Forked from Airbnb's Style Guide

HubSpot's version of a mostly reasonable approach to JavaScript

Other Style Guides (from HubSpot)

• ES5
• React

Other Style Guides (from Airbnb)

• CSS & Sass
• Ruby

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

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

  • string
  • number
  • boolean
  • null
  • undefined

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

• 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 Use const for all of your references; avoid using var. 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, use let instead of var. eslint: no-var jscs: disallowVar

  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.

  // 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 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs: disallowIdentifierNames

  // badconstsuperman={default: {clark: 'kent'},private: true,};// goodconstsuperman={defaults: {clark: 'kent'},hidden: true,};

• 3.3 Use readable synonyms in place of reserved words. jscs: disallowIdentifierNames

  // badconstsuperman={class: 'alien',};// badconstsuperman={klass: 'alien',};// goodconstsuperman={type: 'alien',};

• 3.4 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,};

• 3.5 Use object method shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

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

• 3.6 Use property value shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

  Why? It is shorter to write and descriptive.

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

• 3.7 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,};

• 3.8 Only quote properties that are invalid identifiers. eslint: quote-props jscs: disallowQuotedKeysInObjects

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,};

• 3.9 Don't wrap object literals in extra parentheses except for arrow function returns

// badconstbad=({hello: 'world',foo: 'bar'});// goodconstgood={hello: 'world',foo: 'bar'};// okayconstokay=()=>({hello: 'world',foo: 'bar'});

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

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

• 4.2 Use Array#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++){itemsCopy[i]=items[i];}// goodconstitemsCopy=[...items];

• 4.4 To convert an array-like object to an array, use Array#from.

  constfoo=document.querySelectorAll('.foo');constnodes=Array.from(foo);

• 4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement 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);// badconstflat={};[[0,1],[2,3],[4,5]].reduce((memo,item,index)=>{constflatten=memo.concat(item);flat[index]=memo.concat(item);});// goodconstflat={};[[0,1],[2,3],[4,5]].reduce((memo,item,index)=>{constflatten=memo.concat(item);flat[index]=flatten;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;});

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• 5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring

  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. jscs: requireArrayDestructuring

  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. jscs: disallowArrayDestructuringReturn

  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 jscs: validateQuoteMarks

  // badconstname="Capt. Janeway";// goodconstname='Capt. Janeway';

• 6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.

• 6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.

  // 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.';

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

  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}?`;}

• 6.5 Never use eval() on a string, it opens too many vulnerabilities.

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• 7.1 Use function declarations instead of function expressions. jscs: requireFunctionDeclarations

  Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.

  // badconstfoo=function(){};// goodfunctionfoo(){}

• 7.2 Immediately invoked function expressions: eslint: wrap-iife jscs: requireParenthesesAroundIIFE

  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. Read ECMA-262's note on this issue.

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

• 7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

  // badfunctionnope(name,options,arguments){// ...stuff...}// goodfunctionyup(name,options,args){// ...stuff...}

• 7.6 Never use arguments, opt to use rest syntax ... instead. prefer-rest-params

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

  // 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.

  Why? Creating a function in this way evaluates a string similarly to eval(), 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.

  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 the arguments 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){}

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• 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

  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 function declaration.

  // 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 expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

  Why? Syntactic sugar. It reads well when multiple functions are chained together.

  Why not? If you plan on returning an object.

  // 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}.`);// good[1,2,3].map((number)=>{constnextNumber=number+1;return`A string containing the ${nextNumber}.`;});

• 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[1,2,3].map(number=>'As time went by, the string containing the '+`${number} became much longer. So we needed to break it over multiple `+'lines.');// good[1,2,3].map(number=>(`As time went by, the string containing the ${number} became much `+'longer. So we needed to break it over multiple lines.'));

• 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

  Why? Less visual clutter.

  // bad[1,2,3].map((x)=>x*x);// good[1,2,3].map(x=>x*x);// good[1,2,3].map(number=>(`A long string with the ${number}. It’s so long that we’ve broken it `+'over multiple lines!'));// 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)=>{returnitem.height>256 ? item.largeSize : item.smallSize;}

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• 9.1 Always use class. Avoid manipulating prototype 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 Use extends for inheritance.

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

  // 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 return this 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 custom toString() 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. no-useless-constructor

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

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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;

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• 11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of. eslint: no-iterator

  Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

  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;

• 11.2 Don't use generators for now.

  Why? They don't transpile well to ES5.

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

  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');

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• 13.1 Always use const 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.

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

• 13.2 Use one const declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl

  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 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 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;}

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• 14.1var declarations get hoisted to the top of their 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.

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

• For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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• 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.

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

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

• 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

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.

eslint rules: no-case-declarations.

```javascript // bad switch (foo){case 1: let x = 1; break; case 2: const y = 2; break; case 3: function f(){} break; default: class C{} } // good switch (foo){case 1:{let x = 1; break} case 2:{const y = 2; break} case 3:{function f(){} break} case 4: bar(); break; default:{class C{} } } ``` 

• 15.6 Ternaries should not be nested and generally be single line expressions.

  eslint rules: no-nested-ternary.

  // badconstfoo=maybe1>maybe2 ? "bar" : value1>value2 ? "baz" : null;// betterconstmaybeNull=value1>value2 ? 'baz' : null;constfoo=maybe1>maybe2 ? 'bar' : maybeNull;// bestconstmaybeNull=value1>value2 ? 'baz' : null;constfoo=maybe1>maybe2 ? 'bar' : maybeNull;

• 15.7 Avoid unneeded ternary statements.

  eslint rules: no-unneeded-ternary.

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

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• 16.1 Use braces with all multi-line blocks.

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

• 16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

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

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• 17.1 Use /** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

  // bad// make() returns a new element// based on the passed in tag name//// @param{String} tag// @return{Element} elementfunctionmake(tag){// ...stuff...returnelement;}// good/** * make() returns a new element * based on the passed in tag name * * @param{String} tag * @return{Element} element */functionmake(tag){// ...stuff...returnelement;}

• 17.2 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;}

• 17.3 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.

• 17.4 Use // FIXME: to annotate problems.

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

• 17.5 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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• 18.1 Use soft tabs set to 2 spaces. eslint: indent jscs: validateIndentation

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

• 18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

  // 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',});

• 18.3 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: space-after-keywords, space-before-keywords jscs: requireSpaceAfterKeywords

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

• 18.4 Set off operators with spaces. eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators

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

• 18.5 End files with a single newline character.

  // bad(function(global){// ...stuff...})(this);

  // bad(function(global){// ...stuff...})(this);↵↵

  // good(function(global){// ...stuff...})(this);↵

• 18.6 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

  // 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);

• 18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks

  // badif(foo){returnbar;}returnbaz;// goodif(foo){returnbar;}returnbaz;// badconstobj={foo(){},bar(){},};returnobj;// goodconstobj={foo(){},bar(){},};returnobj;// badconstarr=[functionfoo(){},functionbar(){},];returnarr;// goodconstarr=[functionfoo(){},functionbar(){},];returnarr;

• 18.8 Do not pad your blocks with blank lines. eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

  // badfunctionbar(){console.log(foo);}// also badif(baz){console.log(qux);}else{console.log(foo);}// goodfunctionbar(){console.log(foo);}// goodif(baz){console.log(qux);}else{console.log(foo);}

• 18.9 Do not add spaces inside parentheses. eslint: space-in-parens jscs: disallowSpacesInsideParentheses

  // badfunctionbar(foo){returnfoo;}// goodfunctionbar(foo){returnfoo;}// badif(foo){console.log(foo);}// goodif(foo){console.log(foo);}

• 18.10 Do not add spaces inside brackets. eslint: array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets

  // badconstfoo=[1,2,3];console.log(foo[0]);// goodconstfoo=[1,2,3];console.log(foo[0]);

• 18.11 Add spaces inside curly braces. eslint: object-curly-spacing jscs: disallowSpacesInsideObjectBrackets

  // badconstfoo={clark: 'kent'};// goodconstfoo={clark: 'kent'};

• 18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len jscs: maximumLineLength

  Why? This ensures readability and maintainability.

  // badconstfoo='Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';// bad$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'}}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));// goodconstfoo='Whatever national crop flips the window. The cartoon reverts within the screw. '+'Whatever wizard constrains a helpful ally. The counterpart ascends!';// good$.ajax({method: 'POST',url: 'https://airbnb.com/',data: {name: 'John'},}).done(()=>console.log('Congratulations!')).fail(()=>console.log('You have failed this city.'));

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• 19.1 Leading commas: Nope. eslint: comma-style jscs: requireCommaBeforeLineBreak

  // badconststory=[once,upon,aTime];// goodconststory=[once,upon,aTime,];// badconsthero={firstName: 'Ada',lastName: 'Lovelace',birthYear: 1815,superPower: 'computers'};// goodconsthero={firstName: 'Ada',lastName: 'Lovelace',birthYear: 1815,superPower: 'computers',};

• 19.2 Additional trailing comma: Yup. eslint: comma-dangle jscs: requireTrailingComma

  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.

  // bad - git diff without trailing commaconsthero={firstName: 'Florence',-lastName: 'Nightingale'+lastName: 'Nightingale',+inventorOf: ['coxcomb graph','modern nursing']};// good - git diff with trailing commaconsthero={firstName: 'Florence',lastName: 'Nightingale',+inventorOf: ['coxcomb chart','modern nursing'],};// badconsthero={firstName: 'Dana',lastName: 'Scully'};constheroes=['Batman','Superman'];// goodconsthero={firstName: 'Dana',lastName: 'Scully',};constheroes=['Batman','Superman',];

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• 20.1Yup. eslint: semi jscs: requireSemicolons

  // bad(function(){constname='Skywalker'returnname})()// good(()=>{constname='Skywalker';returnname;}());// good (guards against the function becoming an argument when two files with IIFEs are concatenated);(()=>{constname='Skywalker';returnname;}());

  Read more.

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• 21.1 Perform type coercion at the beginning of the statement.

• 21.2 Strings:

  // => this.reviewScore = 9;// badconsttotalScore=this.reviewScore+'';// invokes this.reviewScore.valueOf()// badconsttotalScore=this.reviewScore.toString();// isn't guaranteed to return a string// goodconsttotalScore=String(this.reviewScore);

• 21.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix

  constinputValue='4';// badconstval=newNumber(inputValue);// badconstval=+inputValue;// badconstval=inputValue>>0;// badconstval=parseInt(inputValue);// goodconstval=Number(inputValue);// goodconstval=parseInt(inputValue,10);

• 21.4 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;

• 21.5Note: 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:

  2147483647>>0//=> 21474836472147483648>>0//=> -21474836482147483649>>0//=> -2147483647

• 21.6 Booleans:

  constage=0;// badconsthasAge=newBoolean(age);// goodconsthasAge=Boolean(age);// goodconsthasAge=!!age;

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• 22.1 Avoid single letter names. Be descriptive with your naming.

  // badfunctionq(){// ...stuff...}// goodfunctionquery(){// ..stuff..}

• 22.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase jscs: requireCamelCaseOrUpperCaseIdentifiers

  // badconstOBJEcttsssss={};constthis_is_my_object={};functionc(){}// goodconstthisIsMyObject={};functionthisIsMyFunction(){}

• 22.3 Use PascalCase only when naming constructors or classes. eslint: new-cap jscs: requireCapitalizedConstructors

  // badfunctionuser(options){this.name=options.name;}constbad=newuser({name: 'nope',});// goodclassUser{constructor(options){this.name=options.name;}}constgood=newUser({name: 'yup',});

• 22.4 Use a leading underscore _ when naming private properties. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores

  // badthis.__firstName__='Panda';this.firstName_='Panda';// goodthis._firstName='Panda';

• 22.5 Don't save references to this. Use arrow functions or Function#bind. jscs: disallowNodeTypes

  // badfunctionfoo(){constself=this;returnfunction(){console.log(self);};}// badfunctionfoo(){constthat=this;returnfunction(){console.log(that);};}// goodfunctionfoo(){return()=>{console.log(this);};}

• 22.6 If your file exports a single class, your filename should be exactly the name of the class.

  // file contentsclassCheckBox{// ...}exportdefaultCheckBox;// in some other file// badimportCheckBoxfrom'./checkBox';// badimportCheckBoxfrom'./check_box';// goodimportCheckBoxfrom'./CheckBox';

• 22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.

  functionmakeStyleGuide(){}exportdefaultmakeStyleGuide;

• 22.8 Use PascalCase when you export a singleton / function library / bare object.

  constAirbnbStyleGuide={es6: {}};exportdefaultAirbnbStyleGuide;

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• 23.1 Accessor functions for properties are not required.

• 23.2 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').

  // baddragon.age();// gooddragon.getAge();// baddragon.age(25);// gooddragon.setAge(25);

• 23.3 If the property/method is a boolean, use isVal() or hasVal().

  // badif(!dragon.age()){returnfalse;}// goodif(!dragon.hasAge()){returnfalse;}

• 23.4 It's okay to create get() and set() functions, but be consistent.

  classJedi{constructor(options={}){constlightsaber=options.lightsaber||'blue';this.set('lightsaber',lightsaber);}set(key,val){this[key]=val;}get(key){returnthis[key];}}

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• 24.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass 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});

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• 25.1 Prefix jQuery object variables with a $. jscs: requireDollarBeforejQueryAssignment

  // badconstsidebar=$('.sidebar');// goodconst$sidebar=$('.sidebar');// goodconst$sidebarBtn=$('.sidebar-btn');

• 25.2 Cache jQuery lookups.

  // badfunctionsetSidebar(){$('.sidebar').hide();// ...stuff...$('.sidebar').css({'background-color': 'pink'});}// goodfunctionsetSidebar(){const$sidebar=$('.sidebar');$sidebar.hide();// ...stuff...$sidebar.css({'background-color': 'pink'});}

• 25.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

• 25.4 Use find with scoped jQuery object queries.

  // bad$('ul','.sidebar').hide();// bad$('.sidebar').find('ul').hide();// good$('.sidebar ul').hide();// good$('.sidebar > ul').hide();// good$sidebar.find('ul').hide();

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• 26.1 Refer to Kangax's ES5 compatibility table.

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• 27.1 This is a collection of links to the various ES6 features.

1. Arrow Functions
2. Classes
3. Object Shorthand
4. Object Concise
5. Object Computed Properties
6. Template Strings
7. Destructuring
8. Default Parameters
9. Rest
10. Array Spreads
11. Let and Const
12. Iterators and Generators
13. Modules

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• 28.1Yup.

  functionfoo(){returntrue;}

• 28.2No, but seriously:
• Whichever testing framework you use, you should be writing tests!
• Strive to write many small pure functions, and minimize where mutations occur.
• Be cautious about stubs and mocks - they can make your tests more brittle.
• We primarily use mocha at Airbnb. tape is also used occasionally for small, separate modules.
• 100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
• Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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• 29.1 Addendum to Additional trailing comma: Don't care.

Airbnb encourages multiline trailing commas and makes a good point about it below, but either should be okay as long it's consistent.

```javascript // okay const hero ={firstName: 'Dana', lastName: 'Scully' }; const heroes = [ 'Batman', 'Superman' ]; // okay const hero ={firstName: 'Dana', lastName: 'Scully', }; const heroes = [ 'Batman', 'Superman', ]; ``` 

• 29.2 Addendum to using PascalCase. Constructors require a capital letters, but functions can be capitalize too. The main reason for the latter is for libraries like Immutable which allow (and encourage?) initializing objects without new

  // okayconstmapInstance=newMap();// okayconstmapInstance=Map();

• 29.3 Addendum to add spaces inside curly braces. Some prefer spacing, others don't. If you do use spacing, make sure there is a leading and trailing space and at most one space.

  // badconstgreetings={hello: 'world'};constgreetings={hello: 'world'};// badconstgreetings={hello: 'world'};// okayconstgreetings={hello: 'world'};// okayconstgreetings={hello: 'world'};

• 29.4 Addendum to spacing in a function signature. Some prefer the leading space, others don't in anonymous functions. Just be consistent and use, at most, one space. However, do omit the space for named functions.

  // badconstf=function(){};constg=function(){};consth=functiona(){};// okayconstz=function(){};consty=function(){};constz=functionb(){};

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• On Layout & Web Performance
• String vs Array Concat
• Try/Catch Cost In a Loop
• Bang Function
• jQuery Find vs Context, Selector
• innerHTML vs textContent for script text
• Long String Concatenation
• Loading...

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Learning ES6

• Draft ECMA 2015 (ES6) Spec
• ExploringJS
• ES6 Compatibility Table
• Comprehensive Overview of ES6 Features

Read This

• Standard ECMA-262

Tools

• Code Style Linters
  • ESlint - HubSpot Style .eslintrc
  • ESlint - Airbnb Style .eslintrc
  • JSHint - Airbnb Style .jshintrc
  • JSCS - Airbnb Style Preset

Other Style Guides

• Google JavaScript Style Guide
• jQuery Core Style Guidelines
• Principles of Writing Consistent, Idiomatic JavaScript

Other Styles

• Naming this in nested functions - Christian Johansen
• Conditional Callbacks - Ross Allen
• Popular JavaScript Coding Conventions on Github - JeongHoon Byun
• Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

• Understanding JavaScript Closures - Angus Croll
• Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
• You Might Not Need jQuery - Zack Bloom & Adam Schwartz
• ES6 Features - Luke Hoban
• Frontend Guidelines - Benjamin De Cock

Books

• JavaScript: The Good Parts - Douglas Crockford
• JavaScript Patterns - Stoyan Stefanov
• Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
• High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
• Maintainable JavaScript - Nicholas C. Zakas
• JavaScript Web Applications - Alex MacCaw
• Pro JavaScript Techniques - John Resig
• Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
• Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
• Human JavaScript - Henrik Joreteg
• Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
• JSBooks - Julien Bouquillon
• Third Party JavaScript - Ben Vinegar and Anton Kovalyov
• Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
• Eloquent JavaScript - Marijn Haverbeke
• You Don't Know JS: ES6 & Beyond - Kyle Simpson

Blogs

• DailyJS
• JavaScript Weekly
• JavaScript, JavaScript...
• Bocoup Weblog
• Adequately Good
• NCZOnline
• Perfection Kills
• Ben Alman
• Dmitry Baranovskiy
• Dustin Diaz
• nettuts

Podcasts

• JavaScript Jabber

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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.

• Aan Zee: AanZee/javascript
• Adult Swim: adult-swim/javascript
• Airbnb: airbnb/javascript
• Apartmint: apartmint/javascript
• Ascribe: ascribe/javascript
• Avalara: avalara/javascript
• Avant: avantcredit/javascript
• Billabong: billabong/javascript
• Bisk: bisk/javascript
• Blendle: blendle/javascript
• Brainshark: brainshark/javascript
• ComparaOnline: comparaonline/javascript
• Compass Learning: compasslearning/javascript-style-guide
• DailyMotion: dailymotion/javascript
• Digitpaintdigitpaint/javascript
• Ecosia: ecosia/javascript
• Evernote: evernote/javascript-style-guide
• Evolution Gaming: evolution-gaming/javascript
• ExactTarget: ExactTarget/javascript
• ExpensifyExpensify/Style-Guide
• Flexberry: Flexberry/javascript-style-guide
• Gawker Media: gawkermedia/javascript
• General Electric: GeneralElectric/javascript
• GoodData: gooddata/gdc-js-style
• Grooveshark: grooveshark/javascript
• How About We: howaboutwe/javascript
• Huballin: huballin/javascript
• HubSpot: HubSpot/javascript
• Hyper: hyperoslo/javascript-playbook
• InfoJobs: InfoJobs/JavaScript-Style-Guide
• Intent Media: intentmedia/javascript
• Jam3: Jam3/Javascript-Code-Conventions
• JeopardyBot: kesne/jeopardy-bot
• JSSolutions: JSSolutions/javascript
• Kinetica Solutions: kinetica/javascript
• Mighty Spring: mightyspring/javascript
• MinnPost: MinnPost/javascript
• MitocGroup: MitocGroup/javascript
• ModCloth: modcloth/javascript
• Money Advice Service: moneyadviceservice/javascript
• Muber: muber/javascript
• National Geographic: natgeo/javascript
• National Park Service: nationalparkservice/javascript
• Nimbl3: nimbl3/javascript
• Orion Health: orionhealth/javascript
• OutBoxSoft: OutBoxSoft/javascript
• Peerby: Peerby/javascript
• Razorfish: razorfish/javascript-style-guide
• reddit: reddit/styleguide/javascript
• React: /facebook/react/blob/master/CONTRIBUTING.md#style-guide
• REI: reidev/js-style-guide
• Ripple: ripple/javascript-style-guide
• SeekingAlpha: seekingalpha/javascript-style-guide
• Shutterfly: shutterfly/javascript
• Springload: springload/javascript
• StudentSphere: studentsphere/javascript
• SysGarage: sysgarage/javascript-style-guide
• Target: target/javascript
• TheLadders: TheLadders/javascript
• T4R Technology: T4R-Technology/javascript
• VoxFeed: VoxFeed/javascript-style-guide
• WeBox Studio: weboxstudio/javascript
• Weggo: Weggo/javascript
• Zillow: zillow/javascript
• ZocDoc: ZocDoc/javascript