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Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

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Table of Contents

  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. Performance
  30. Resources
  31. In the Wild
  32. Translation
  33. The JavaScript Style Guide Guide
  34. Chat With Us About JavaScript
  35. Contributors
  36. License

Types

  • 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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References

  • 2.1 Use const for all of your references; avoid using var.

    Why? This ensures that you can't reassign your references, which can lead to bugs and difficult to comprehend code.

    eslint rules: prefer-const, no-const-assign.

    // badvara=1;varb=2;// goodconsta=1;constb=2;

  • 2.2 If you must reassign references, use let instead of var.

    Why? let is block-scoped rather than function-scoped like var.

    eslint rules: no-var.

    jscs rules: disallowVar.

    // 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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Objects

  • 3.1 Use the literal syntax for object creation.

    eslint rules: 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 rules: disallowIdentiferNames.

```javascript
// bad
const superman = {
default: { clark: 'kent' },
private: true,
};
// good
const superman = {
defaults: { clark: 'kent' },
hidden: true,
};
```
  • 3.3 Use readable synonyms in place of reserved words.

jscs rules: disallowIdentiferNames.

```javascript
// bad
const superman = {
class: 'alien',
};
// bad
const superman = {
klass: 'alien',
};
// good
const superman = {
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 rules: object-shorthand.

    jscs rules: requireEnhancedObjectLiterals.

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

  • 3.6 Use property value shorthand.

    Why? It is shorter to write and descriptive.

    eslint rules: object-shorthand.

    jscs rules: requireEnhancedObjectLiterals.

    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.

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

eslint rules: quote-props.

jscs rules: disallowQuotedKeysInObjects.

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

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Arrays

  • 4.1 Use the literal syntax for array creation.

    eslint rules: 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);

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Destructuring

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

    Why? Destructuring saves you from creating temporary references for those properties.

    jscs rules: requireObjectDestructuring.

    // 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 rules: requireArrayDestructuring.

```javascript
const arr = [1, 2, 3, 4];
// bad
const first = arr[0];
const second = arr[1];
// good
const [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, right }=processInput(input);

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Strings

  • 6.1 Use single quotes '' for strings.

    eslint rules: quotes.

    jscs rules: 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.

    Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

    eslint rules: prefer-template.

    jscs rules: requireTemplateStrings.

    // badfunctionsayHi(name){return'How are you, '+name+'?';}// badfunctionsayHi(name){return['How are you, ',name,'?'].join();}// goodfunctionsayHi(name){return`How are you, ${name}?`;}

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

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Functions

  • 7.1 Use function declarations instead of function expressions.

    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.

    jscs rules: requireFunctionDeclarations.

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

  • 7.2 Immediately invoked function expressions:

    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.

    eslint rules: wrap-iife.

    jscs rules: requireParenthesesAroundIIFE.

    // 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 rules: 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.

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

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

    eslint rules: no-param-reassign.

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

  • 7.13 Never reassign parameters.

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

    eslint rules: no-param-reassign.

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

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Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation.

    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.

    eslint rules: prefer-arrow-callback, arrow-spacing.

    jscs rules: requireArrowFunctions.

    // 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.

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

    Why not? If you plan on returning an object.

    eslint rules: arrow-parens, arrow-body-style.

    jscs rules: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions.

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

    Why? Less visual clutter.

    eslint rules: arrow-parens.

    jscs rules: disallowParenthesesAroundArrowParam.

    // 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;});

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Constructors

  • 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()}`;}}

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Modules

  • 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';

  • 10.3And 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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Iterators and Generators

  • 11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of.

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

    eslint rules: no-iterator.

    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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Properties

  • 12.1 Use dot notation when accessing properties.

    eslint rules: dot-notation.

    jscs rules: requireDotNotation.

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

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

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

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Variables

  • 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.

    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.

    eslint rules: one-var.

    jscs rules: disallowMultipleVarDecl.

    // 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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Hoisting

  • 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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Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=.

  • 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:

    eslint rules: eqeqeq.

    • 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.

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Blocks

  • 16.1 Use braces with all multi-line blocks.

    // badif(test)returnfalse;// goodif(test)returnfalse;// goodif(test){returnfalse;}// badfunction(){returnfalse;}// goodfunction(){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 rules: brace-style.

    jscs rules: disallowNewlineBeforeBlockStatements.

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

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Comments

  • 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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Whitespace

  • 18.1 Use soft tabs set to 2 spaces.

    eslint rules: indent.

    jscs rules: validateIndentation.

    // badfunction(){∙∙∙∙constname;}// badfunction(){∙constname;}// goodfunction(){∙∙constname;}

  • 18.2 Place 1 space before the leading brace.

    eslint rules: space-before-blocks.

    jscs rules: 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 rules: space-after-keywords, space-before-keywords.

    jscs rules: requireSpaceAfterKeywords.

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

  • 18.4 Set off operators with spaces.

    eslint rules: space-infix-ops.

    jscs rules: 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. Use a leading dot, which emphasizes that the line is a method call, not a new statement.

    // 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').class('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);

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

    jscs rules: 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 rules: padded-blocks.

    jscs rules: 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 rules: space-in-parens.

    jscs rules: 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 rules: array-bracket-spacing.

    jscs rules: 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 rules: object-curly-spacing.

    jscs rules: disallowSpacesInsideObjectBrackets.

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

  • 18.12 Avoid having lines of code that are longer than 100 characters (including whitespace).

    Why? This ensures readability and maintainability.

    eslint rules: max-len.

    jscs rules: maximumLineLength.

    // 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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Commas

  • 19.1 Leading commas: Nope.

    eslint rules: comma-style.

    jscs rules: 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 rules: comma-dangle.

    jscs rules: 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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Semicolons

  • 20.1Yup.

    eslint rules: semi.

    jscs rules: 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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Type Casting & Coercion

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

  • 21.2 Strings:

    // => this.reviewScore = 9;// badconsttotalScore=this.reviewScore+'';// goodconsttotalScore=String(this.reviewScore);

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

    eslint rules: 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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Naming Conventions

  • 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 rules: camelcase.

    jscs rules: requireCamelCaseOrUpperCaseIdentifiers.

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

  • 22.3 Use PascalCase when naming constructors or classes.

    eslint rules: new-cap.

    jscs rules: 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 rules: no-underscore-dangle.

    jscs rules: 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 rules: 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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Accessors

  • 23.1 Accessor functions for properties are not required.

  • 23.2 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 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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Events

  • 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',function(e,listingId){// do something with listingId});

    prefer:

    // good$(this).trigger('listingUpdated',{listingId: listing.id});
...
$(this).on('listingUpdated',function(e,data){// do something with data.listingId});

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jQuery

  • 25.1 Prefix jQuery object variables with a $.

    jscs rules: 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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ECMAScript 5 Compatibility

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ECMAScript 6 Styles

  • 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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Testing

  • 28.1Yup.

    function(){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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Performance

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Resources

Learning ES6

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.

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Translation

This style guide is also available in other languages:

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

(The MIT License)

Copyright (c) 2014-2016 Airbnb

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Amendments

We encourage you to fork this guide and change the rules to fit your team's style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

};

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JavaScript Style Guide

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