HowAboutWe JavaScript Style Guide

A mostly reasonable approach to JavaScript

Objects
Arrays
Strings
Functions
Properties
Variables
Conditional Expressions & Equality
Blocks
Comments
Whitespace
Leading Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Constructors
jQuery
Modules
ES5 Compatibility
Performance
Resources
In the Wild
Translation
The JavaScript Style Guide Guide
Contributors
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

[⬆]

Resources

Read This

Annotated ECMAScript 5.1

Other Styleguides

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

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)

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.

[⬆]

Name		Name	Last commit message	Last commit date
Latest commit History 135 Commits
linters/SublimeLinter		linters/SublimeLinter
README.md		README.md

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HowAboutWe JavaScript Style Guide

Table of Contents

Objects

Arrays

Strings

Functions

Properties

Variables

Conditional Expressions & Equality

Blocks

Comments

Whitespace

Leading Commas

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Constructors

jQuery

Modules

ECMAScript 5 Compatibility

Performance

Resources

In the Wild

Translation

The JavaScript Style Guide Guide

License

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