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A mostly reasonable approach to JavaScript

Note: this guide assumes you are usingBabel, and requires that you usebabel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, withairbnb-browser-shims or the equivalent.

This guide is available in other languages too. SeeTranslation

Other Style Guides

• ES5 (Deprecated)
• React
• CSS-in-JavaScript
• CSS & Sass
• Ruby

1. Types
2. References
3. Objects
4. Arrays
5. Destructuring
6. Strings
7. Functions
8. Arrow Functions
9. Classes & Constructors
10. Modules
11. Iterators and Generators
12. Properties
13. Variables
14. Hoisting
15. Comparison Operators & Equality
16. Blocks
17. Control Statements
18. Comments
19. Whitespace
20. Commas
21. Semicolons
22. Type Casting & Coercion
23. Naming Conventions
24. Accessors
25. Events
26. jQuery
27. ECMAScript 5 Compatibility
28. ECMAScript 6+ (ES 2015+) Styles
29. Standard Library
30. Testing
31. Performance
32. Resources
33. In the Wild
34. Translation
35. The JavaScript Style Guide Guide
36. Chat With Us About JavaScript
37. Contributors
38. License
39. Amendments

• 1.1Primitives: When you access a primitive type you work directly on its value.

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol
  • bigint

  constfoo=1;letbar=foo;bar=9;console.log(foo,bar);// => 1, 9

  • Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

• 1.2Complex: When you access a complex type you work on a reference to its value.

  • object
  • array
  • function

  constfoo=[1,2];constbar=foo;bar[0]=9;console.log(foo[0],bar[0]);// => 9, 9

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• 2.1 Useconst for all of your references; avoid usingvar. eslint:prefer-const,no-const-assign

  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, uselet instead ofvar. eslint:no-var

  Why?let is block-scoped rather than function-scoped likevar.

  // badvarcount=1;if(true){count+=1;}// good, use the let.letcount=1;if(true){count+=1;}

• 2.3 Note that bothlet andconst are block-scoped.

  // const and let only exist in the blocks they are defined in.{leta=1;constb=1;}console.log(a);// ReferenceErrorconsole.log(b);// ReferenceError

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• 3.1 Use the literal syntax for object creation. eslint:no-new-object

  // badconstitem=newObject();// goodconstitem={};

• 3.2 Use computed property names when creating objects with dynamic property names.

  Why? They allow you to define all the properties of an object in one place.

  functiongetKey(k){return`a key named${k}`;}// badconstobj={id:5,name:'San Francisco',};obj[getKey('enabled')]=true;// goodconstobj={id:5,name:'San Francisco',[getKey('enabled')]:true,};

• 3.3 Use object method shorthand. eslint:object-shorthand

  // badconstatom={value:1,addValue:function(value){returnatom.value+value;},};// goodconstatom={value:1,addValue(value){returnatom.value+value;},};

• 3.4 Use property value shorthand. eslint:object-shorthand

  Why? It is shorter and descriptive.

  constlukeSkywalker='Luke Skywalker';// badconstobj={lukeSkywalker:lukeSkywalker,};// goodconstobj={  lukeSkywalker,};

• 3.5 Group your shorthand properties at the beginning of your object declaration.

  Why? It’s easier to tell which properties are using the shorthand.

  constanakinSkywalker='Anakin Skywalker';constlukeSkywalker='Luke Skywalker';// badconstobj={episodeOne:1,twoJediWalkIntoACantina:2,  lukeSkywalker,episodeThree:3,mayTheFourth:4,  anakinSkywalker,};// goodconstobj={  lukeSkywalker,  anakinSkywalker,episodeOne:1,twoJediWalkIntoACantina:2,episodeThree:3,mayTheFourth:4,};

• 3.6 Only quote properties that are invalid identifiers. eslint:quote-props

  Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

  // badconstbad={'foo':3,'bar':4,'data-blah':5,};// goodconstgood={foo:3,bar:4,'data-blah':5,};

• 3.7 Do not callObject.prototype methods directly, such ashasOwnProperty,propertyIsEnumerable, andisPrototypeOf. 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));

• 3.8 Prefer the object spread operator overObject.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

  // very badconstoriginal={a:1,b:2};constcopy=Object.assign(original,{c:3});// this mutates `original` ಠ_ಠdeletecopy.a;// so does this// badconstoriginal={a:1,b:2};constcopy=Object.assign({},original,{c:3});// copy => { a: 1, b: 2, c: 3 }// goodconstoriginal={a:1,b:2};constcopy={ ...original,c:3};// copy => { a: 1, b: 2, c: 3 }const{ a, ...noA}=copy;// noA => { b: 2, c: 3 }

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• 4.1 Use the literal syntax for array creation. eslint:no-array-constructor

  // badconstitems=newArray();// goodconstitems=[];

• 4.2 UseArray#push instead of direct assignment to add items to an array.

  constsomeStack=[];// badsomeStack[someStack.length]='abracadabra';// goodsomeStack.push('abracadabra');

• 4.3 Use array spreads... to copy arrays.

  // badconstlen=items.length;constitemsCopy=[];leti;for(i=0;i<len;i+=1){itemsCopy[i]=items[i];}// goodconstitemsCopy=[...items];

• 4.4 To convert an iterable object to an array, use spreads... instead ofArray.from.

  constfoo=document.querySelectorAll('.foo');// goodconstnodes=Array.from(foo);// bestconstnodes=[...foo];

• 4.5 UseArray.from for converting an array-like object to an array.

  constarrLike={0:'foo',1:'bar',2:'baz',length:3};// badconstarr=Array.prototype.slice.call(arrLike);// goodconstarr=Array.from(arrLike);

• 4.6 UseArray.from instead of spread... for mapping over iterables, because it avoids creating an intermediate array.

  // badconstbaz=[...foo].map(bar);// goodconstbaz=Array.from(foo,bar);

• 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, following8.2. 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

  // badconstarr=[[0,1],[2,3],[4,5],];constobjectInArray=[{id:1,},{id:2,}];constnumberInArray=[1,2,];// goodconstarr=[[0,1],[2,3],[4,5]];constobjectInArray=[{id:1,},{id:2,},];constnumberInArray=[1,2,];

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• 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.

  // badfunctiongetFullName(user){constfirstName=user.firstName;constlastName=user.lastName;return`${firstName}${lastName}`;}// goodfunctiongetFullName(user){const{ firstName, lastName}=user;return`${firstName}${lastName}`;}// bestfunctiongetFullName({ firstName, lastName}){return`${firstName}${lastName}`;}

• 5.2 Use array destructuring. eslint:prefer-destructuring

  constarr=[1,2,3,4];// badconstfirst=arr[0];constsecond=arr[1];// goodconst[first,second]=arr;

• 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);

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• 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.';

• 6.3 When programmatically building up strings, use template strings instead of concatenation. eslint:prefer-templatetemplate-curly-spacing

  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 useeval() 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}'`;

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• 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 ablock as a list of statements. A function declaration is not a statement.

  // badif(currentUser){functiontest(){console.log('Nope.');}}// goodlettest;if(currentUser){test=()=>{console.log('Yup.');};}

• 7.5 Never name a parameterarguments. This will take precedence over thearguments object that is given to every function scope.

  // badfunctionfoo(name,options,arguments){// ...}// goodfunctionfoo(name,options,args){// ...}

• 7.6 Never usearguments, opt to use rest syntax... instead. eslint:prefer-rest-params

  Why?... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like likearguments.

  // badfunctionconcatenateAll(){constargs=Array.prototype.slice.call(arguments);returnargs.join('');}// goodfunctionconcatenateAll(...args){returnargs.join('');}

• 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={}){// ...}

• 7.8 Avoid side effects with default parameters.

  Why? They are confusing to reason about.

  varb=1;// badfunctioncount(a=b++){console.log(a);}count();// 1count();// 2count(3);// 3count();// 3

• 7.9 Always put default parameters last.

  // badfunctionhandleThings(opts={},name){// ...}// goodfunctionhandleThings(name,opts={}){// ...}

• 7.10 Never use the Function constructor to create a new function. eslint:no-new-func

  Why? Creating a function in this way evaluates a string similarly toeval(), which opens vulnerabilities.

  // badvaradd=newFunction('a','b','return a + b');// still badvarsubtract=Function('a','b','return a - b');

• 7.11 Spacing in a function signature. eslint:space-before-function-parenspace-before-blocks

  Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

  // badconstf=function(){};constg=function(){};consth=function(){};// goodconstx=function(){};consty=functiona(){};

• 7.12 Never mutate parameters. eslint:no-param-reassign

  Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

  // badfunctionf1(obj){obj.key=1;}// goodfunctionf2(obj){constkey=Object.prototype.hasOwnProperty.call(obj,'key') ?obj.key :1;}

• 7.13 Never reassign parameters. eslint:no-param-reassign

  Why? Reassigning parameters can lead to unexpected behavior, especially when accessing thearguments object. It can also cause optimization issues, especially in V8.

  // badfunctionf1(a){a=1;// ...}functionf2(a){if(!a){a=1;}// ...}// goodfunctionf3(a){constb=a||1;// ...}functionf4(a=1){// ...}

• 7.14 Prefer the use of the spread operator... to call variadic functions. eslint:prefer-spread

  Why? It’s cleaner, you don’t need to supply a context, and you can not easily composenew withapply.

  // badconstx=[1,2,3,4,5];console.log.apply(console,x);// goodconstx=[1,2,3,4,5];console.log(...x);// badnew(Function.prototype.bind.apply(Date,[null,2016,8,5]));// goodnewDate(...[2016,8,5]);

• 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

  // badfunctionfoo(bar,baz,quux){// ...}// goodfunctionfoo(bar,baz,quux,){// ...}// badconsole.log(foo,bar,baz);// goodconsole.log(foo,bar,baz,);

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• 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 ofthis, 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.

  // bad[1,2,3].map(function(x){consty=x+1;returnx*y;});// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});

• 8.2 If the function body consists of a single statement returning anexpression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use areturn 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.

  // bad['get','post','put'].map((httpMethod)=>Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName,httpMethod,));// good['get','post','put'].map((httpMethod)=>(Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName,httpMethod,)));

• 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

  // badconstitemHeight=(item)=>item.height<=256 ?item.largeSize :item.smallSize;// badconstitemHeight=(item)=>item.height>=256 ?item.largeSize :item.smallSize;// goodconstitemHeight=(item)=>(item.height<=256 ?item.largeSize :item.smallSize);// goodconstitemHeight=(item)=>{const{ height, largeSize, smallSize}=item;returnheight<=256 ?largeSize :smallSize;};

• 8.6 Enforce the location of arrow function bodies with implicit returns. eslint:implicit-arrow-linebreak

  // bad(foo)=>bar;(foo)=>(bar);// good(foo)=>bar;(foo)=>(bar);(foo)=>(bar)

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• 9.1 Always useclass. Avoid manipulatingprototype directly.

  Why?class syntax is more concise and easier to reason about.

  // badfunctionQueue(contents=[]){this.queue=[...contents];}Queue.prototype.pop=function(){constvalue=this.queue[0];this.queue.splice(0,1);returnvalue;};// goodclassQueue{constructor(contents=[]){this.queue=[...contents];}pop(){constvalue=this.queue[0];this.queue.splice(0,1);returnvalue;}}

• 9.2 Useextends for inheritance.

  Why? It is a built-in way to inherit prototype functionality without breakinginstanceof.

  // badconstinherits=require('inherits');functionPeekableQueue(contents){Queue.apply(this,contents);}inherits(PeekableQueue,Queue);PeekableQueue.prototype.peek=function(){returnthis.queue[0];};// goodclassPeekableQueueextendsQueue{peek(){returnthis.queue[0];}}

• 9.3 Methods can returnthis to help with method chaining.

  // badJedi.prototype.jump=function(){this.jumping=true;returntrue;};Jedi.prototype.setHeight=function(height){this.height=height;};constluke=newJedi();luke.jump();// => trueluke.setHeight(20);// => undefined// goodclassJedi{jump(){this.jumping=true;returnthis;}setHeight(height){this.height=height;returnthis;}}constluke=newJedi();luke.jump().setHeight(20);

• 9.4 It’s okay to write a customtoString() method, just make sure it works successfully and causes no side effects.

  classJedi{constructor(options={}){this.name=options.name||'no name';}getName(){returnthis.name;}toString(){return`Jedi -${this.getName()}`;}}

• 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

  // badclassJedi{constructor(){}getName(){returnthis.name;}}// badclassReyextendsJedi{constructor(...args){super(...args);}}// goodclassReyextendsJedi{constructor(...args){super(...args);this.name='Rey';}}

• 9.6 Avoid duplicate class members. eslint:no-dupe-class-members

  Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

  // badclassFoo{bar(){return1;}bar(){return2;}}// goodclassFoo{bar(){return1;}}// goodclassFoo{bar(){return2;}}

• 9.7 Class methods should usethis or be made into a static method unless an external library or framework requires to use 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');}}

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• 10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

  Why? Modules are the future, let’s start using the future now.

  // badconstAirbnbStyleGuide=require('./AirbnbStyleGuide');module.exports=AirbnbStyleGuide.es6;// okimportAirbnbStyleGuidefrom'./AirbnbStyleGuide';exportdefaultAirbnbStyleGuide.es6;// bestimport{es6}from'./AirbnbStyleGuide';exportdefaultes6;

• 10.2 Do not use wildcard imports.

  Why? This makes sure you have a single default export.

  // badimport*asAirbnbStyleGuidefrom'./AirbnbStyleGuide';// goodimportAirbnbStyleGuidefrom'./AirbnbStyleGuide';

• 10.3 And do not export directly from an import.

  Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

  // bad// filename es6.jsexport{es6asdefault}from'./AirbnbStyleGuide';// good// filename es6.jsimport{es6}from'./AirbnbStyleGuide';exportdefaultes6;

• 10.4 Only import from a path in one place.eslint:no-duplicate-imports

  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';

• 10.5 Do not export mutable bindings.eslint:import/no-mutable-exports

  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.

  // badletfoo=3;export{foo};// goodconstfoo=3;export{foo};

• 10.6 In modules with a single export, prefer default export over named export.eslint:import/prefer-default-export

  Why? To encourage more files that only ever export one thing, which is better for readability and maintainability.

  // badexportfunctionfoo(){}// goodexportdefaultfunctionfoo(){}

• 10.7 Put allimports above non-import statements.eslint:import/first

  Why? Sinceimports are hoisted, keeping them all at the top prevents surprising behavior.

  // badimportfoofrom'foo';foo.init();importbarfrom'bar';// goodimportfoofrom'foo';importbarfrom'bar';foo.init();

• 10.8 Multiline imports should be indented just like multiline array and object literals.eslint:object-curly-newline

  Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

  // badimport{longNameA,longNameB,longNameC,longNameD,longNameE}from'path';// goodimport{longNameA,longNameB,longNameC,longNameD,longNameE,}from'path';

• 10.9 Disallow Webpack loader syntax in module import statements.eslint:import/no-webpack-loader-syntax

  Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax inwebpack.config.js.

  // badimportfooSassfrom'css!sass!foo.scss';importbarCssfrom'style!css!bar.css';// goodimportfooSassfrom'foo.scss';importbarCssfrom'bar.css';

• 10.10 Do not include JavaScript filename extensionseslint:import/extensions

  Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

  // badimportfoofrom'./foo.js';importbarfrom'./bar.jsx';importbazfrom'./baz/index.jsx';// goodimportfoofrom'./foo';importbarfrom'./bar';importbazfrom'./baz';

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• 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops likefor-in orfor-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.

  Usemap() /every() /filter() /find() /findIndex() /reduce() /some() / ... to iterate over arrays, andObject.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);

• 11.2 Don’t use generators for now.

  Why? They don’t transpile well to ES5.

• 11.3 If you must use generators, or if you disregardour 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 forfunction,function* is a unique construct, different fromfunction.

  // badfunction*foo(){// ...}// badconstbar=function*(){// ...};// badconstbaz=function*(){// ...};// badconstquux=function*(){// ...};// badfunction*foo(){// ...}// badfunction*foo(){// ...}// very badfunction*foo(){// ...}// very badconstwat=function*(){// ...};// goodfunction*foo(){// ...}// goodconstfoo=function*(){// ...};

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• 12.1 Use dot notation when accessing properties. eslint:dot-notation

  constluke={jedi:true,age:28,};// badconstisJedi=luke['jedi'];// goodconstisJedi=luke.jedi;

• 12.2 Use bracket notation[] when accessing properties with a variable.

  constluke={jedi:true,age:28,};functiongetProp(prop){returnluke[prop];}constisJedi=getProp('jedi');

• 12.3 Use exponentiation operator** when calculating exponentiations. eslint:no-restricted-properties.

  // badconstbinary=Math.pow(2,10);// goodconstbinary=2**10;

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• 13.1 Always useconst 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-undefprefer-const

  // badsuperPower=newSuperPower();// goodconstsuperPower=newSuperPower();

• 13.2 Use oneconst orlet 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 yourconsts and then group all yourlets.

  Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

  // badleti,len,dragonball,items=getItems(),goSportsTeam=true;// badleti;constitems=getItems();letdragonball;constgoSportsTeam=true;letlen;// goodconstgoSportsTeam=true;constitems=getItems();letdragonball;leti;letlength;

• 13.4 Assign variables where you need them, but place them in a reasonable place.

  Why?let andconst 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;}

• 13.5 Don’t chain variable assignments. eslint:no-multi-assign

  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 (++,--). eslintno-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 likenum += 1 instead ofnum++ ornum ++. 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.

  // badconstarray=[1,2,3];letnum=1;num++;--num;letsum=0;lettruthyCount=0;for(leti=0;i<array.length;i++){letvalue=array[i];sum+=value;if(value){truthyCount++;}}// goodconstarray=[1,2,3];letnum=1;num+=1;num-=1;constsum=array.reduce((a,b)=>a+b,0);consttruthyCount=array.filter(Boolean).length;

• 13.7 Avoid linebreaks before or after= in an assignment. If your assignment violatesmax-len, surround the value in parens. eslintoperator-linebreak.

  Why? Linebreaks surrounding= can obfuscate the value of an assignment.

  // badconstfoo=superLongLongLongLongLongLongLongLongFunctionName();// badconstfoo='superLongLongLongLongLongLongLongLongString';// goodconstfoo=(superLongLongLongLongLongLongLongLongFunctionName());// goodconstfoo='superLongLongLongLongLongLongLongLongString';

• 13.8 Disallow unused variables. eslint:no-unused-vars

  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.

  // badvarsome_unused_var=42;// Write-only variables are not considered as used.vary=10;y=5;// A read for a modification of itself is not considered as used.varz=0;z=z+1;// Unused function arguments.functiongetX(x,y){returnx;}// goodfunctiongetXPlusY(x,y){returnx+y;}varx=1;vary=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.var{ type, ...coords}=data;// 'coords' is now the 'data' object without its 'type' property.

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• 14.1var 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 calledTemporal Dead Zones (TDZ). It’s important to know whytypeof 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.

  functionexample(){superPower();// => FlyingfunctionsuperPower(){console.log('Flying');}}

• For more information refer toJavaScript Scoping & Hoisting byBen Cherry.

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• 15.1 Use=== and!== over== and!=. eslint:eqeqeq

• 15.2 Conditional statements such as theif statement evaluate their expression using coercion with theToBoolean abstract method and always follow these simple rules:

  • Objects evaluate totrue
  • Undefined evaluates tofalse
  • Null evaluates tofalse
  • Booleans evaluate tothe value of the boolean
  • Numbers evaluate tofalse if+0, -0, or NaN, otherwisetrue
  • Strings evaluate tofalse if an empty string'', otherwisetrue

  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.

  // badif(isValid===true){// ...}// goodif(isValid){// ...}// badif(name){// ...}// goodif(name!==''){// ...}// badif(collection.length){// ...}// goodif(collection.length>0){// ...}

• 15.4 For more information seeTruth Equality and JavaScript by Angus Croll.

• 15.5 Use braces to create blocks incase anddefault clauses that contain lexical declarations (e.g.let,const,function, andclass). eslint:no-case-declarations

  Why? Lexical declarations are visible in the entireswitch block but only get initialized when assigned, which only happens when itscase is reached. This causes problems when multiplecase clauses attempt to define the same thing.

  // badswitch(foo){case1:letx=1;break;case2:consty=2;break;case3:functionf(){// ...}break;default:classC{}}// goodswitch(foo){case1:{letx=1;break;}case2:{consty=2;break;}case3:{functionf(){// ...}break;}case4:bar();break;default:{classC{}}}

• 15.6 Ternaries should not be nested and generally be single line expressions. eslint:no-nested-ternary

  // badconstfoo=maybe1>maybe2  ?"bar"  :value1>value2 ?"baz" :null;// split into 2 separated ternary expressionsconstmaybeNull=value1>value2 ?'baz' :null;// betterconstfoo=maybe1>maybe2  ?'bar'  :maybeNull;// bestconstfoo=maybe1>maybe2 ?'bar' :maybeNull;

• 15.7 Avoid unneeded ternary statements. eslint:no-unneeded-ternary

  // badconstfoo=a ?a :b;constbar=c ?true :false;constbaz=c ?false :true;// goodconstfoo=a||b;constbar=!!c;constbaz=!c;

• 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;

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• 16.1 Use braces with all multiline blocks. eslint:nonblock-statement-body-position

  // badif(test)returnfalse;// goodif(test)returnfalse;// goodif(test){returnfalse;}// badfunctionfoo(){returnfalse;}// goodfunctionbar(){returnfalse;}

• 16.2 If you’re using multiline blocks withif andelse, putelse on the same line as yourif block’s closing brace. eslint:brace-style

  // badif(test){thing1();thing2();}else{thing3();}// goodif(test){thing1();thing2();}else{thing3();}

• 16.3 If anif 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

  // badfunctionfoo(){if(x){returnx;}else{returny;}}// badfunctioncats(){if(x){returnx;}elseif(y){returny;}}// badfunctiondogs(){if(x){returnx;}else{if(y){returny;}}}// goodfunctionfoo(){if(x){returnx;}returny;}// goodfunctioncats(){if(x){returnx;}if(y){returny;}}// goodfunctiondogs(x){if(x){if(z){returny;}}else{returnz;}}

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• 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.

  // badif((foo===123||bar==='abc')&&doesItLookGoodWhenItBecomesThatLong()&&isThisReallyHappening()){thing1();}// badif(foo===123&&bar==='abc'){thing1();}// badif(foo===123&&bar==='abc'){thing1();}// badif(foo===123&&bar==='abc'){thing1();}// goodif(foo===123&&bar==='abc'){thing1();}// goodif((foo===123||bar==='abc')&&doesItLookGoodWhenItBecomesThatLong()&&isThisReallyHappening()){thing1();}// goodif(foo===123&&bar==='abc'){thing1();}

• 17.2 Don't use selection operators in place of control statements.

  // bad!isRunning&&startRunning();// goodif(!isRunning){startRunning();}

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• 18.1 Use/** ... */ for multiline comments.

  // 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 withFIXME 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.

  classCalculatorextendsAbacus{constructor(){super();// FIXME: shouldn’t use a global heretotal=0;}}

• 18.6 Use// TODO: to annotate solutions to problems.

  classCalculatorextendsAbacus{constructor(){super();// TODO: total should be configurable by an options paramthis.total=0;}}

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• 19.1 Use soft tabs (space character) set to 2 spaces. eslint:indent

  // badfunctionfoo(){∙∙∙∙letname;}// badfunctionbar(){∙letname;}// goodfunctionbaz(){∙∙letname;}

• 19.2 Place 1 space before the leading brace. eslint:space-before-blocks

  // badfunctiontest(){console.log('test');}// goodfunctiontest(){console.log('test');}// baddog.set('attr',{age:'1 year',breed:'Bernese Mountain Dog',});// gooddog.set('attr',{age:'1 year',breed:'Bernese Mountain Dog',});

• 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

  // badif(isJedi){fight();}// goodif(isJedi){fight();}// badfunctionfight(){console.log('Swooosh!');}// goodfunctionfight(){console.log('Swooosh!');}

• 19.4 Set off operators with spaces. eslint:space-infix-ops

  // badconstx=y+5;// goodconstx=y+5;

• 19.5 End files with a single newline character. eslint:eol-last

  // badimport{es6}from'./AirbnbStyleGuide';// ...exportdefaultes6;

  // badimport{es6}from'./AirbnbStyleGuide';// ...exportdefaultes6;↵↵

  // goodimport{es6}from'./AirbnbStyleGuide';// ...exportdefaultes6;↵

• 19.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, whichemphasizes that the line is a method call, not a new statement. eslint:newline-per-chained-callno-whitespace-before-property

  // bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// good$('#items').find('.selected').highlight().end().find('.open').updateCount();// badconstleds=stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led',true).attr('width',(radius+margin)*2).append('svg:g').attr('transform',`translate(${radius+margin},${radius+margin})`).call(tron.led);// goodconstleds=stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led',true).attr('width',(radius+margin)*2).append('svg:g').attr('transform',`translate(${radius+margin},${radius+margin})`).call(tron.led);// goodconstleds=stage.selectAll('.led').data(data);