Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Table of Contents

1. Types
2. Objects
3. Arrays
4. Strings
5. Functions
6. Properties
7. Variables
8. Hoisting
9. Conditional Expressions & Equality
10. Blocks
11. Comments
12. Whitespace
13. Leading Commas
14. Semicolons
15. Type Casting & Coercion
16. Naming Conventions
17. Accessors
18. Constructors
19. Modules
20. jQuery
21. ES5 Compatibility
22. Testing
23. Performance
24. Resources
25. In the Wild
26. Translation
27. The JavaScript Style Guide Guide
28. Contributors
29. License

Types

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

  • string
  • number
  • boolean
  • null
  • undefined

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

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

  • object
  • array
  • function

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

  [⬆]

Objects

• Use the literal syntax for object creation.

  // badvaritem=newObject();// goodvaritem={};

• Don't use reserved words as keys.

  // badvarsuperman={class: 'superhero',default: {clark: 'kent'},private: true};// goodvarsuperman={klass: 'superhero',defaults: {clark: 'kent'},hidden: true};

  [⬆]

Arrays

• Use the literal syntax for array creation

  // badvaritems=newArray();// goodvaritems=[];

• If you don't know array length use Array#push.

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

• When you need to copy an array use Array#slice. jsPerf

  varlen=items.length,itemsCopy=[],i;// badfor(i=0;i<len;i++){itemsCopy[i]=items[i];}// gooditemsCopy=Array.prototype.slice.call(items);

  [⬆]

Strings

• Use single quotes '' for strings

  // badvarname="Bob Parr";// goodvarname='Bob Parr';// badvarfullName="Bob "+this.lastName;// goodvarfullName='Bob '+this.lastName;

• Strings longer than 80 characters should be written across multiple lines using string concatenation.

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

  // badvarerrorMessage='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.';// badvarerrorMessage='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.';// goodvarerrorMessage='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.';

• When programatically building up a string, use Array#join instead of string concatenation. Mostly for IE: jsPerf.

  varitems,messages,length,i;messages=[{state: 'success',message: 'This one worked.'},{state: 'success',message: 'This one worked as well.'},{state: 'error',message: 'This one did not work.'}];length=messages.length;// badfunctioninbox(messages){items='<ul>';for(i=0;i<length;i++){items+='<li>'+messages[i].message+'</li>';}returnitems+'</ul>';}// goodfunctioninbox(messages){items=[];for(i=0;i<length;i++){items[i]=messages[i].message;}return'<ul><li>'+items.join('</li><li>')+'</li></ul>';}

  [⬆]

Functions

• Function expressions:

  // anonymous function expressionvaranonymous=function(){returntrue;};// named function expressionvarnamed=functionnamed(){returntrue;};// immediately-invoked function expression (IIFE)(function(){console.log('Welcome to the Internet. Please follow me.');})();

• 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 where bad news bears.

• Note: 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.');}}// goodif(currentUser){vartest=functiontest(){console.log('Yup.');};}

• 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...}

  [⬆]

Properties

• Use dot notation when accessing properties.

  varluke={jedi: true,age: 28};// badvarisJedi=luke['jedi'];// goodvarisJedi=luke.jedi;

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

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

  [⬆]

Variables

• Always use var 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();// goodvarsuperPower=newSuperPower();

• Use one var declaration for multiple variables and declare each variable on a newline.

  // badvaritems=getItems();vargoSportsTeam=true;vardragonball='z';// goodvaritems=getItems(),goSportsTeam=true,dragonball='z';

• Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

  // badvari,len,dragonball,items=getItems(),goSportsTeam=true;// badvari,items=getItems(),dragonball,goSportsTeam=true,len;// goodvaritems=getItems(),goSportsTeam=true,dragonball,length,i;

• Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.

  // badfunction(){test();console.log('doing stuff..');//..other stuff..varname=getName();if(name==='test'){returnfalse;}returnname;}// goodfunction(){varname=getName();test();console.log('doing stuff..');//..other stuff..if(name==='test'){returnfalse;}returnname;}// badfunction(){varname=getName();if(!arguments.length){returnfalse;}returntrue;}// goodfunction(){if(!arguments.length){returnfalse;}varname=getName();returntrue;}

  [⬆]

Hoisting

• Variable declarations get hoisted to the top of their scope, their assignment does not.

  // 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(){vardeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}

• 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');};}

• 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');};}}

• 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

  [⬆]

Conditional Expressions & Equality

• Use === and !== over == and !=.

• Conditional expressions are evaluated using coercion with the ToBoolean 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 evalute to false if +0, -0, or NaN, otherwise true
  • Strings evaluate to false if an empty string '', otherwise true

  if([0]){// true// An array is an object, objects evaluate to true}

• Use shortcuts.

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

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

  [⬆]

Blocks

• Use braces with all multi-line blocks.

  // badif(test)returnfalse;// goodif(test)returnfalse;// goodif(test){returnfalse;}// badfunction(){returnfalse;}// goodfunction(){returnfalse;}

  [⬆]

Comments

• Use /** ... */ for multiline 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;}

• Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an emptyline before the comment.

  // badvaractive=true;// is current tab// good// is current tabvaractive=true;// badfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}// goodfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}

  [⬆]

Whitespace

• Use soft tabs set to 2 spaces

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

• Place 1 space before the leading brace.

  // 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'});

• Place an empty newline at the end of the file.

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

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

• Use indentation when making long method chains.

  // bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// good$('#items').find('.selected').highlight().end().find('.open').updateCount();// badvarleds=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);// goodvarleds=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);

  [⬆]

Leading Commas

• Nope.

  // badvaronce,upon,aTime;// goodvaronce,upon,aTime;// badvarhero={firstName: 'Bob',lastName: 'Parr',heroName: 'Mr. Incredible',superPower: 'strength'};// goodvarhero={firstName: 'Bob',lastName: 'Parr',heroName: 'Mr. Incredible',superPower: 'strength'};

  [⬆]

Semicolons

• Yup.

  // bad(function(){varname='Skywalker'returnname})()// good(function(){varname='Skywalker';returnname;})();// good;(function(){varname='Skywalker';returnname;})();

  [⬆]

Type Casting & Coercion

• Perform type coercion at the beginning of the statement.

• Strings:

  // => this.reviewScore = 9;// badvartotalScore=this.reviewScore+'';// goodvartotalScore=''+this.reviewScore;// badvartotalScore=''+this.reviewScore+' total score';// goodvartotalScore=this.reviewScore+' total score';

• Use parseInt for Numbers and always with a radix for type casting.

• 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.

  varinputValue='4';// badvarval=newNumber(inputValue);// badvarval=+inputValue;// badvarval=inputValue>>0;// badvarval=parseInt(inputValue);// goodvarval=Number(inputValue);// goodvarval=parseInt(inputValue,10);// good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */varval=inputValue>>0;

• Booleans:

  varage=0;// badvarhasAge=newBoolean(age);// goodvarhasAge=Boolean(age);// goodvarhasAge=!!age;

  [⬆]

Naming Conventions

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

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

• Use camelCase when naming objects, functions, and instances

  // badvarOBJEcttsssss={};varthis_is_my_object={};varthis-is-my-object={};functionc(){};varu=newuser({name: 'Bob Parr'});// goodvarthisIsMyObject={};functionthisIsMyFunction(){};varuser=newUser({name: 'Bob Parr'});

• Use PascalCase when naming constructors or classes

  // badfunctionuser(options){this.name=options.name;}varbad=newuser({name: 'nope'});// goodfunctionUser(options){this.name=options.name;}vargood=newUser({name: 'yup'});

• Use a leading underscore _ when naming private properties

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

• When saving a reference to this use _this.

  // badfunction(){varself=this;returnfunction(){console.log(self);};}// badfunction(){varthat=this;returnfunction(){console.log(that);};}// goodfunction(){var_this=this;returnfunction(){console.log(_this);};}

• Name your functions. This is helpful for stack traces.

  // badvarlog=function(msg){console.log(msg);};// goodvarlog=functionlog(msg){console.log(msg);};

  [⬆]

Accessors

• Accessor functions for properties are not required

• If you do make accessor functions use getVal() and setVal('hello')

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

• If the property is a boolean, use isVal() or hasVal()

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

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

  functionJedi(options){options||(options={});varlightsaber=options.lightsaber||'blue';this.set('lightsaber',lightsaber);}Jedi.prototype.set=function(key,val){this[key]=val;};Jedi.prototype.get=function(key){returnthis[key];};

  [⬆]

Constructors

• Assign methods to the prototype object, instead of overwriting the prototype with a new object. Overwriting the prototype makes inheritance impossible: by resetting the prototype you'll overwrite the base!

  functionJedi(){console.log('new jedi');}// badJedi.prototype={fight: functionfight(){console.log('fighting');},block: functionblock(){console.log('blocking');}};// goodJedi.prototype.fight=functionfight(){console.log('fighting');};Jedi.prototype.block=functionblock(){console.log('blocking');};

• 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;};varluke=newJedi();luke.jump();// => trueluke.setHeight(20)// => undefined// goodJedi.prototype.jump=function(){this.jumping=true;returnthis;};Jedi.prototype.setHeight=function(height){this.height=height;returnthis;};varluke=newJedi();luke.jump().setHeight(20);

• It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

  functionJedi(options){options||(options={});this.name=options.name||'no name';}Jedi.prototype.getName=functiongetName(){returnthis.name;};Jedi.prototype.toString=functiontoString(){return'Jedi - '+this.getName();};

  [⬆]

Modules

• The module should start with a !. This ensures that if a malformed module forgets to include a final semicolon there aren't errors in production when the scripts get concatenated.

• The file should be named with camelCase, live in a folder with the same name, and match the name of the single export.

• Add a method called noConflict() that sets the exported module to the previous version and returns this one.

• Always declare 'use strict'; at the top of the module.

  // fancyInput/fancyInput.js!function(global){'use strict';varpreviousFancyInput=global.FancyInput;functionFancyInput(options){this.options=options||{};}FancyInput.noConflict=functionnoConflict(){global.FancyInput=previousFancyInput;returnFancyInput;};global.FancyInput=FancyInput;}(this);

  [⬆]

jQuery

• Prefix jQuery object variables with a $.

  // badvarsidebar=$('.sidebar');// goodvar$sidebar=$('.sidebar');

• Cache jQuery lookups.

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

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

• Use find with scoped jQuery object queries.

  // bad$('.sidebar','ul').hide();// bad$('.sidebar').find('ul').hide();// good$('.sidebar ul').hide();// good$('.sidebar > ul').hide();// good (slower)$sidebar.find('ul');// good (faster)$($sidebar[0]).find('ul');

  [⬆]

ECMAScript 5 Compatibility

• Refer to Kangax's ES5 compatibility table

[⬆]

Testing

• Yup.

  function(){returntrue;}

  [⬆]

Performance

• 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...

[⬆]

Resources

Read This

• Annotated ECMAScript 5.1

Other Styleguides

• 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

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

Blogs

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

[⬆]

In the Wild

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

• Airbnb: airbnb/javascript
• American Insitutes for Research: AIRAST/javascript
• ExactTarget: ExactTarget/javascript
• GoCardless: gocardless/javascript
• GoodData: gooddata/gdc-js-style
• How About We: howaboutwe/javascript
• MinnPost: MinnPost/javascript
• National Geographic: natgeo/javascript
• Razorfish: razorfish/javascript-style-guide
• Shutterfly: shutterfly/javascript

Translation

This style guide is also available in other languages:

• 🇩🇪 German: timofurrer/javascript-style-guide
• 🇯🇵 Japanese: mitsuruog/javacript-style-guide

The JavaScript Style Guide Guide

• Reference

Contributors

• View Contributors

License

(The MIT License)

Copyright (c) 2012 Airbnb

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

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

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

[⬆]

};