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JavaScript Style Guide

A mostly reasonable approach to JavaScript

Types
Objects
Arrays
Strings
Functions
Properties
Variables
Hoisting
Conditional Expressions & Equality
Blocks
Comments
Whitespace
Leading Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Modules
Constructors and Classes
Accessors
jQuery
ES5 Compatibility
Performance
Resources
About
License

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.

// 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 precendence 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 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.
```
// bad
superPower = new SuperPower();

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

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

// bad
var items = getItems();
var goSportsTeam = true;
var dragonball = 'z';

// good
var 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.

// bad
var i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
var i, items = getItems(),
    dragonball,
    goSportsTeam = true,
    len;

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

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;
}

[⬆]

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 interpretor 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 expression 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 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...
}

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

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

For more information see Truth Equality and JavaScript by Angus Croll

[⬆]

Blocks

Use braces with all multi-line blocks.

// bad
if (test)
    return false;

// good
if (test) return false;

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

// bad
function() { return false; }

// good
function() {
    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> 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 emptyline 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;
}

[⬆]

Whitespace

Use soft tabs set to 4 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'
});

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();

Use indentation when making long method calls. Indent once for every nested method call.

// bad
self._actionQueue.unshift(self._createExecution(actionConfig.dependencies[i], params, cancelationToken, false));

// good
self._actionQueue
    .unshift(
        self._createExecution(
            actionConfig.dependencies[i],
            params,
            cancelationToken,
            false));

[⬆]

Leading Commas

Nope.

// bad
var once
  , upon
  , aTime;

// good
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;
})();

[⬆]

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';

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;

// bad
var val = inputValue >> 0;

// bad
var val = parseInt(inputValue);

// good
var val = Number(inputValue);

// good
var val = 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.
 */
var val = inputValue >> 0;

Booleans:

var age = 0;

// bad
var hasAge = new Boolean(age);

// good
var hasAge = Boolean(age);

// good
var hasAge = !!age;

[⬆]

Naming Conventions

Avoid single letter names. Be descriptive with your naming.

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

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

Use camelCase when naming 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'
});

Use PascalCase when naming constructors or classes

// 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 a leading underscore _ when naming private properties

// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';

// good
this._firstName = 'Panda';

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);
};

[⬆]

Modules

All modules should follow the AMD pattern.
Each module should export a constructor function or an instance of an object for 'singleton' modules.
Every module should be in its own file with no other modules inside. The 'main' module is an exception.
The file should be named with underscore separated lowercased letters that match the name of the constructor function.

Always declare 'use strict'; at the top module function.

// view_models/user_vm.js

define([
    //libs
    'jquery'
], function ($, ko, Class) {
    'use strict';

    var MyModuleConstructor = function () {};
    return MyModuleConstructor;
});

[⬆]

Constructors and Classes

As javascript does not provide a class system we create a convention based class system taken from BackboneJS 'Class.extend'.

Always use 'Class.extend' to create constructor functions for classes.

// bad
function Jedi() {
    console.log('new jedi');
}

Jedi.prototype.fight = function fight() {
    console.log('fighting');
};

Jedi.prototype.block = function block() {
    console.log('blocking');
};

// good
var Jedi = Class.extend({
    constructor: function () {
        console.log('new jedi');
    },
    fight: function fight() {
        console.log('fighting');
    },
    block: function block() {
        console.log('blocking');
    }
});

Always use a saved reference to 'this' named 'self' when accessing it inside 'Class' methods.

// bad
var Jedi = Class.extend({
    constructor: function () {
        this._height = height;
    }
});

// good
var Jedi = Class.extend({
    constructor: function () {
        var self = this;

        self._height = height;
    }
});

Always initialize all public/private properties in the constructor for easier maintainance and performace reasons Initialize all object members in constructor functions.

// bad
var Jedi = Class.extend({
    constructor: function () {
    },
    height: function(height) {
        var self = this;

        self._height = height;
    }
});

// good
var Jedi = Class.extend({
    constructor: function () {
        var self = this;

        self._height = undefined;
    },
    height: function(height) {
        var self = this;

        self._height = height;
    }
});

[⬆]

Accessors

Accessor functions for private properties are not required.
Always create accessor functions for public properties.
If you do make accessor functions use a single function - val() and val('newval') instead of getVal() and setVal('hello').
```
// bad
dragon.getAge();

// good
dragon.age();

// bad
dragon.setAge(25);

// good
dragon.age(25);
```

Use 'ko.observable()' and 'ko.observableArray()' to create accessor functions for properties. Its easier that way.

var Dragon = Class.extend({
    constructor: function () {
        var self = this;

        self.age = ko.observable();
    }
});

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.

var Jedi = Class.extend({
    constructor: function (options) {
        var self = this,
            lightsaber;
        
        options || (options = {});
        lightsaber = options.lightsaber || 'blue';

        self.set('lightsaber', lightsaber);
    },
    set: function(key, val) {
        this[key] = val;
    },
    get: function(key) {
        return this[key];
    }
});

[⬆]

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'
    });
}

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');