howaboutwe / javascript-style-guide

JavaScript Style Guide

HowAboutWe JavaScript Style Guide

A mostly reasonable approach to JavaScript

Table of Contents

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

Objects

• Use the literal syntax for object creation.

  // bad
  var item = new Object();
  
  // good
  var item = {};

• Don't use reserved words as keys.

  // bad
  var superman = {
    class: 'superhero',
    default: { clark: 'kent' },
    private: true
  };
  
  // good
  var superman = {
    klass: 'superhero',
    defaults: { clark: 'kent' },
    hidden: true
  };

  [⬆]

Arrays

• Use the literal syntax for array creation

  // bad
  var items = new Array();
  
  // good
  var items = [];

• For clarity and performance reasons, cache the length of your arrays in variables outside of the loop

  // bad
  for (var i = 0; i < items.length; i++) {
    // ...stuff...
  }
  
  // bad
  for (var i = 0, len = items.length; i < len; i++) {
    // ...stuff...
  }
  
  // good
  var len = items.length;
  var i;
  
  for (i = 0; i < len; i++) {
    // ...stuff...
  }

• Always use array#push when appending new values to an array.

  var someStack = [];
  
  // bad
  someStack[someStack.length] = 'abracadabra';
  
  // good
  someStack.push('abracadabra');

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

  var len = items.length;
  var itemsCopy = [];
  var i;
  
  // bad
  for (i = 0; i < len; i++) {
    itemsCopy[i] = items[i];
  }
  
  // good
  itemsCopy = 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

  // bad
  var name = "Bob Parr";
  
  // good
  var name = 'Bob Parr';
  
  // bad
  var fullName = "Bob " + this.lastName;
  
  // good
  var fullName = 'Bob ' + this.lastName;
  
  // bad
  var fullName = "<a href=\"/name\">Bob " + this.lastName + "</a>";
  
  // good
  var fullName = '<a href="/name">Bob ' + this.lastName + '</a>';

• Use double quotes "" for interpreted strings inside templates.

  // bad
  var name = users['#{ getUser }'];
  
  // good
  var name = users["#{ getUser }"];

• 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

  // bad
  var 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.';
  
  // bad
  var 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.';
  
  
  // good
  var 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;
  var messages;
  var length;
  var 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;
  
  // bad
  function inbox(messages) {
    items = '<ul>';
  
    for (i = 0; i < length; i++) {
      items += '<li>' + messages[i].message + '</li>';
    }
  
    return items + '</ul>';
  }
  
  // good
  function 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 expression
  var anonymous = function() {
    return true;
  };
  
  // named function expression
  var 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.

  // bad
  if (currentUser) {
    function test() {
      console.log('Nope.');
    }
  }
  
  // good
  if (currentUser) {
    var 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.

  // bad
  function nope(name, options, arguments) {
    // ...stuff...
  }
  
  // good
  function yup(name, options, args) {
    // ...stuff...
  }

  [⬆]

Properties

• Use dot notation when accessing properties.

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

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

  var users = {
    jane: {},
    john: {}
  };
  
  var userName = getUsername();
  var userObject = users[userName];

  [⬆]

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.

  // bad
  superPower = new SuperPower();
  
  // good
  var superPower = new SuperPower();

• Use additional var declarations for multiple variables and declare each variable on a newline. This is useful when reordering variables and avoiding simple syntax mistakes.

  // bad
  var items = getItems(),
      goSportsTeam = true,
      dragonball = 'z';
  
  // bad
  var items = getItems()
      , goSportsTeam = true
      , dragonball = 'z';
  
  // good
  var items = getItems();
  var goSportsTeam = true;
  var dragonball = 'z';
  var length;
  var i;

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

  // bad
  function() {
    test();
    console.log('doing stuff..');
  
    //..other stuff..
  
    var name = getName();
  
    if (name === 'test') {
      return false;
    }
  
    return name;
  }
  
  // good
  function() {
    var name = getName();
  
    test();
    console.log('doing stuff..');
  
    //..other stuff..
  
    if (name === 'test') {
      return false;
    }
  
    return name;
  }
  
  // bad
  function() {
    var name = getName();
  
    if (!arguments.length) {
      return false;
    }
  
    return true;
  }
  
  // good
  function() {
    if (!arguments.length) {
      return false;
    }
  
    var name = getName();
  
    return true;
  }

  [⬆]

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.

  // bad
  if (name !== '') {
    // ...stuff...
  }
  
  // good
  if (name) {
    // ...stuff...
  }

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

  [⬆]

Blocks

• Use braces with all blocks.

  // bad
  if (test)
    return false;
  
  // bad
  if (test) return false;
  
  // good
  if (test) {
    return false;
  }

• Always put blocks and curlys on their own lines.

  // bad
  if (test) { return false; }
  
  // good
  if (test) {
    return false;
  }
  
  // bad
  function() { return false; }
  
  // bad
  function() {
      return false; }
  
  // good
  function() {
    return false;
  }

  [⬆]

Comments

• Documentation is highly recommended on Utility Modules. It is optional on all other types of Modules.

• Don't over document. Opt for descriptive names of variables, functions, and returns over documenting the obvious.

• Use JSDoc notation for commenting guidelines.

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

  // bad
  // make() returns a new element
  // based on the passed in tag name
  //
  // @param {String} tag
  // @return {Element} element
  function 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. Always place single line comments on a newline above the subject of the comment. Put an empty line before the comment.

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

• Always specify types and values for all parameters and return values. Ideally, provide a description for the function and parameters as well.

```javascript
// bad
function make(tag) {
  // ...stuff...
}

// good
/**
 * @param {String} tag
 * @return {Element} element
 */
function make(tag) {
  // ...stuff...
}

// best
/**
 * Returns a new element based on the passed in tag name.
 *
 * @param {String} tag
 *  Tag to create. -eg 'a', 'span', 'strong'
 * @return {Element} element
 *  DOM element object.
 */
function make(tag) {

  // ...stuff...

  return element;
}
```

**[[⬆]](#TOC)**

Whitespace

• Use soft tabs set to 2 spaces

  // bad
  function() {
  ∙∙∙∙var name;
  }
  
  // bad
  function() {
  ∙var name;
  }
  
  // good
  function() {
  ∙∙var name;
  }

• Place 1 space before the leading brace.

  // bad
  function test(){
    console.log('test');
  }
  
  // good
  function test() {
    console.log('test');
  }
  
  // bad
  dog.set('attr',{
    age: '1 year',
    breed: 'Bernese Mountain Dog'
  });
  
  // good
  dog.set('attr', {
    age: '1 year',
    breed: 'Bernese Mountain Dog'
  });

• 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();
  
  // bad
  var 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);
  
  // good
  var 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);

  [⬆]

Leading Commas

• Nope.

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

  [⬆]

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;
  
  // bad
  var totalScore = this.reviewScore + '';
  
  // good
  var totalScore = '' + this.reviewScore;
  
  // bad
  var totalScore = '' + this.reviewScore + ' total score';
  
  // good
  var totalScore = this.reviewScore + ' total score';

• Numbers:

• 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.

  var inputValue = '4';
  
  // bad
  var val = new Number(inputValue);
  
  // bad
  var val = +inputValue;
  
  // good
  var val = parseInt(inputValue, 10);

  [⬆]

Naming Conventions

• Avoid single letter names. Be descriptive with your naming. These get Uglified anyways.

  // bad
  function q() {
    // ...stuff...
  }
  
  // good
  function query() {
    // ..stuff..
  }

• Use PascalCase when naming Views, Constructors, Classes, Models, and Collections.

  // bad
  function user(options) {
    this.name = options.name;
  }
  
  var bad = new user({
    name: 'nope'
  });
  
  // good
  function User(options) {
    this.name = options.name;
  }
  
  var good = new User({
    name: 'yup'
  });

• Use lower camelCase for everything else (e.g.- objects, functions, and instances).

  // bad
  var OBJEcttsssss = {};
  var this_is_my_object = {};
  var this-is-my-object = {};
  function c() {};
  var u = new user({
    name: 'Bob Parr'
  });
  
  // good
  var thisIsMyObject = {};
  function thisIsMyFunction() {};
  var user = new User({
    name: 'Bob Parr'
  });

• When saving a reference to this use self.

  // bad
  function() {
    var that = this;
    return function() {
      console.log(that);
    };
  }
  
  // good
  function() {
    var self = this;
    return function() {
      console.log(self);
    };
  }

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

  // bad
  var log = function(msg) {
    console.log(msg);
  };
  
  // good
  var log = function log(msg) {
    console.log(msg);
  };

  [⬆]

Accessors

• Accessor functions for properties are not required

• If you do make accessor functions, always prepend with get and set. Eg- getVal() and setVal('hello').

  // bad
  dragon.age();
  
  // good
  dragon.getAge();
  
  // bad
  dragon.age(25);
  
  // good
  dragon.setAge(25);

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

  // bad
  if (!dragon.age()) {
    return false;
  }
  
  // good
  if (!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');
  }
  
  // bad
  Jedi.prototype = {
    fight: function fight() {
      console.log('fighting');
    },
  
    block: function block() {
      console.log('blocking');
    }
  };
  
  // good
  Jedi.prototype.fight = function fight() {
    console.log('fighting');
  };
  
  Jedi.prototype.block = function block() {
    console.log('blocking');
  };

• Methods can return this to help with method chaining.

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

  [⬆]

jQuery

• Prefix jQuery object variables with a $.

  // bad
  var sidebar = $('.sidebar');
  
  // good
  var $sidebar = $('.sidebar');
  
  // bad
  var $primary_nav = $('#primary-nav');
  
  // good
  var $primaryNav = $('#primary-nav');

• If a jQuery lookup is performed more than once, cache the jQuery object.

  // bad
  function setSidebar() {
    $('.sidebar').hide();
  
    // ...stuff...
  
    $('.sidebar').css({
      'background-color': 'pink'
    });
  }
  
  // bad
  function getSideBarHeight() {
    var $sidebar = $('.sidebar');
  
    $sidebar.height();
  }
  
  // good
  function getSideBarHeight() {
    $('.sidebar').height();
  }
  
  // good
  function 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
  $('.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');

• Don't combine elements with a class or ID as part of the selector. If this results in selecting elements you don't want, you probably need to refactor your DOM.

  // bad
  $('form#new_user').submit();
  
  // bad
  $('li.user').hide();
  
  // good
  $('#new_user').submit();
  
  // good
  $('.user').hide();

  [⬆]

Modules

• We use the JavaScript Module Pattern, as defined by Ben Cherry. Read up on it here.

• Files should be named with camelCase, live in their appropriate folder (views/ utils/ etc), and match the name of the single export.

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

• Always return the original module object.

• If you need to reference any other global objects, pass it to the module's arguments to help provide faster lookups and prevent linting errors.

  /*jshint forin:true, noarg:true, noempty:true, eqeqeq:true, bitwise:true, strict:true, undef:true, unused:true, curly:true, browser:true, jquery:true, indent:2, maxerr:100 */
  
  /*
    @namespace viewName
    @memberOf haw.views
  */
  var haw = (function (module, $) {
    'use strict';
  
    // Module Namespace Extension
    var views = module.views = module.views || {};
    var viewName = views.viewName = views.viewName || {};
  
  
    // Private Variables
    var foo = "bar";
  
    // Private Method
    function privateMethod () {}
  
    // Public API
    viewName.publicMethod = function () {};
  
    // Return the extended module
    return module;
  
  }(haw || {}, jQuery));

  [⬆]

ECMAScript 5 Compatibility

• Refer to Kangax's ES5 compatibility table

[⬆]

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

Additional Articles

• JavaScript Scoping & Hoisting - Ben Cherry
• JavaScript Module Pattern: In-Depth - Ben Cherry
• Named function expressions demystified - Juriy Zaytsev
• Function Declarations vs. Function Expressions - Angus Croll
• Essential JavaScript Namespacing Patterns - Addy Osmani

[⬆]

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
• GeneralElectric: GeneralElectric/javascript
• GoodData: gooddata/gdc-js-style
• How About We: howaboutwe/javascript
• MinnPost: MinnPost/javascript
• ModCloth: modcloth/javascript
• National Geographic: natgeo/javascript
• Razorfish: razorfish/javascript-style-guide
• Shutterfly: shutterfly/javascript
• Userify: userify/javascript
• Zillow: zillow/javascript

Translation

This style guide is also available in other languages:

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

The JavaScript Style Guide Guide

• Reference

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.

[⬆]