Javascript style guide

Types

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

  • string
  • number
  • boolean
  • null
  • undefined

  var foo = 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

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

  [⬆]

Objects

• Use the literal syntax for object creation.

  // badvar item = new Object();// goodvar item = {};

• Don't use reserved words as keys. It won't work in IE8. More info

  // badvar superman = {  default: { clark: 'kent' },  private: true};// goodvar superman = {  defaults: { clark: 'kent' },  hidden: true};

• Use readable synonyms in place of reserved words.

  // badvar superman = {  class: 'alien'};// badvar superman = {  klass: 'alien'};// goodvar superman = {  type: 'alien'};

  [⬆]

Arrays

• Use the literal syntax for array creation

  // badvar items = new Array();// goodvar items = [];

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

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

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

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

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

  function trigger() {  var args = Array.prototype.slice.call(arguments);  ...}

  [⬆]

Strings

• Use single quotes '' for strings

  // badvar name = "Bob Parr";// goodvar name = 'Bob Parr';// badvar fullName = "Bob " + this.lastName;// goodvar fullName = '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

  // badvar errorMessage = '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.';// badvar errorMessage = '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.';// goodvar errorMessage = '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.

  var items,    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;// badfunction inbox(messages) {  items = '<ul>';  for (i = 0; i < length; i++) {    items += '<li>' + messages[i].message + '</li>';  }  return items + '</ul>';}// goodfunction inbox(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 expressionvar anonymous = function() {  return true;};// named function expressionvar named = function named() {  return true;};// 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 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) {  function test() {    console.log('Nope.');  }}// goodvar test;if (currentUser) {  test = function test() {    console.log('Yup.');  };}

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

  // badfunction nope(name, options, arguments) {  // ...stuff...}// goodfunction yup(name, options, args) {  // ...stuff...}

  [⬆]

Properties

• Use dot notation when accessing properties.

  var luke = {  jedi: true,  age: 28};// badvar isJedi = luke['jedi'];// goodvar isJedi = luke.jedi;

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

  var luke = {  jedi: true,  age: 28};function getProp(prop) {  return luke[prop];}var isJedi = 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 = new SuperPower();// goodvar superPower = new SuperPower();

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

  // badvar items = getItems();var goSportsTeam = true;var dragonball = 'z';// goodvar items = 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.

  // badvar i, len, dragonball,    items = getItems(),    goSportsTeam = true;// badvar i, items = getItems(),    dragonball,    goSportsTeam = true,    len;// goodvar items = 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..  var name = getName();  if (name === 'test') {    return false;  }  return name;}// goodfunction() {  var name = getName();  test();  console.log('doing stuff..');  //..other stuff..  if (name === 'test') {    return false;  }  return name;}// badfunction() {  var name = getName();  if (!arguments.length) {    return false;  }  return true;}// goodfunction() {  if (!arguments.length) {    return false;  }  var name = getName();  return true;}

  [⬆]

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)function example() {  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.function example() {  console.log(declaredButNotAssigned); // => undefined  var declaredButNotAssigned = true;}// The interpreter is hoisting the variable// declaration to the top of the scope.// Which means our example could be rewritten as:function example() {  var declaredButNotAssigned;  console.log(declaredButNotAssigned); // => undefined  declaredButNotAssigned = true;}

• Anonymous function expressions hoist their variable name, but not the function assignment.

  function example() {  console.log(anonymous); // => undefined  anonymous(); // => TypeError anonymous is not a function  var anonymous = function() {    console.log('anonymous function expression');  };}

• Named function expressions hoist the variable name, not the function name or the function body.

  function example() {  console.log(named); // => undefined  named(); // => TypeError named is not a function  superPower(); // => ReferenceError superPower is not defined  var named = function superPower() {    console.log('Flying');  };}// the same is true when the function name// is the same as the variable name.function example() {  console.log(named); // => undefined  named(); // => TypeError named is not a function  var named = function named() {    console.log('named');  }}

• Function declarations hoist their name and the function body.

  function example() {  superPower(); // => Flying  function superPower() {    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 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 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)  return false;// goodif (test) return false;// goodif (test) {  return false;}// badfunction() { return false; }// goodfunction() {  return false;}

  [⬆]

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> elementfunction make(tag) {  // ...stuff...  return element;}// good/** * make() returns a new element * based on the passed in tag name * * @param <String> tag * @return <Element> element */function make(tag) {  // ...stuff...  return element;}

• 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.

  // badvar active = true;  // is current tab// good// is current tabvar active = true;// badfunction getType() {  console.log('fetching type...');  // set the default type to 'no type'  var type = this._type || 'no type';  return type;}// goodfunction getType() {  console.log('fetching type...');  // set the default type to 'no type'  var type = this._type || 'no type';  return type;}

• 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.

• Use // FIXME: to annotate problems

  function Calculator() {  // FIXME: shouldn't use a global here  total = 0;  return this;}

• Use // TODO: to annotate solutions to problems

  function Calculator() {  // TODO: total should be configurable by an options param  this.total = 0;  return this;}

  [⬆]

Whitespace

• Use soft tabs set to 2 spaces

  // badfunction() {∙∙∙∙var name;}// badfunction() {∙var name;}// goodfunction() {∙∙var name;}

• Place 1 space before the leading brace.

  // badfunction test(){  console.log('test');}// goodfunction test() {  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();// badvar leds = 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);// goodvar leds = 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);

  [⬆]

Commas

• Leading commas: Nope.

  // badvar once  , upon  , aTime;// goodvar once,    upon,    aTime;// badvar hero = {    firstName: 'Bob'  , lastName: 'Parr'  , heroName: 'Mr. Incredible'  , superPower: 'strength'};// goodvar hero = {  firstName: 'Bob',  lastName: 'Parr',  heroName: 'Mr. Incredible',  superPower: 'strength'};

• Additional trailing comma: Nope. This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 (source):

  Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.

    // bad  var hero = {    firstName: 'Kevin',    lastName: 'Flynn',  };  var heroes = [    'Batman',    'Superman',  ];  // good  var hero = {    firstName: 'Kevin',    lastName: 'Flynn'  };  var heroes = [    'Batman',    'Superman'  ];

  [⬆]

Semicolons

• Yup.

  // bad(function() {  var name = 'Skywalker'  return name})()// good(function() {  var name = 'Skywalker';  return name;})();// good;(function() {  var name = 'Skywalker';  return name;})();

  [⬆]

Type Casting & Coercion

• Perform type coercion at the beginning of the statement.

• Strings:

  //  => this.reviewScore = 9;// badvar totalScore = this.reviewScore + '';// goodvar totalScore = '' + this.reviewScore;// badvar totalScore = '' + this.reviewScore + ' total score';// goodvar totalScore = this.reviewScore + ' total score';

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

  var inputValue = '4';// badvar val = new Number(inputValue);// badvar val = +inputValue;// badvar val = inputValue >> 0;// badvar val = parseInt(inputValue);// goodvar val = Number(inputValue);// goodvar val = parseInt(inputValue, 10);

• 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.

• Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion

  // good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */var val = inputValue >> 0;

• Booleans:

  var age = 0;// badvar hasAge = new Boolean(age);// goodvar hasAge = Boolean(age);// goodvar hasAge = !!age;

  [⬆]

Naming Conventions

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

  // badfunction q() {  // ...stuff...}// goodfunction query() {  // ..stuff..}

• Use camelCase when naming objects, functions, and instances

  // badvar OBJEcttsssss = {};var this_is_my_object = {};function c() {};var u = new user({  name: 'Bob Parr'});// goodvar thisIsMyObject = {};function thisIsMyFunction() {};var user = new User({  name: 'Bob Parr'});

• Use PascalCase when naming constructors or classes

  // badfunction user(options) {  this.name = options.name;}var bad = new user({  name: 'nope'});// goodfunction User(options) {  this.name = options.name;}var good = new User({  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() {  var self = this;  return function() {    console.log(self);  };}// badfunction() {  var that = this;  return function() {    console.log(that);  };}// goodfunction() {  var _this = this;  return function() {    console.log(_this);  };}

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

  // badvar log = function(msg) {  console.log(msg);};// goodvar log = function log(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()) {  return false;}// goodif (!dragon.hasAge()) {  return false;}

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

  function Jedi(options) {  options || (options = {});  var lightsaber = options.lightsaber || 'blue';  this.set('lightsaber', lightsaber);}Jedi.prototype.set = function(key, val) {  this[key] = val;};Jedi.prototype.get = function(key) {  return this[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!

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

• Methods can return this to help with method chaining.

  // badJedi.prototype.jump = function() {  this.jumping = true;  return true;};Jedi.prototype.setHeight = function(height) {  this.height = height;};var luke = new Jedi();luke.jump(); // => trueluke.setHeight(20) // => undefined// goodJedi.prototype.jump = function() {  this.jumping = true;  return this;};Jedi.prototype.setHeight = function(height) {  this.height = height;  return this;};var luke = new Jedi();luke.jump()  .setHeight(20);

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

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

  [⬆]

Events

• 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});

  [⬆]

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. Explanation
• 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';  var previousFancyInput = global.FancyInput;  function FancyInput(options) {    this.options = options || {};  }  FancyInput.noConflict = function noConflict() {    global.FancyInput = previousFancyInput;    return FancyInput;  };  global.FancyInput = FancyInput;}(this);

  [⬆]

jQuery

• Prefix jQuery object variables with a $.

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

• Cache jQuery lookups.

  // badfunction setSidebar() {  $('.sidebar').hide();  // ...stuff...  $('.sidebar').css({    'background-color': 'pink'  });}// goodfunction setSidebar() {  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$('ul', '.sidebar').hide();// bad$('.sidebar').find('ul').hide();// good$('.sidebar ul').hide();// good$('.sidebar > ul').hide();// good$sidebar.find('ul');

  [⬆]

ECMAScript 5 Compatibility

• Refer to Kangax's ES5 compatibility table

  [⬆]

Testing

• Yup.

  function() {  return true;}

  [⬆]

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
• Popular JavaScript Coding Conventions on Github

Further Reading

• Understanding JavaScript Closures - Angus Croll
• Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer

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

(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.

[⬆]