JavaScript Style Guide

Freely inspired from Airbnb’s 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

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Objects

Use the literal syntax for object creation.

// bad
var models = new Object();

// good
var models = {};

Try not to use reserved words as keys. It won’t work in IE8. More info

// bad
var superman = {
		default: { clark: 'kent' },
		private: true
	};

// good
var superman = {
		defaults: { clark: 'kent' },
		hidden: true
	};

Use readable synonyms in place of reserved words.

// bad
var superman = {
		class: 'alien'
	};

// bad
var superman = {
		klass: 'alien'
	};

// good
var superman = {
		type: 'alien'
	};

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Arrays

Use the literal syntax for array creation

// bad
var families = new Array();

// good
var families = [];

If you don’t know the length of an array, use Array#push.

var spareNumbers = [];


// bad
spareNumbers[spareNumbers.length] = 'abracadabra';

// good
spareNumbers.push('abracadabra');

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

var len = numbers.length,
	numbersCopy = [],
	i;

// bad
for (i = 0; i < len; i++) {
  numbersCopy[i] = numbers[i];
}

// good
numbersCopy = numbers.slice();

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

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

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Strings

Use single quotes '' for strings

// bad
var fullName = "John Doe";

// good
var fullName = 'John Doe';

// bad
var fullName = "John " + user.lastName;

// good
var fullName = 'John ' + user.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

// 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 programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE: jsPerf.

var messages,
	length,
	items,
	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>';
}

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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
var 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.

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

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

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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 luke = {
		jedi: true,
		age: 28
	};

function getProp(prop) {
	return luke[prop];
}

var isJedi = getProp('jedi');

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Variables

Use a single var keyword to declare several variables.

// bad
var cardType = getCardType(cardNumber);
var activate = true;
var result = '';

// good
var cardType = getCardType(cardNumber),
	activate = true,
	result = '';

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.

// bad
var i, len, result,
	cardType = getCardType(cardNumber),
	activate = true;

// good
var cardType = getCardType(cardNumber),
	activate = true,
	result,
	len,
	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;
}

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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

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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.

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

// good
if (name) {
	// ...stuff...
}

// bad
if (collection.length > 0) {
	// ...stuff...
}

// good
if (collection.length) {
	// ...stuff...
}

For more information see Truth Equality and JavaScript by Angus Croll

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Blocks

Use braces with all multi-line blocks.

// bad
if (test)
	return false;

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

// bad
function() { return false; }

// good
function() {
	return false;
}

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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} 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. 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 getCardType(number) {
	console.log('fetching type...');
	// set the default type to 'no type'
	var type = type || 'no type';

	return type;
}

// good
function getCardType(number) {
	console.log('fetching type...');

	// set the default type to 'no type'
	var type = 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 buildRequest(options) {

	// FIXME: don't know if options is defined
	options = $.extend(true, options, defaultOptions);

	return options;
}

Use // TODO: to annotate solutions to problems

function buildRequest(options) {

	// TODO: should check if options is undefined
	options = $.extend(true, options, defaultOptions);

	return options;
}

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Whitespace

Place 1 space before the leading brace.

// bad
function error(){
	console.log('error');
}

// good
function error()∙{
	console.log('error');
}

// bad
monster.pub('auth.initApp',{
	app: self,
	callbackSuccess: function callbackSuccess() { /* ...stuff... */ }
});

// good
monster.pub('auth.initApp',∙{
	app: self,
	callbackSuccess: function callbackSuccess() { /* ...stuff... */ }
});

Set off operators with spaces.

// bad
var username=firstName+lastName;

// good
var username∙=∙firstName∙+∙lastName;

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);

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Commas

Leading commas: Nope.

// bad
var story = [
		once
	  , upon
	  , aTime
	];

// good
var story = [
		once,
		upon,
		aTime
	];

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

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

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 = [
		'Superman',
		'Batman'
	];

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Semicolons

Although not mandatory in javascript, every statement should be terminated with a semicolon.

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

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

// good (guards against the function becoming an argument when two files with IIFEs are concatenated)
;(function() {
	var name = 'Skywalker';
	return name;
})();

Type Casting & Coercion

Perform type coercion at the beginning of the statement.
Strings:

//  => discount = 9;

// bad
var totalDiscount = discount + '';

// good
var totalDiscount = '' + discount;

// bad
var totalDiscount = '' + discount + ' total discount';

// good
var totalDiscount = discount + ' total discount';

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

var inputValue = '4';

// bad
var val = new Number(inputValue);

// bad
var val = +inputValue;

// bad
var val = inputValue >> 0;

// bad
var val = parseInt(inputValue);

// good
var val = Number(inputValue);

// good
var val = parseInt(inputValue, 10);

Booleans:

var type = 0;

// bad
var hasType = new Boolean(type);

// good
var hasType = Boolean(type);

// good
var hasType = !!type;

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Naming Conventions

Avoid single letter names. Be descriptive with your naming.

// bad
function r() {
	// ...stuff...
}

// good
function request() {
	// ..stuff..
}

Use camelCase when naming objects, variables and functions, even for acronyms.

// bad
var formatted_user_data = {};
function ParseURL() {}
var n = {
		id: '+14151234568'
	};

// good
var formattedUserData = {};
function parseUrl() {}
var number = {
		id: '+14151234568'
	};

When saving a reference to this use self.

// bad
function() {
	var _this = this;
	return function() {
		console.log(_this);
	};
}

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

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

All our applications are defined as an object. By convention, every function defined at the root of the application should declare a variable self to reference the scope of the application (this).

{
	bindEvents: function() {
		var self = this;
		// Some code...
	}
}

The data returned by the APIs, does not follow the same conventions. Most properties will be lowercased, each word separated by an underscore. For obvious reasons, these properties must not be renamed.

{
	getUserName: function(accountId, userId) {
		var self = this;

		self.callApi({
			resource: 'user.get',
			data: {
				accountId: accountId,
				userId: userId
			},
			success: function(data) {
				var user = data.data;

				return user.first_name + ' ' + user.last_name;
			}
		});
	}
	/* ... */
}

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);
	};

Note: IE8 and below exhibit some quirks with named function expressions. See http://kangax.github.io/nfe/ for more info.

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jQuery

Prefix jQuery object variables with a $.

// bad
var sidebar = $('.sidebar');

// good
var $sidebar = $('.sidebar');

Cache jQuery lookups.

// bad
function setSidebar() {
	$('.sidebar').hide();

	// ...stuff...

	$('.sidebar').css({
		'background-color': 'pink'
	});
}

// good
function setSidebar() {
	var $sidebar = $('.sidebar');
	$sidebar.hide();

	// ...stuff...

	$sidebar.css({
		'background-color': 'pink'
	});
}

When accessing $(this) multiple times, reference it as a $this variable to call it only once.

// bad
$.each($appList, function(index, value) {
	var id = $(this).data('id');

	$(this).hide();

	// ...stuff..
});

// good
$.each($appList, function(index, value) {
	var $this = $(this),
		id = $this.data('id');

	$this.hide();

	// ...stuff..
});

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').hide();

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Lodash

Prefer using Lodash helpers when manipulating JavaScript objects (non-DOM elements).

var collection: {
		one: 1,
		two: 2,
		three: 3
	};

// bad
$.each(collection, function(index, value) {
	// ...stuff...
});

// good
_.each(collection, function(value, key, list) {
	// ...stuff...
});

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JavaScript Style Guide

Types

Objects

Arrays

Strings

Functions

Properties

Variables

Hoisting

Conditional Expressions & Equality

Blocks

Comments

Whitespace

Commas

Semicolons

Type Casting & Coercion

Naming Conventions

jQuery

Lodash

};

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