is called by logging the result(1) and then return a promise with the value of result * 2..then
handler by logging the result(2) and return a promise with result * 3..then
handler by logging the result(6) and return a promise with result * 4.What is promise.all
Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,
Note: Remember that the order of the promises(output the result) is maintained as per input order.
What is the purpose of the race method in promise
Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first
What is a strict mode in javascript
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression
"use strict";
instructs the browser to use the javascript code in the Strict mode.Why do you need strict mode
Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.
How do you declare strict mode
The strict mode is declared by adding "use strict"; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.
and if you declare inside a function, it has local scope
What is the purpose of double exclamation
The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, it will be true. For example, you can test IE version using this expression as below,
If you don't use this expression then it returns the original value.
Note: The expression !! is not an operator, but it is just twice of ! operator.
What is the purpose of the delete operator
The delete keyword is used to delete the property as well as its value.
What is typeof operator
You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.
What is undefined property
The undefined property indicates that a variable has not been assigned a value, or declared but not initialized at all. The type of undefined value is undefined too.
Any variable can be emptied by setting the value to undefined.
What is null value
The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object. You can empty the variable by setting the value to null.
What is the difference between null and undefined
Below are the main differences between null and undefined,
What is eval
The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.
What is the difference between window and document
Below are the main differences between window and document,
How do you access history in javascript
The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.
Note: You can also access history without window prefix.
How do you detect caps lock key turned on or not
The
mouseEvent getModifierState()
is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.Let's take an input element to detect the CapsLock on/off behavior with an example,
What is isNaN
The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.
What are the differences between undeclared and undefined variables
Below are the major differences between undeclared(not defined) and undefined variables,
What are global variables
Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable
What are the problems with global variables
The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.
What is NaN property
The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases
What is the purpose of isFinite function
The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.
What is an event flow
Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow
What is event bubbling
Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.
What is event capturing
Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.
How do you submit a form using JavaScript
You can submit a form using
document.forms[0].submit()
. All the form input's information is submitted using onsubmit event handlerHow do you find operating system details
The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,
What is the difference between document load and DOMContentLoaded events
The
DOMContentLoaded
event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).What is the difference between native, host and user objects
Native objects
are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec.Host objects
are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects.User objects
are objects defined in the javascript code. For example, User objects created for profile information.What are the tools or techniques used for debugging JavaScript code
You can use below tools or techniques for debugging javascript
What are the pros and cons of promises over callbacks
Below are the list of pros and cons of promises over callbacks,
Pros:
Cons:
What is the difference between an attribute and a property
Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,
You can retrieve the attribute value as below,
And after you change the value of the text field to "Good evening", it becomes like
What is same-origin policy
The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).
What is the purpose of void 0
Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an
<a>
element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the pageIs JavaScript a compiled or interpreted language
JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.
Is JavaScript a case-sensitive language
Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.
Is there any relation between Java and JavaScript
No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).
What are events
Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can
react
on these events. Some of the examples of HTML events are,Let's describe the behavior of click event for button element,
Who created javascript
JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name
Mocha
, but later the language was officially calledLiveScript
when it first shipped in beta releases of Netscape.What is the use of preventDefault method
The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.
Note: Remember that not all events are cancelable.
What is the use of stopPropagation method
The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)
What are the steps involved in return false usage
The return false statement in event handlers performs the below steps,
What is BOM
The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.
What is the use of setTimeout
The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,
What is the use of setInterval
The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,
Why is JavaScript treated as Single threaded
JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.
What is an event delegation
Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.
For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,
What is ECMAScript
ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.
What is JSON
JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.
What are the syntax rules of JSON
Below are the list of syntax rules of JSON
What is the purpose JSON stringify
When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.
How do you parse JSON string
When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.
Why do you need JSON
When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.
What are PWAs
Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.
What is the purpose of clearTimeout method
The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.
For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.
What is the purpose of clearInterval method
The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.
For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.
How do you redirect new page in javascript
In vanilla javascript, you can redirect to a new page using the
location
property of window object. The syntax would be as follows,How do you check whether a string contains a substring
There are 3 possible ways to check whether a string contains a substring or not,
String.prototype.includes
method to test a string contains a substringString.prototype.indexOf
which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.RegExp.test
), which allows for testing for against regular expressionsHow do you validate an email in javascript
You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.
The above regular expression accepts unicode characters.
How do you get the current url with javascript
You can use
window.location.href
expression to get the current url path and you can use the same expression for updating the URL too. You can also usedocument.URL
for read-only purposes but this solution has issues in FF.What are the various url properties of location object
The below
Location
object properties can be used to access URL components of the page,How do get query string values in javascript
You can use URLSearchParams to get query string values in javascript. Let's see an example to get the client code value from URL query string,
How do you check if a key exists in an object
You can check whether a key exists in an object or not using three approaches,
and If you want to check if a key doesn't exist, remember to use parenthesis,
hasOwnProperty
to particularly test for properties of the object instance (and not inherited properties)How do you loop through or enumerate javascript object
You can use the
for-in
loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype usinghasOwnProperty
method.How do you test for an empty object
There are different solutions based on ECMAScript versions
What is an arguments object
The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,
Note: You can't apply array methods on arguments object. But you can convert into a regular array as below.
How do you make first letter of the string in an uppercase
You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.
What are the pros and cons of for loop
The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons
Pros
Cons
How do you display the current date in javascript
You can use
new Date()
to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyyHow do you compare two date objects
You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators)
How do you check if a string starts with another string
You can use ECMAScript 6's
String.prototype.startsWith()
method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,How do you trim a string in javascript
JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.
If your browser(<IE9) doesn't support this method then you can use below polyfill.
How do you add a key value pair in javascript
There are two possible solutions to add new properties to an object. Let's take a simple object to explain these solutions.
Is the !-- notation represents a special operator
No,that's not a special operator. But it is a combination of 2 standard operators one after the other,
At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.
How do you assign default values to variables
You can use the logical or operator
||
in an assignment expression to provide a default value. The syntax looks like as below,As per the above expression, variable 'a 'will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.
How do you define multiline strings
You can define multiline string literals using the '\' character followed by line terminator.
But if you have a space after the '\' character, the code will look exactly the same, but it will raise a SyntaxError.
What is an app shell model
An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users' screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.
Can we define properties for functions
Yes, We can define properties for functions because functions are also objects.
What is the way to find the number of parameters expected by a function
You can use
function.length
syntax to find the number of parameters expected by a function. Let's take an example ofsum
function to calculate the sum of numbers,What is a polyfill
A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.
What are break and continue statements
The break statement is used to "jump out" of a loop. i.e, It breaks the loop and continues executing the code after the loop.
The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
What are js labels
The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,
What are the benefits of keeping declarations at the top
It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,
What are the benefits of initializing variables
It is recommended to initialize variables because of the below benefits,
What are the recommendations to create new object
It is recommended to avoid creating new objects using
new Object()
. Instead you can initialize values based on it's type to create the objects.You can define them as an example,
How do you define JSON arrays
JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,
How do you generate random integers
You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,
Note: Math.random() returns a random number between 0 (inclusive), and 1 (exclusive)
Can you write a random integers function to print integers with in a range
Yes, you can create a proper random function to return a random number between min and max (both included)
What is tree shaking
Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler
rollup
.What is the need of tree shaking
Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.
Is it recommended to use eval
No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.
What is a Regular Expression
A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,
For example, the regular expression or search pattern with case-insensitive username would be,
What are the string methods available in Regular expression
Regular Expressions has two string methods: search() and replace(). The search() method uses an expression to search for a match, and returns the position of the match.
The replace() method is used to return a modified string where the pattern is replaced.
What are modifiers in regular expression
Modifiers can be used to perform case-insensitive and global searches. Let's list down some of the modifiers,
Let's take an example of global modifier,
What are regular expression patterns
Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,
What is a RegExp object
RegExp object is a regular expression object with predefined properties and methods. Let's see the simple usage of RegExp object,
How do you search a string for a pattern
You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result.
What is the purpose of exec method
The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.
How do you change the style of a HTML element
You can change inline style or classname of a HTML element using javascript
What would be the result of 1+2+'3'
The output is going to be
33
. Since1
and2
are numeric values, the result of the first two digits is going to be a numeric value3
. The next digit is a string type value because of that the addition of numeric value3
and string type value3
is just going to be a concatenation value33
.What is a debugger statement
The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect. For example, in the below function a debugger statement has been inserted. So execution is paused at the debugger statement just like a breakpoint in the script source.
What is the purpose of breakpoints in debugging
You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.
Can I use reserved words as identifiers
No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example,
How do you detect a mobile browser
You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.
How do you detect a mobile browser without regexp
You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,
How do you get the image width and height using JS
You can programmatically get the image and check the dimensions(width and height) using Javascript.
How do you make synchronous HTTP request
Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript
How do you make asynchronous HTTP request
Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.
How do you convert date to another timezone in javascript
You can use the toLocaleString() method to convert dates in one timezone to another. For example, let's convert current date to British English timezone as below,
What are the properties used to get size of window
You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let's use them combination of these properties to calculate the size of a window or document,
What is a conditional operator in javascript
The conditional (ternary) operator is the only JavaScript operator that takes three operands which acts as a shortcut for if statements.
Can you apply chaining on conditional operator
Yes, you can apply chaining on conditional operators similar to if … else if … else if … else chain. The syntax is going to be as below,
What are the ways to execute javascript after page load
You can execute javascript after page load in many different ways,
What is the difference between proto and prototype
The
__proto__
object is the actual object that is used in the lookup chain to resolve methods, etc. Whereasprototype
is the object that is used to build__proto__
when you create an object with new.There are few more differences,
Give an example where do you really need semicolon
It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error ".. is not a function" at runtime due to missing semicolon.
and it will be interpreted as
In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.
What is a freeze method
The freeze() method is used to freeze an object. Freezing an object does not allow adding new properties to an object,prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.
Remember freezing is only applied to the top-level properties in objects but not for nested objects. For example, let's try to freeze user object which has employment details as nested object and observe that details have been changed.
Note: It causes a TypeError if the argument passed is not an object.
What is the purpose of freeze method
Below are the main benefits of using freeze method,
Why do I need to use freeze method
In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the
final
keyword which is used in various languages.How do you detect a browser language preference
You can use navigator object to detect a browser language preference as below,
How to convert string to title case with javascript
Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,
How do you detect javascript disabled in the page
You can use the
<noscript>
tag to detect javascript disabled or not. The code block inside<noscript>
gets executed when JavaScript is disabled, and is typically used to display alternative content when the page generated in JavaScript.What are various operators supported by javascript
An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,
typeof variable
What is a rest parameter
Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,
For example, let's take a sum example to calculate on dynamic number of parameters,
Note: Rest parameter is added in ES2015 or ES6
What happens if you do not use rest parameter as a last argument
The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.
What are the bitwise operators available in javascript
Below are the list of bitwise logical operators used in JavaScript
What is a spread operator
Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior,
How do you determine whether object is frozen or not
Object.isFrozen() method is used to determine if an object is frozen or not.An object is frozen if all of the below conditions hold true,
How do you determine two values same or not using object
The Object.is() method determines whether two values are the same value. For example, the usage with different types of values would be,
Two values are the same if one of the following holds:
What is the purpose of using object is method
Some of the applications of Object's
is
method are follows,How do you copy properties from one object to other
You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects. The syntax would be as below,
Let's take example with one source and one target object,
As observed in the above code, there is a common property(
b
) from source to target so it's value has been overwritten.What are the applications of assign method
Below are the some of main applications of Object.assign() method,
What is a proxy object
The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
Let's take an example of proxy object,
In the above code, it uses
get
handler which define the behavior of the proxy when an operation is performed on itWhat is the purpose of seal method
The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method
What are the applications of seal method
Below are the main applications of Object.seal() method,
What are the differences between freeze and seal methods
If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.
How do you determine if an object is sealed or not
The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true
How do you get enumerable key and value pairs
The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example,
Note: The order is not guaranteed as object defined.
What is the main difference between Object.values and Object.entries method
The Object.values() method's behavior is similar to Object.entries() method but it returns an array of values instead [key,value] pairs.
How can you get the list of keys of any object
You can use the
Object.keys()
method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,How do you create an object with prototype
The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.
What is a WeakSet
WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,
Let's see the below example to explain it's behavior,
What are the differences between WeakSet and Set
The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it. Other differences are,
List down the collection of methods available on WeakSet
Below are the list of methods available on WeakSet,
Let's see the functionality of all the above methods in an example,
What is a WeakMap
The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
Let's see the below example to explain it's behavior,
What are the differences between WeakMap and Map
The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it. Other differences are,
List down the collection of methods available on WeakMap
Below are the list of methods available on WeakMap,
What is the purpose of uneval
The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,
How do you encode an URL
The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.
How do you decode an URL
The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().
How do you print the contents of web page
The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,
Note: In most browsers, it will block while the print dialog is open.
What is the difference between uneval and eval
The
uneval
function returns the source of a given object; whereas theeval
function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,What is an anonymous function
An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,
Let's see the above anonymous function in an example,
What is the precedence order between local and global variables
A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example.
What are javascript accessors
ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the
get
keyword whereas Setters uses theset
keyword.How do you define property on Object constructor
The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,
What is the difference between get and defineProperty
Both have similar results until unless you use classes. If you use
get
the property will be defined on the prototype of the object whereas usingObject.defineProperty()
the property will be defined on the instance it is applied to.What are the advantages of Getters and Setters
Below are the list of benefits of Getters and Setters,
Can I add getters and setters using defineProperty method
Yes, You can use the
Object.defineProperty()
method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,What is the purpose of switch-case
The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.
What are the conventions to be followed for the usage of switch case
Below are the list of conventions should be taken care,
What are primitive data types
A primitive data type is data that has a primitive value (which has no properties or methods). There are 7 types of primitive data types.
What are the different ways to access object properties
There are 3 possible ways for accessing the property of an object.
What are the function parameter rules
JavaScript functions follow below rules for parameters,
What is an error object
An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
When you get a syntax error
A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
What are the different error names from error object
There are 6 different types of error names returned from error object,
What are the various statements in error handling
Below are the list of statements used in an error handling,
What are the two types of loops in javascript
What is nodejs
Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.
What is an Intl object
The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.
How do you perform language specific date and time formatting
You can use the
Intl.DateTimeFormat
object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,What is an Iterator
An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a
next()
method which returns an object with two properties:value
(the next value in the sequence) anddone
(which is true if the last value in the sequence has been consumed).How does synchronous iteration works
Synchronous iteration was introduced in ES6 and it works with below set of components,
Iterable: It is an object which can be iterated over via a method whose key is Symbol.iterator. Iterator: It is an object returned by invoking
[Symbol.iterator]()
on an iterable. This iterator object wraps each iterated element in an object and returns it vianext()
method one by one. IteratorResult: It is an object returned bynext()
method. The object contains two properties; thevalue
property contains an iterated element and thedone
property determines whether the element is the last element or not.Let's demonstrate synchronous iteration with an array as below,
What is an event loop
The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn't start processing the event loop until the async function has finished executing the code. Note: It allows Node.js to perform non-blocking I/O operations even though JavaScript is single-threaded.
What is call stack
Call Stack is a data structure for javascript interpreters to keep track of function calls(creates execution context) in the program. It has two major actions,
Let's take an example and it's state representation in a diagram format
The above code processed in a call stack as below,
hungry()
function to the call stack list and execute the code.eatFruits()
function to the call stack list and execute the code.eatFruits()
function from our call stack list.hungry()
function from the call stack list since there are no items anymore.What is an event queue
The event queue follows the queue data structure. It stores async callbacks to be added to the call stack. It is also known as the Callback Queue or Macrotask Queue.
Whenever the call stack receives an async function, it is moved into the Web API. Based on the function, Web API executes it and awaits the result. Once it is finished, it moves the callback into the event queue (the callback of the promise is moved into the microtask queue).
The event queue constantly checks whether or not the call stack is empty. Once the call stack is empty and there is a callback in the event queue, the event queue moves the callback into the call stack. If there is a callback in the microtask queue as well, it is moved first. The microtask queue has a higher priority than the event queue.
What is a decorator
A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,
What are the properties of Intl object
Below are the list of properties available on Intl object,
What is an Unary operator
The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action.
How do you sort elements in an array
The sort() method is used to sort the elements of an array in place and returns the sorted array. The example usage would be as below,
What is the purpose of compareFunction while sorting arrays
The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value. Let's take an example to see the usage of compareFunction,
How do you reversing an array
You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of reverse() method in an example,
How do you find min and max value in an array
You can use
Math.min
andMath.max
methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array,How do you find min and max values without Math functions
You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values,
What is an empty statement and purpose of it
The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it's usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,
How do you get metadata of a module
You can use the
import.meta
object which is a meta-property exposing context-specific meta data to a JavaScript module. It contains information about the current module, such as the module's URL. In browsers, you might get different meta data than NodeJS.What is a comma operator
The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,
What is the advantage of a comma operator
It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a
for
loop. For example, the below for loop uses multiple expressions in a single location using comma operator,You can also use the comma operator in a return statement where it processes before returning.
What is typescript
TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as
Let's see a simple example of TypeScript usage,
The greeting method allows only string type as argument.
What are the differences between javascript and typescript
Below are the list of differences between javascript and typescript,
What are the advantages of typescript over javascript
Below are some of the advantages of typescript over javascript,
What is an object initializer
An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.
What is a constructor method
The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,
What happens if you write constructor more than once in a class
The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a
SyntaxError
error.How do you call the constructor of a parent class
You can use the
super
keyword to call the constructor of a parent class. Remember thatsuper()
must be called before using 'this' reference. Otherwise it will cause a reference error. Let's the usage of it,How do you get the prototype of an object
You can use the
Object.getPrototypeOf(obj)
method to return the prototype of the specified object. i.e. The value of the internalprototype
property. If there are no inherited properties thennull
value is returned.What happens If I pass string type for getPrototype method
In ES5, it will throw a TypeError exception if the obj parameter isn't an object. Whereas in ES2015, the parameter will be coerced to an
Object
.How do you set prototype of one object to another
You can use the
Object.setPrototypeOf()
method that sets the prototype (i.e., the internalPrototype
property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,How do you check whether an object can be extendable or not
The
Object.isExtensible()
method is used to determine if an object is extendable or not. i.e, Whether it can have new properties added to it or not.Note: By default, all the objects are extendable. i.e, The new properties can be added or modified.
How do you prevent an object to extend
The
Object.preventExtensions()
method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let's see the usage of this property,What are the different ways to make an object non-extensible
You can mark an object non-extensible in 3 ways,
How do you define multiple properties on an object
The
Object.defineProperties()
method is used to define new or modify existing properties directly on an object and returning the object. Let's define multiple properties on an empty object,What is MEAN in javascript
The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.
What Is Obfuscation in javascript
Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it. Let's see the below function before Obfuscation,
And after the code Obfuscation, it would be appeared as below,
Why do you need Obfuscation
Below are the few reasons for Obfuscation,
What is Minification
Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation .
What are the advantages of minification
Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits,
What are the differences between Obfuscation and Encryption
Below are the main differences between Obfuscation and Encryption,
What are the common tools used for minification
There are many online/offline tools to minify the javascript files,
How do you perform form validation using javascript
JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted. Lets' perform user login in an html form,
And the validation on user login is below,
How do you perform form validation without javascript
You can perform HTML form validation automatically without using javascript. The validation enabled by applying the
required
attribute to prevent form submission when the input is empty.Note: Automatic form validation does not work in Internet Explorer 9 or earlier.
What are the DOM methods available for constraint validation
The below DOM methods are available for constraint validation on an invalid input,
What are the available constraint validation DOM properties
Below are the list of some of the constraint validation DOM properties available,
What are the list of validity properties
The validity property of an input element provides a set of properties related to the validity of data.
Give an example usage of rangeOverflow property
If an element's value is greater than its max attribute then rangeOverflow property returns true. For example, the below form submission throws an error if the value is more than 100,
Is enums feature available in javascript
No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,
What is an enum
An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.
How do you list all properties of an object
You can use the
Object.getOwnPropertyNames()
method which returns an array of all properties found directly in a given object. Let's the usage of it in an example,How do you get property descriptors of an object
You can use the
Object.getOwnPropertyDescriptors()
method which returns all own property descriptors of a given object. The example usage of this method is below,What are the attributes provided by a property descriptor
A property descriptor is a record which has the following attributes
How do you extend classes
The
extends
keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,Let's take an example of Square subclass from Polygon parent class,
How do I modify the url without reloading the page
The
window.location.href
property will be helpful to modify the url but it reloads the page. HTML5 introduced thehistory.pushState()
andhistory.replaceState()
methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,How do you check whether an array includes a particular value or not
The
Array#includes()
method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let's see an example to find an element(numeric and string) within an array.How do you compare scalar arrays
You can use length and every method of arrays to compare two scalar(compared directly using ===) arrays. The combination of these expressions can give the expected result,
If you would like to compare arrays irrespective of order then you should sort them before,
How to get the value from get parameters
The
new URL()
object accepts the url string andsearchParams
property of this object can be used to access the get parameters. Remember that you may need to use polyfill orwindow.location
to access the URL in older browsers(including IE).How do you print numbers with commas as thousand separators
You can use the
Number.prototype.toLocaleString()
method which returns a string with a language-sensitive representation such as thousand separator,currency etc of this number.What is the difference between java and javascript
Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas Javascript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let's see the major differences in a tabular format,
Does JavaScript supports namespace
JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace,
It always calls the second function definition. In this case, namespace will solve the name collision problem.
How do you declare namespace
Even though JavaScript lacks namespaces, we can use Objects , IIFE to create namespaces.
How do you invoke javascript code in an iframe from parent page
Initially iFrame needs to be accessed using either
document.getElementBy
orwindow.frames
. After thatcontentWindow
property of iFrame gives the access for targetFunctionHow do get the timezone offset from date
You can use the
getTimezoneOffset
method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTCHow do you load CSS and JS files dynamically
You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below,
What are the different methods to find HTML elements in DOM
If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,
What is jQuery
jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,
Note: You can download it from jquery's official site or install it from CDNs, like google.
What is V8 JavaScript engine
V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors. Note: It can run standalone, or can be embedded into any C++ application.
Why do we call javascript as dynamic language
JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.
What is a void operator
The
void
operator evaluates the given expression and then returns undefined(i.e, without returning value). The syntax would be as below,Let's display a message without any redirection or reload
Note: This operator is often used to obtain the undefined primitive value, using "void(0)".
How to set the cursor to wait
The cursor can be set to wait in JavaScript by using the property "cursor". Let's perform this behavior on page load using the below function.
and this function invoked on page load
How do you create an infinite loop
You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,
Why do you need to avoid with statement
JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let's take an example where it is used to avoid redundancy when accessing an object several times.
Using
with
it turns this into:But this
with
statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.What is the output of below for loops
The output of the above for loops is 4 4 4 4 and 0 1 2 3
Explanation: Due to the event queue/loop of javascript, the
setTimeout
callback function is called after the loop has been executed. Since the variable i is declared with thevar
keyword it became a global variable and the value was equal to 4 using iteration when the timesetTimeout
function is invoked. Hence, the output of the first loop is4 4 4 4
.Whereas in the second loop, the variable i is declared as the
let
keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is0 1 2 3
.List down some of the features of ES6
Below are the list of some new features of ES6,
What is ES6
ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.
Can I redeclare let and const variables
No, you cannot redeclare let and const variables. If you do, it throws below error
Uncaught SyntaxError: Identifier 'someVariable' has already been declared
Explanation: The variable declaration with
var
keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.The block-scoped multi-declaration throws syntax error,
Is const variable makes the value immutable
No, the const variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)
What are default parameters
In E5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let's compare the behavior with an examples,
The default parameters makes the initialization more simpler,
What are template literals
Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes. In E6, this feature enables using dynamic expressions as below,
In ES5, you need break string like below,
Note: You can use multi-line strings and string interpolation features with template literals.
How do you write multi-line strings in template literals
In ES5, you would have to use newline escape characters('\n') and concatenation symbols(+) in order to get multi-line strings.
Whereas in ES6, You don't need to mention any newline sequence character,
What are nesting templates
The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,
You can write the above use case without nesting template features as well. However, the nesting template feature is more compact and readable.
What are tagged templates
Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let's see the usage of this tagged template behavior of an IT professional skill set in an organization,
What are raw strings
ES6 provides a raw strings feature using the
String.raw()
method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,If you don't use raw strings, the newline character sequence will be processed by displaying the output in multiple lines
Also, the raw property is available on the first argument to the tag function
What is destructuring assignment
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment
and you can get user properties of an object using destructuring assignment,
What are default values in destructuring assignment
A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let's take an example for both arrays and object use cases,
Arrays destructuring:
Objects destructuring:
How do you swap variables in destructuring assignment
If you don't use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let's swap two number variables in array destructuring assignment,
What are enhanced object literals
Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.
What are dynamic imports
The dynamic imports using
import()
function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in stage4 proposal. The main advantage of dynamic imports is reduction of our bundle's sizes, the size/payload response of our requests and overall improvements in the user experience. The syntax of dynamic imports would be as below,What are the use cases for dynamic imports
Below are some of the use cases of using dynamic imports over static imports,
What are typed arrays
Typed arrays are array-like objects from ECMAScript 6 API for handling binary data. JavaScript provides 8 Typed array types,
For example, you can create an array of 8-bit signed integers as below
What are the advantages of module loaders
The module loaders provides the below features,
What is collation
Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let's take comparison and sorting features,
What is for...of statement
The for...of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for...of statement on arrays would be as below,
What is the output of below spread operator array
The output of the array is ['J', 'o', 'h', 'n', '', 'R', 'e', 's', 'i', 'g'] Explanation: The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an Array.
Is PostMessage secure
Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.
What are the problems with postmessage target origin as wildcard
The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “*” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities.
How do you avoid receiving postMessages from attackers
Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver's end using the “message.origin” attribute. For examples, let's check the sender's origin http://www.some-sender.com on receiver side www.some-receiver.com,
Can I avoid using postMessages completely
You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.
Is postMessages synchronous
The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.
What paradigm is Javascript
JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.
What is the difference between internal and external javascript
Internal JavaScript: It is the source code within the script tag. External JavaScript: The source code is stored in an external file(stored with .js extension) and referred with in the tag.
Is JavaScript faster than server side script
Yes, JavaScript is faster than server side script. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.
How do you get the status of a checkbox
You can apply the
checked
property on the selected checkbox in the DOM. If the value istrue
means the checkbox is checked otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below,What is the purpose of double tilde operator
The double tilde operator(~~) is known as double NOT bitwise operator. This operator is going to be a quicker substitute for Math.floor().
How do you convert character to ASCII code
You can use the
String.prototype.charCodeAt()
method to convert string characters to ASCII numbers. For example, let's find ASCII code for the first letter of 'ABC' string,Whereas
String.fromCharCode()
method converts numbers to equal ASCII characters.What is ArrayBuffer
An ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can create it as below,
To manipulate an ArrayBuffer, we need to use a “view” object.
What is the output of below string expression
The output of the above expression is "W". Explanation: The bracket notation with specific index on a string returns the character at a specific location. Hence, it returns the character "W" of the string. Since this is not supported in IE7 and below versions, you may need to use the .charAt() method to get the desired result.
What is the purpose of Error object
The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,
You can throw user defined exceptions or errors using Error object in try...catch block as below,
What is the purpose of EvalError object
The EvalError object indicates an error regarding the global
eval()
function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,You can throw EvalError with in try...catch block as below,
What are the list of cases error thrown from non-strict mode to strict mode
When you apply 'use strict'; syntax, some of the below cases will throw a SyntaxError before executing the script
with
statementHence, the errors from above cases are helpful to avoid errors in development/production environments.
Do all objects have prototypes
No. All objects have prototypes except for the base object which is created by the user, or an object that is created using the new keyword.
What is the difference between a parameter and an argument
Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function
What is the purpose of some method in arrays
The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let's take an example to test for any odd elements,
How do you combine two or more arrays
The concat() method is used to join two or more arrays by returning a new array containing all the elements. The syntax would be as below,
Let's take an example of array's concatenation with veggies and fruits arrays,
What is the difference between Shallow and Deep copy
There are two ways to copy an object,
Shallow Copy: Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
Example
to create a duplicate
if we change some property value in the duplicate one like this:
The above statement will also change the name of
empDetails
, since we have a shallow copy. That means we're losing the original data as well.Deep copy: A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.
Example
Create a deep copy by using the properties from the original object into new variable
Now if you change
empDetailsDeepCopy.name
, it will only affectempDetailsDeepCopy
& notempDetails
How do you create specific number of copies of a string
The
repeat()
method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification. Let's take an example of Hello string to repeat it 4 times,How do you return all matching strings against a regular expression
The
matchAll()
method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,How do you trim a string at the beginning or ending
The
trim
method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can usetrimStart/trimLeft
andtrimEnd/trimRight
methods. Let's see an example of these methods on a greeting message,