Note: this guide assumes you are using Babel, and requires that you use babel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, with airbnb-browser-shims or the equivalent.
This guide is available in other languages too. See Translation
Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
// badvara=1;varb=2;// goodconsta=1;constb=2;
2.2 If you must reassign references, use let instead of var. eslint: no-var
Why? let is block-scoped rather than function-scoped like var.
// badvarcount=1;if(true){count+=1;}// good, use the let.letcount=1;if(true){count+=1;}
2.3 Note that both let and const are block-scoped, whereas var is function-scoped.
// const and let only exist in the blocks they are defined in.{leta=1;constb=1;varc=1;}console.log(a);// ReferenceErrorconsole.log(b);// ReferenceErrorconsole.log(c);// Prints 1
In the above code, you can see that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.
3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins
Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
// badconsole.log(object.hasOwnProperty(key));// goodconsole.log(Object.prototype.hasOwnProperty.call(object,key));// bestconsthas=Object.prototype.hasOwnProperty;// cache the lookup once, in module scope.console.log(has.call(object,key));/* or */importhasfrom'has';// https://www.npmjs.com/package/hasconsole.log(has(object,key));/* or */console.log(Object.hasOwn(object,key));// https://www.npmjs.com/package/object.hasown
3.8 Prefer the object spread syntax over Object.assign to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint: prefer-object-spread
4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return
// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});// good[1,2,3].map((x)=>x+1);// bad - no returned value means `acc` becomes undefined after the first iteration[[0,1],[2,3],[4,5]].reduce((acc,item,index)=>{constflatten=acc.concat(item);});// good[[0,1],[2,3],[4,5]].reduce((acc,item,index)=>{constflatten=acc.concat(item);returnflatten;});// badinbox.filter((msg)=>{const{ subject, author }=msg;if(subject==='Mockingbird'){returnauthor==='Harper Lee';}else{returnfalse;}});// goodinbox.filter((msg)=>{const{ subject, author }=msg;if(subject==='Mockingbird'){returnauthor==='Harper Lee';}returnfalse;});
4.8 Use line breaks after open and before close array brackets if an array has multiple lines
5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring
Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
5.3 Use object destructuring for multiple return values, not array destructuring.
Why? You can add new properties over time or change the order of things without breaking call sites.
// badfunctionprocessInput(input){// then a miracle occursreturn[left,right,top,bottom];}// the caller needs to think about the order of return dataconst[left,__,top]=processInput(input);// goodfunctionprocessInput(input){// then a miracle occursreturn{ left, right, top, bottom };}// the caller selects only the data they needconst{ left, top }=processInput(input);
6.1 Use single quotes '' for strings. eslint: quotes
// badconstname="Capt. Janeway";// bad - template literals should contain interpolation or newlinesconstname=`Capt. Janeway`;// goodconstname='Capt. Janeway';
6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.
Why? Broken strings are painful to work with and make code less searchable.
// badconsterrorMessage='This is a super long error that was thrown because \of Batman. When you stop to think about how Batman had anything to do \with this, you would get nowhere \fast.';// badconsterrorMessage='This is a super long error that was thrown because '+'of Batman. When you stop to think about how Batman had anything to do '+'with this, you would get nowhere fast.';// goodconsterrorMessage='This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.
// badfunctionsayHi(name){return'How are you, '+name+'?';}// badfunctionsayHi(name){return['How are you, ',name,'?'].join();}// badfunctionsayHi(name){return`How are you, ${name}?`;}// goodfunctionsayHi(name){return`How are you, ${name}?`;}
6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval
6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape
Why? Backslashes harm readability, thus they should only be present when necessary.
// badconstfoo='\'this\' \i\s \"quoted\"';// goodconstfoo='\'this\' is "quoted"';constfoo=`my name is '${name}'`;
7.1 Use named function expressions instead of function declarations. eslint: func-style
Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. (Discussion)
// badfunctionfoo(){// ...}// badconstfoo=function(){// ...};// good// lexical name distinguished from the variable-referenced invocation(s)constshort=functionlongUniqueMoreDescriptiveLexicalFoo(){// ...};
7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife
Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
// immediately-invoked function expression (IIFE)(function(){console.log('Welcome to the Internet. Please follow me.');}());
7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func
7.4Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.
7.7 Use default parameter syntax rather than mutating function arguments.
// really badfunctionhandleThings(opts){// No! We shouldn’t mutate function arguments.// Double bad: if opts is falsy it'll be set to an object which may// be what you want but it can introduce subtle bugs.opts=opts||{};// ...}// still badfunctionhandleThings(opts){if(opts===void0){opts={};}// ...}// goodfunctionhandleThings(opts={}){// ...}
Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.
7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline
8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing
Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.
Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.
8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style
Why? Syntactic sugar. It reads well when multiple functions are chained together.
// bad[1,2,3].map((number)=>{constnextNumber=number+1;`A string containing the ${nextNumber}.`;});// good[1,2,3].map((number)=>`A string containing the ${number+1}.`);// good[1,2,3].map((number)=>{constnextNumber=number+1;return`A string containing the ${nextNumber}.`;});// good[1,2,3].map((number,index)=>({[index]: number,}));// No implicit return with side effectsfunctionfoo(callback){constval=callback();if(val===true){// Do something if callback returns true}}letbool=false;// badfoo(()=>bool=true);// goodfoo(()=>{bool=true;});
8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
Why? It shows clearly where the function starts and ends.
8.4 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens
Why? Minimizes diff churn when adding or removing arguments.
// bad[1,2,3].map(x=>x*x);// good[1,2,3].map((x)=>x*x);// bad[1,2,3].map(number=>(`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`));// good[1,2,3].map((number)=>(`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`));// bad[1,2,3].map(x=>{consty=x+1;returnx*y;});// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});
8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow
9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor
9.7 Class methods should use this or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this
// badclassFoo{bar(){console.log('bar');}}// good - this is usedclassFoo{bar(){console.log(this.bar);}}// good - constructor is exemptclassFoo{constructor(){// ...}}// good - static methods aren't expected to use thisclassFoo{staticbar(){console.log('bar');}}
Why? Having multiple lines that import from the same path can make code harder to maintain.
// badimportfoofrom'foo';// … some other imports … //import{named1,named2}from'foo';// goodimportfoo,{named1,named2}from'foo';// goodimportfoo,{named1,named2,}from'foo';
Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iteratorno-restricted-syntax
Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.
Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.
constnumbers=[1,2,3,4,5];// badletsum=0;for(letnumofnumbers){sum+=num;}sum===15;// goodletsum=0;numbers.forEach((num)=>{sum+=num;});sum===15;// best (use the functional force)constsum=numbers.reduce((total,num)=>total+num,0);sum===15;// badconstincreasedByOne=[];for(leti=0;i<numbers.length;i++){increasedByOne.push(numbers[i]+1);}// goodconstincreasedByOne=[];numbers.forEach((num)=>{increasedByOne.push(num+1);});// best (keeping it functional)constincreasedByOne=numbers.map((num)=>num+1);
11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: generator-star-spacing
Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.
13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undefprefer-const
13.2 Use one const or let declaration per variable or assignment. eslint: one-var
Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.
// badconstitems=getItems(),goSportsTeam=true,dragonball='z';// bad// (compare to above, and try to spot the mistake)constitems=getItems(),goSportsTeam=true;dragonball='z';// goodconstitems=getItems();constgoSportsTeam=true;constdragonball='z';
13.3 Group all your consts and then group all your lets.
Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.
13.4 Assign variables where you need them, but place them in a reasonable place.
Why? let and const are block scoped and not function scoped.
// bad - unnecessary function callfunctioncheckName(hasName){constname=getName();if(hasName==='test'){returnfalse;}if(name==='test'){this.setName('');returnfalse;}returnname;}// goodfunctioncheckName(hasName){if(hasName==='test'){returnfalse;}constname=getName();if(name==='test'){this.setName('');returnfalse;}returnname;}
Why? Chaining variable assignments creates implicit global variables.
// bad(functionexample(){// JavaScript interprets this as// let a = ( b = ( c = 1 ) );// The let keyword only applies to variable a; variables b and c become// global variables.leta=b=c=1;}());console.log(a);// throws ReferenceErrorconsole.log(b);// 1console.log(c);// 1// good(functionexample(){leta=1;letb=a;letc=a;}());console.log(a);// throws ReferenceErrorconsole.log(b);// throws ReferenceErrorconsole.log(c);// throws ReferenceError// the same applies for `const`
13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus
Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.
Why? Linebreaks surrounding = can obfuscate the value of an assignment.
Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.
// badconstsome_unused_var=42;// Write-only variables are not considered as used.lety=10;y=5;// A read for a modification of itself is not considered as used.letz=0;z=z+1;// Unused function arguments.functiongetX(x,y){returnx;}// goodfunctiongetXPlusY(x,y){returnx+y;}constx=1;consty=a+2;alert(getXPlusY(x,y));// 'type' is ignored even if unused because it has a rest property sibling.// This is a form of extracting an object that omits the specified keys.const{ type, ...coords}=data;// 'coords' is now the 'data' object without its 'type' property.
14.1var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.
// we know this wouldn’t work (assuming there// is no notDefined global variable)functionexample(){console.log(notDefined);// => throws a ReferenceError}// creating a variable declaration after you// reference the variable will work due to// variable hoisting. Note: the assignment// value of `true` is not hoisted.functionexample(){console.log(declaredButNotAssigned);// => undefinedvardeclaredButNotAssigned=true;}// the interpreter is hoisting the variable// declaration to the top of the scope,// which means our example could be rewritten as:functionexample(){letdeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}// using const and letfunctionexample(){console.log(declaredButNotAssigned);// => throws a ReferenceErrorconsole.log(typeofdeclaredButNotAssigned);// => throws a ReferenceErrorconstdeclaredButNotAssigned=true;}
14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
functionexample(){console.log(anonymous);// => undefinedanonymous();// => TypeError anonymous is not a functionvaranonymous=function(){console.log('anonymous function expression');};}
14.3 Named function expressions hoist the variable name, not the function name or the function body.
functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionsuperPower();// => ReferenceError superPower is not definedvarnamed=functionsuperPower(){console.log('Flying');};}// the same is true when the function name// is the same as the variable name.functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionvarnamed=functionnamed(){console.log('named');};}
14.4 Function declarations hoist their name and the function body.
15.1 Use === and !== over == and !=. eslint: eqeqeq
15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
Objects evaluate to true
Undefined evaluates to false
Null evaluates to false
Booleans evaluate to the value of the boolean
Numbers evaluate to false if +0, -0, or NaN, otherwise true
Strings evaluate to false if an empty string '', otherwise true
if([0]&&[]){// true// an array (even an empty one) is an object, objects will evaluate to true}
15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.
15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations
Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.
// badconstfoo=a ? a : b;constbar=c ? true : false;constbaz=c ? false : true;constquux=a!=null ? a : b;// goodconstfoo=a||b;constbar=!!c;constbaz=!c;constquux=a??b;
15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed.
eslint: no-mixed-operators
Why? This improves readability and clarifies the developer’s intention.
// badconstfoo=a&&b<0||c>0||d+1===0;// badconstbar=a**b-5%d;// bad// one may be confused into thinking (a || b) && cif(a||b&&c){returnd;}// badconstbar=a+b/c*d;// goodconstfoo=(a&&b<0)||c>0||(d+1===0);// goodconstbar=a**b-(5%d);// goodif(a||(b&&c)){returnd;}// goodconstbar=a+(b/c)*d;
16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return
17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.
Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.
// bad// make() returns a new element// based on the passed in tag name//// @param {String} tag// @return {Element} elementfunctionmake(tag){// ...returnelement;}// good/** * make() returns a new element * based on the passed-in tag name */functionmake(tag){// ...returnelement;}
18.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.
// badconstactive=true;// is current tab// good// is current tabconstactive=true;// badfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'consttype=this.type||'no type';returntype;}// goodfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'consttype=this.type||'no type';returntype;}// also goodfunctiongetType(){// set the default type to 'no type'consttype=this.type||'no type';returntype;}
18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment
// bad//is current tabconstactive=true;// good// is current tabconstactive=true;// bad/** *make() returns a new element *based on the passed-in tag name */functionmake(tag){// ...returnelement;}// good/** * make() returns a new element * based on the passed-in tag name */functionmake(tag){// ...returnelement;}
18.4 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.
19.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing
19.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which
emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-callno-whitespace-before-property
19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len
Why? This ensures readability and maintainability.
// badconstfoo=jsonData&&jsonData.foo&&jsonData.foo.bar&&jsonData.foo.bar.baz&&jsonData.foo.bar.baz.quux&&jsonData.foo.bar.baz.quux.xyzzy;// bad$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'}}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));// goodconstfoo=jsonData&&jsonData.foo&&jsonData.foo.bar&&jsonData.foo.bar.baz&&jsonData.foo.bar.baz.quux&&jsonData.foo.bar.baz.quux.xyzzy;// betterconstfoo=jsonData?.foo?.bar?.baz?.quux?.xyzzy;// good$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'},}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));
19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing
19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines
// bad - multiple empty linesconstx=1;consty=2;// bad - 2+ newlines at end of fileconstx=1;consty=2;// bad - 1+ newline(s) at beginning of fileconstx=1;consty=2;// goodconstx=1;consty=2;
Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.
// badconsthero={firstName: 'Dana',lastName: 'Scully'};constheroes=['Batman','Superman'];// goodconsthero={firstName: 'Dana',lastName: 'Scully',};constheroes=['Batman','Superman',];// badfunctioncreateHero(firstName,lastName,inventorOf){// does nothing}// goodfunctioncreateHero(firstName,lastName,inventorOf,){// does nothing}// good (note that a comma must not appear after a "rest" element)functioncreateHero(firstName,lastName,inventorOf,
...heroArgs){// does nothing}// badcreateHero(firstName,lastName,inventorOf);// goodcreateHero(firstName,lastName,inventorOf,);// good (note that a comma must not appear after a "rest" element)createHero(firstName,lastName,inventorOf,
...heroArgs);
Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.
// bad - raises exceptionconstluke={}constleia={}[luke,leia].forEach((jedi)=>jedi.father='vader')// bad - raises exceptionconstreaction="No! That’s impossible!"(asyncfunctionmeanwhileOnTheFalcon(){// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`// ...}())// bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!functionfoo(){return'search your feelings, you know it to be foo'}// goodconstluke={};constleia={};[luke,leia].forEach((jedi)=>{jedi.father='vader';});// goodconstreaction='No! That’s impossible!';(asyncfunctionmeanwhileOnTheFalcon(){// handle `leia`, `lando`, `chewie`, `r2`, `c3p0`// ...}());// goodfunctionfoo(){return'search your feelings, you know it to be foo';}
// => this.reviewScore = 9;// badconsttotalScore=newString(this.reviewScore);// typeof totalScore is "object" not "string"// badconsttotalScore=this.reviewScore+'';// invokes this.reviewScore.valueOf()// badconsttotalScore=this.reviewScore.toString();// isn’t guaranteed to return a string// goodconsttotalScore=String(this.reviewScore);
22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radixno-new-wrappers
Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.
22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.
// good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */constval=inputValue>>0;
22.5Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
// badthis.__firstName__='Panda';this.firstName_='Panda';this._firstName='Panda';// goodthis.firstName='Panda';// good, in environments where WeakMaps are available// see https://kangax.github.io/compat-table/es6/#test-WeakMapconstfirstNames=newWeakMap();firstNames.set(this,'Panda');
23.5 Don’t save references to this. Use arrow functions or Function#bind.
23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.
Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.
What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
// badconstPRIVATE_VARIABLE='should not be unnecessarily uppercased within a file';// badexportconstTHING_TO_BE_CHANGED='should obviously not be uppercased';// badexportletREASSIGNABLE_VARIABLE='do not use let with uppercase variables';// ---// allowed but does not supply semantic valueexportconstapiKey='SOMEKEY';// better in most casesexportconstAPI_KEY='SOMEKEY';// ---// bad - unnecessarily uppercases key while adding no semantic valueexportconstMAPPING={KEY: 'value'};// goodexportconstMAPPING={key: 'value',};
24.1 Accessor functions for properties are not required.
24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').
25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
// bad$(this).trigger('listingUpdated',listing.id);// ...$(this).on('listingUpdated',(e,listingID)=>{// do something with listingID});
prefer:
// good$(this).trigger('listingUpdated',{listingID: listing.id});// ...$(this).on('listingUpdated',(e,data)=>{// do something with data.listingID});
Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.
Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number.
If this behavior is desired, make it explicit.
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
'Software'), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.
airbnb/javascript
Airbnb JavaScript Style Guide() {
A mostly reasonable approach to JavaScript
This guide is available in other languages too. See Translation
Other Style Guides
Table of Contents
Types
1.1 Primitives: When you access a primitive type you work directly on its value.
string
number
boolean
null
undefined
symbol
bigint
1.2 Complex: When you access a complex type you work on a reference to its value.
object
array
function
References
2.1 Use
const
for all of your references; avoid usingvar
. eslint:prefer-const
,no-const-assign
2.2 If you must reassign references, use
let
instead ofvar
. eslint:no-var
2.3 Note that both
let
andconst
are block-scoped, whereasvar
is function-scoped.In the above code, you can see that referencing
a
andb
will produce a ReferenceError, whilec
contains the number. This is becausea
andb
are block scoped, whilec
is scoped to the containing function.Objects
3.1 Use the literal syntax for object creation. eslint:
no-new-object
3.2 Use computed property names when creating objects with dynamic property names.
3.3 Use object method shorthand. eslint:
object-shorthand
3.4 Use property value shorthand. eslint:
object-shorthand
3.5 Group your shorthand properties at the beginning of your object declaration.
3.6 Only quote properties that are invalid identifiers. eslint:
quote-props
3.7 Do not call
Object.prototype
methods directly, such ashasOwnProperty
,propertyIsEnumerable
, andisPrototypeOf
. eslint:no-prototype-builtins
3.8 Prefer the object spread syntax over
Object.assign
to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint:prefer-object-spread
Arrays
4.1 Use the literal syntax for array creation. eslint:
no-array-constructor
4.2 Use Array#push instead of direct assignment to add items to an array.
4.3 Use array spreads
...
to copy arrays.4.4 To convert an iterable object to an array, use spreads
...
instead ofArray.from
4.5 Use
Array.from
for converting an array-like object to an array.4.6 Use
Array.from
instead of spread...
for mapping over iterables, because it avoids creating an intermediate array.4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint:
array-callback-return
4.8 Use line breaks after open and before close array brackets if an array has multiple lines
Destructuring
5.1 Use object destructuring when accessing and using multiple properties of an object. eslint:
prefer-destructuring
5.2 Use array destructuring. eslint:
prefer-destructuring
5.3 Use object destructuring for multiple return values, not array destructuring.
Strings
6.1 Use single quotes
''
for strings. eslint:quotes
6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.
6.3 When programmatically building up strings, use template strings instead of concatenation. eslint:
prefer-template
template-curly-spacing
eval()
on a string, it opens too many vulnerabilities. eslint:no-eval
6.5 Do not unnecessarily escape characters in strings. eslint:
no-useless-escape
Functions
7.1 Use named function expressions instead of function declarations. eslint:
func-style
7.2 Wrap immediately invoked function expressions in parentheses. eslint:
wrap-iife
if
,while
, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint:no-loop-func
7.4 Note: ECMA-262 defines a
block
as a list of statements. A function declaration is not a statement.7.5 Never name a parameter
arguments
. This will take precedence over thearguments
object that is given to every function scope.7.6 Never use
arguments
, opt to use rest syntax...
instead. eslint:prefer-rest-params
7.7 Use default parameter syntax rather than mutating function arguments.
7.8 Avoid side effects with default parameters.
7.9 Always put default parameters last. eslint:
default-param-last
7.10 Never use the Function constructor to create a new function. eslint:
no-new-func
7.11 Spacing in a function signature. eslint:
space-before-function-paren
space-before-blocks
7.12 Never mutate parameters. eslint:
no-param-reassign
7.13 Never reassign parameters. eslint:
no-param-reassign
7.14 Prefer the use of the spread syntax
...
to call variadic functions. eslint:prefer-spread
7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint:
function-paren-newline
Arrow Functions
8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint:
prefer-arrow-callback
,arrow-spacing
8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a
return
statement. eslint:arrow-parens
,arrow-body-style
8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.
8.4 Always include parentheses around arguments for clarity and consistency. eslint:
arrow-parens
8.5 Avoid confusing arrow function syntax (
=>
) with comparison operators (<=
,>=
). eslint:no-confusing-arrow
8.6 Enforce the location of arrow function bodies with implicit returns. eslint:
implicit-arrow-linebreak
Classes & Constructors
9.1 Always use
class
. Avoid manipulatingprototype
directly.9.2 Use
extends
for inheritance.9.3 Methods can return
this
to help with method chaining.9.4 It’s okay to write a custom
toString()
method, just make sure it works successfully and causes no side effects.9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint:
no-useless-constructor
9.6 Avoid duplicate class members. eslint:
no-dupe-class-members
9.7 Class methods should use
this
or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint:class-methods-use-this
Modules
10.1 Always use modules (
import
/export
) over a non-standard module system. You can always transpile to your preferred module system.10.2 Do not use wildcard imports.
10.3 And do not export directly from an import.
10.4 Only import from a path in one place. eslint:
no-duplicate-imports
10.5 Do not export mutable bindings. eslint:
import/no-mutable-exports
10.6 In modules with a single export, prefer default export over named export. eslint:
import/prefer-default-export
10.7 Put all
import
s above non-import statements. eslint:import/first
10.8 Multiline imports should be indented just like multiline array and object literals. eslint:
object-curly-newline
10.9 Disallow Webpack loader syntax in module import statements. eslint:
import/no-webpack-loader-syntax
10.10 Do not include JavaScript filename extensions eslint:
import/extensions
Iterators and Generators
11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like
for-in
orfor-of
. eslint:no-iterator
no-restricted-syntax
11.2 Don’t use generators for now.
11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint:
generator-star-spacing
Properties
12.1 Use dot notation when accessing properties. eslint:
dot-notation
12.2 Use bracket notation
[]
when accessing properties with a variable.12.3 Use exponentiation operator
**
when calculating exponentiations. eslint:no-restricted-properties
.Variables
13.1 Always use
const
orlet
to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint:no-undef
prefer-const
13.2 Use one
const
orlet
declaration per variable or assignment. eslint:one-var
13.3 Group all your
const
s and then group all yourlet
s.13.4 Assign variables where you need them, but place them in a reasonable place.
13.5 Don’t chain variable assignments. eslint:
no-multi-assign
13.6 Avoid using unary increments and decrements (
++
,--
). eslintno-plusplus
13.7 Avoid linebreaks before or after
=
in an assignment. If your assignment violatesmax-len
, surround the value in parens. eslintoperator-linebreak
.13.8 Disallow unused variables. eslint:
no-unused-vars
Hoisting
14.1
var
declarations get hoisted to the top of their closest enclosing function scope, their assignment does not.const
andlet
declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.14.2 Anonymous function expressions hoist their variable name, but not the function assignment.
14.3 Named function expressions hoist the variable name, not the function name or the function body.
14.4 Function declarations hoist their name and the function body.
For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.
Comparison Operators & Equality
===
and!==
over==
and!=
. eslint:eqeqeq
15.2 Conditional statements such as the
if
statement evaluate their expression using coercion with theToBoolean
abstract method and always follow these simple rules:''
, otherwise true15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.
15.5 Use braces to create blocks in
case
anddefault
clauses that contain lexical declarations (e.g.let
,const
,function
, andclass
). eslint:no-case-declarations
15.6 Ternaries should not be nested and generally be single line expressions. eslint:
no-nested-ternary
15.7 Avoid unneeded ternary statements. eslint:
no-unneeded-ternary
15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators:
+
,-
, and**
since their precedence is broadly understood. We recommend enclosing/
and*
in parentheses because their precedence can be ambiguous when they are mixed. eslint:no-mixed-operators
Blocks
16.1 Use braces with all multiline blocks. eslint:
nonblock-statement-body-position
16.2 If you’re using multiline blocks with
if
andelse
, putelse
on the same line as yourif
block’s closing brace. eslint:brace-style
16.3 If an
if
block always executes areturn
statement, the subsequentelse
block is unnecessary. Areturn
in anelse if
block following anif
block that contains areturn
can be separated into multipleif
blocks. eslint:no-else-return
Control Statements
17.1 In case your control statement (
if
,while
etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.17.2 Don't use selection operators in place of control statements.
Comments
18.1 Use
/** ... */
for multiline comments.18.2 Use
//
for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.18.3 Start all comments with a space to make it easier to read. eslint:
spaced-comment
FIXME
orTODO
helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions areFIXME: -- need to figure this out
orTODO: -- need to implement
.18.5 Use
// FIXME:
to annotate problems.18.6 Use
// TODO:
to annotate solutions to problems.Whitespace
19.1 Use soft tabs (space character) set to 2 spaces. eslint:
indent
19.2 Place 1 space before the leading brace. eslint:
space-before-blocks
19.3 Place 1 space before the opening parenthesis in control statements (
if
,while
etc.). Place no space between the argument list and the function name in function calls and declarations. eslint:keyword-spacing
19.4 Set off operators with spaces. eslint:
space-infix-ops
19.5 End files with a single newline character. eslint:
eol-last
19.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint:
newline-per-chained-call
no-whitespace-before-property
19.7 Leave a blank line after blocks and before the next statement.
19.8 Do not pad your blocks with blank lines. eslint:
padded-blocks
19.9 Do not use multiple blank lines to pad your code. eslint:
no-multiple-empty-lines
19.10 Do not add spaces inside parentheses. eslint:
space-in-parens
19.11 Do not add spaces inside brackets. eslint:
array-bracket-spacing
19.12 Add spaces inside curly braces. eslint:
object-curly-spacing
19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint:
max-len
19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint:
block-spacing
19.15 Avoid spaces before commas and require a space after commas. eslint:
comma-spacing
19.16 Enforce spacing inside of computed property brackets. eslint:
computed-property-spacing
19.17 Avoid spaces between functions and their invocations. eslint:
func-call-spacing
19.18 Enforce spacing between keys and values in object literal properties. eslint:
key-spacing
no-trailing-spaces
19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint:
no-multiple-empty-lines
Commas
20.1 Leading commas: Nope. eslint:
comma-style
20.2 Additional trailing comma: Yup. eslint:
comma-dangle
Semicolons
21.1 Yup. eslint:
semi
Read more.
Type Casting & Coercion
22.2 Strings: eslint:
no-new-wrappers
22.3 Numbers: Use
Number
for type casting andparseInt
always with a radix for parsing strings. eslint:radix
no-new-wrappers
22.4 If for whatever reason you are doing something wild and
parseInt
is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
22.6 Booleans: eslint:
no-new-wrappers
Naming Conventions
23.1 Avoid single letter names. Be descriptive with your naming. eslint:
id-length
23.2 Use camelCase when naming objects, functions, and instances. eslint:
camelcase
23.3 Use PascalCase only when naming constructors or classes. eslint:
new-cap
23.4 Do not use trailing or leading underscores. eslint:
no-underscore-dangle
23.5 Don’t save references to
this
. Use arrow functions or Function#bind.23.6 A base filename should exactly match the name of its default export.
23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
23.9 Acronyms and initialisms should always be all uppercased, or all lowercased.
23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a
const
(it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.const
variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.EXPORTED_OBJECT.key
) and maintain that all nested properties do not change.Accessors
24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use
getVal()
andsetVal('hello')
.24.3 If the property/method is a
boolean
, useisVal()
orhasVal()
.24.4 It’s okay to create
get()
andset()
functions, but be consistent.Events
25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
prefer:
jQuery
26.1 Prefix jQuery object variables with a
$
.26.2 Cache jQuery lookups.
$('.sidebar ul')
or parent > child$('.sidebar > ul')
. jsPerf26.4 Use
find
with scoped jQuery object queries.ECMAScript 5 Compatibility
ECMAScript 6+ (ES 2015+) Styles
28.2 Do not use TC39 proposals that have not reached stage 3.
Standard Library
The Standard Library contains utilities that are functionally broken but remain for legacy reasons.
29.1 Use
Number.isNaN
instead of globalisNaN
. eslint:no-restricted-globals
29.2 Use
Number.isFinite
instead of globalisFinite
. eslint:no-restricted-globals
Testing
30.1 Yup.
mocha
andjest
at Airbnb.tape
is also used occasionally for small, separate modules.Performance
map()
,reduce()
, andfilter()
optimized for traversing arrays?Resources
Learning ES6+
Read This
Tools
Other Style Guides
Other Styles
Further Reading
Books
Blogs
Podcasts
In the Wild
This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.
Translation
This style guide is also available in other languages:
The JavaScript Style Guide Guide
Chat With Us About JavaScript
Contributors
License
(The MIT License)
Copyright (c) 2012 Airbnb
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Amendments
We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.
};