JavaScript Style Guide

A mostly reasonable approach to JavaScript.

Table of Contents generated with DocToc

Best Practices
Syntax
Resources
License

Best Practices

Semicolons`;`

Automatic Semicolon Insertion (ASI) is a complicated feature. But, writing a ; at the end of every line is painful. And the rules aren't that complex to learn. Don't bother with the semi-colons.

Style Checking

Tools like jscs and eslint can strictly enforce a code style. This makes it easy to ensure that developers can look at code they didn't write and more quickly understand what's going on. However, style-checking failures can be painful to deal with. Leave the implementation up to the team working the the project.

Generally, these work well in small projects. A sample .jscrc and .eslintrc is in this repo.

Linting

Linting is always a great idea. Don't use a linter that's super opinionated about how the code should be styled, like jslint. Instead, use something more lenient like jshint or eslint.

A few tips when using JSHint

Declare a .jshintignore file and include node_modules, bower_components, and the like
You should use a .eslintrc file to keep your rules together. See the one in this repo.

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

write all modules in the CommonJs style. We'll use browserify to turn it into a UMD module where necessary.
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
'use strict'

var previousFancyInput = global.FancyInput

function FancyInput(options){
  this.options = options || {}
}

FancyInput.noConflict = function noConflict(){
  global.FancyInput = previousFancyInput
  return FancyInput
}

module.exports = FancyInput

jQuery

Prefix jQuery object variables with a $.

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

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

Cache jQuery lookups.

// bad
function setSidebar(){
  $('.sidebar').addClass('hidden')

  // …stuff…

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

// good
function setSidebar(){
  var $sidebar = $('.sidebar')
  $sidebar.addClass('hidden')

  // …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').addClass('hidden')

// bad
$('.sidebar').find('ul').addClass('hidden')

// good
$('.sidebar ul').addClass('hidden')

// good
$('.sidebar > ul').addClass('hidden')

// good
$sidebar.find('ul')

ECMAScript 5 Compatibility [es5]

Refer to Kangax's ES5 compatibility table
Note that you're probably using a library that includes es5shim, this means that you can probably freely use es5 methods.

Testing

Yup.
```
function(){
  return true
}
```

`console` statements [console statements]

Preferably bake console statements into a service that can easily be disabled in production. Alternatively, don't ship any console.log printing statements to production distributions.

Comments

Comments aren't meant to explain what the code does. Good code is supposed to be self-explanatory. If you're thinking of writing a comment to explain what a piece of code does, chances are you need to change the code itself. The exception to that rule is explaining what a regular expression does. Good comments are supposed to explain why code does something that may not seem to have a clear-cut purpose.

Bad

// create the centered container
var p = $('<p/>')
p.center(div)
p.text('foo')

Good

var $container = $('<p/>')
  , contents = 'foo'

$container.center(parent)
$container.text(contents)

megaphone.on('data', function(value){
  // the megaphone periodically emits updates for container
  container.text(value)
})

var numeric = /\d+/ // one or more digits somewhere in the string
if (numeric.test(text)){
  console.log('so many numbers!')
}

Commenting out entire blocks of code should be avoided entirely, that's why you have version control systems in place!

Variable Naming

Variables must have meaningful names so that you don't have to resort to commenting what a piece of functionality does. Instead, try to be expressive while succinct, and use meaningful variable names.

Bad

function a (x, y, z) {
  return z * y / x
}
a(4, 2, 6);
// <- 3

Good

function ruleOfThree (had, got, have) {
  return have * got / had
}
ruleOfThree(4, 2, 6)
// <- 3

Polyfills

Where possible use the native browser implementation and include a polyfill that provides that behavior for unsupported browsers. This makes the code easier to work with and less involved in hackery to make things just work.

If you can't patch a piece of functionality with a polyfill, then wrap all uses of the patching code in a globally available method that is accessible from everywhere in the application.

Everyday Tricks

Use || to define a default value. If the left-hand value is [falsy][29] then the right-hand value will be used.

function a(value){
  const defaultValue = 33
  const used = value || defaultValue
}

Use .bind to partially-apply functions.

const sum = function sum(a, b){
    return a + b
  }
  , addSeven = sum.bind(null, 7)

addSeven(6)
// <- 13

Use Array.prototype.slice.call to cast array-like objects to true arrays. (Though, it's better to use rest parameters)

// tricky
const args = [].slice.call(arguments)

// better
const fn = (...args) => args

Use event emitters on all the things!

const emitter = new Backbone.Events()

$body.on('click', function onBodyClick(e){
  emitter.trigger('click', e.target)
})

emitter.on('click', function onEmitterClick(elem) {
  console.log(elem)
})

// simulate click
emitter.trigger('click', document.body)

Use Function() as a "no-op".

function onThingHappended(cb){
  setTimeout(cb || Function(), 2000)
}

Performance

Syntax

Types

Primitives: When you access a primitive type you work directly on its value
- string
- number
- boolean
- null
- undefined
```
const foo = 1
let 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

const foo = [1, 2]
const bar = foo

bar[0] = 9

console.log(foo[0], bar[0]) // => 9, 9

References

Use const for all of your references; avoid using var.

Why? This ensures that you can't reassign your references (mutation), which can lead to bugs and difficult to comprehend code.

```javascript
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

If you must mutate references, use let instead of var.

Why? let is block-scoped rather than function-scoped like var.

```javascript
// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}
```

Note that both let and const are block-scoped.

// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

Objects

Use the literal syntax for object creation.

// bad
const item = new Object()

// good
const item = {}

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

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

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

Use readable synonyms in place of reserved words.

// bad
const superman = {
  class: 'alien'
}

// bad
const superman = {
  klass: 'alien'
}

// good
const superman = {
  type: 'alien'
}

Use computed property names when creating objects with dynamic property names.

Why? They allow you to define all the properties of an object in one place.

function getKey(k) {
  return `a key named ${k}`
}

// bad
const obj = {
  id: 5,
  name: 'San Francisco'
}
obj[getKey('enabled')] = true

// good
const obj = {
  id: 5,
  name: 'San Francisco',
  [getKey('enabled')]: true
}

Use object method shorthand.

// bad
const atom = {
  value: 1,

  addValue: function addValue (value) {
    return atom.value + value
  }
}

// good
const atom = {
  value: 1,

  addValue(value) {
    return atom.value + value
  }
}

Use property value shorthand.

Why? It is shorter to write and descriptive.

const lukeSkywalker = 'Luke Skywalker'

// bad
const obj = {
  lukeSkywalker: lukeSkywalker
}

// good
const obj = {
  lukeSkywalker
}

Group your shorthand properties at the beginning of your object declaration.

Why? It's easier to tell which properties are using the shorthand.

const anakinSkywalker = 'Anakin Skywalker'
const lukeSkywalker = 'Luke Skywalker'

// bad
const obj = {
  episodeOne: 1,
  twoJedisWalkIntoACantina: 2,
  lukeSkywalker,
  episodeThree: 3,
  mayTheFourth: 4,
  anakinSkywalker
}

// good
const obj = {
  lukeSkywalker,
  anakinSkywalker,
  episodeOne: 1,
  twoJedisWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4
}

Arrays

Use the literal syntax for array creation

// bad
const items = new Array()

// good
const items = []

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

const someStack = []

// bad
someStack[someStack.length] = 'abracadabra'

// good
someStack.push('abracadabra')

Use array spreads ... to shallow copy arrays.

// bad
const len = items.length
const itemsCopy = []

for (let i = 0; i < len; i++) {
  itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

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

const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);

Destructuring

Use object destructuring when accessing and using multiple properties of an object.

Why? Destructuring saves you from creating temporary references for those properties.

```javascript
// bad
function getFullName(user) {
  const firstName = user.firstName
  const lastName = user.lastName

  return `${firstName} ${lastName}`
}

// good
function getFullName(obj) {
  const { firstName, lastName } = obj
  return `${firstName} ${lastName}`
}

// best
function getFullName({ firstName, lastName }) {
  return `${firstName} ${lastName}`
}
```

Use array destructuring.

const arr = [1, 2, 3, 4]

// bad
const first = arr[0]
const second = arr[1]

// good
const [first, second] = arr

Use object destructuring for multiple return values, not array destructuring.

Why? You can add new properties over time or change the order of things without breaking call sites.

```javascript
// bad
function processInput(input) {
  // then a miracle occurs
  return [left, right, top, bottom]
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input)

// good
function processInput(input) {
  // then a miracle occurs
  return { left, right, top, bottom }
}

// the caller selects only the data they need
const { left, right } = processInput(input)
```

Strings

Use single quotes '' for strings

// bad
const name = "Bob Parr"

// good
const name = 'Bob Parr'

// bad
const fullName = "Bob " + this.lastName

// good
const fullName = `Bob ${this.lastName}`

Strings longer than 80 characters should use string templates.

// bad
const 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
const 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
const 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
const 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 strings, use template strings instead of concatenation.

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

```javascript
// bad
function sayHi(name) {
  return 'How are you, ' + name + '?'
}

// bad
function sayHi(name) {
  return ['How are you, ', name, '?'].join()
}

// good
function sayHi(name) {
  return `How are you, ${name}?`
}
```

Functions

Use function declarations instead of function expressions.

Why? Function declarations are named, so they're easier to identify in call stacks. Function declarations that aren't assigned to a variable are hoisted and can lead to expected bugs.

// bad
const foo = function () { }

// bad
function foo() { }

// good
const foo = function foo () { }

Function expressions:

// immediately-invoked function expression (IIFE)
(() => {
  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. It's also slower if in a loop or function call because the function has to be created on each call.

// bad
for (let i = 0, l = 10; i < l; i++){
  const square = (num) => num * num
  console.log(square(i))
}

// bad
[1, 2, 3].forEach((num) => {
  const square = (num) => num * num
  console.log(square(i))
})

// good
const square = (num) => num * num
for (let i = 0, l = 10; i < l; i++){
  console.log(square(i))
}

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
let test;
if (currentUser) {
  test = () => {
    console.log('Yup.')
  }
}

Never name a parameter arguments, this will take precedence over the arguments object that is given to every function scope. It's also more efficient to use rest parameters.

Why? ... is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like like arguments.

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

// bad
const concatenateAll = function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('')
}

// good
const yup = function yup(name, options, ...args) {
  return args.join('')
}

Use default parameter syntax rather than mutating function arguments.

// really bad
const handleThings = function handleThings(opts) {
  // No! We shouldn't mutate function arguments.
  // Double bad: if opts is falsey it'll be set to an object which may
  // be what you want but it can introduce subtle bugs.
  opts = opts || {}
  // ...
}

// still bad
const handleThings = function handleThings(opts) {
  if (opts === void 0) {
    opts = {}
  }
  // ...
}

// good
const handleThings = function handleThings(opts = {}) {
  // ...
}

Never put spaces immediately after an opening parens or immediately before a closing parens.

// bad
var bad = function ( badness, devils, misery ) {

}

// good
var good = function good (happiness, heaven, cheesecake){

}

Avoid creating functions that accept more than 3 arguments. You should probably pass a single options argument instead.

// bad
var manyThings = function manyThings (first, second, moar, evenMoar){

}

// good
var manyThings = function manyThings (options){
  /*
  options.first
  options.second
  etc…
  */
}

When you must use function expressions (as when passing an anonymous function), use arrow function notation.

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.

// bad
[1, 2, 3].map(function square (x) {
  return x * x
})

// good
[1, 2, 3].map((x) => x * x)

// good
const square = function square (x) {
  return x * x
}
[1, 2, 3].map(square)

If the function body fits on one line, feel free to omit the braces and use implicit return. Otherwise, add the braces and use a return statement.

Why? Syntactic sugar. It reads well when multiple functions are chained together.

Why not? If you plan on returning an object.

// good
[1, 2, 3].map((x) => x * x)

// good
[1, 2, 3].map((x) => {
  return { number: x }
})

Always use parentheses around the arguments. Omitting the parentheses makes the functions less readable and only works for single arguments.

Why? These declarations read better with parentheses. They are also required when you have multiple parameters so this enforces consistency.

// bad
[1, 2, 3].map(x => x * x)

// good
[1, 2, 3].map((x) => x * x)

Constructors

Always use class. Avoid manipulating prototype directly.

Why? class syntax is more concise and easier to reason about.

```javascript
// bad
function Queue(contents = []) {
  this._queue = [...contents]
}
Queue.prototype.pop = function() {
  const value = this._queue[0]
  this._queue.splice(0, 1)
  return value
}


// good
class Queue {
  constructor(contents = []) {
    this._queue = [...contents]
  }
  , pop() {
    const value = this._queue[0]
    this._queue.splice(0, 1)
    return value
  }
}
```

Use extends for inheritance.

Why? It is a built-in way to inherit prototype functionality without breaking instanceof. Though, you probably want to use composition instead on inheritance.

```javascript
// bad
const inherits = require('inherits')
function PeekableQueue(contents) {
  Queue.apply(this, contents)
}
inherits(PeekableQueue, Queue)
PeekableQueue.prototype.peek = function() {
  return this._queue[0]
}

// good
class PeekableQueue extends Queue {
  peek() {
    return this._queue[0]
  }
}
```

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
}

const luke = new Jedi()
luke.jump() // => true
luke.setHeight(20) // => undefined

// good
class Jedi {
  jump() {
    this.jumping = true
    return this
  }

  , setHeight(height) {
    this.height = height
    return this
  }
}

const 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.

class Jedi {
  contructor(options = {name: 'no name'}) {
    this.name = options.name
  }

  , getName() {
    return this.name
  }

  , toString() {
    return `Jedi - ${this.getName()}`
  }
}

Iterators and Generators

Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of.

Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side-effects.

```javascript
const numbers = [1, 2, 3, 4, 5]

// bad
let sum = 0
for (let num of numbers) {
  sum += num
}

sum === 15

// good
let sum = 0
numbers.forEach((num) => sum += num)
sum === 15

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0)
sum === 15
```

Don't use generators.

Why? They don't transpile well to ES5.

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 const 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
const superPower = new SuperPower()

Use one const declaration per variable.

Why? It's easier to add new variable declarations this way, and you never have to worry about a , or introducing punctuation-only diffs.

// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z'

// bad
// (compare to above, and try to spot the mistake)
const items = getItems(),
    goSportsTeam = true
    dragonball = 'z'

// good
const items = getItems()
const goSportsTeam = true
const dragonball = 'z'

Group all your consts and then group all your lets.

Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

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

// bad
let i
const items = getItems()
let dragonball
const goSportsTeam = true
let len

// good
const goSportsTeam = true
const items = getItems()
let dragonball
let i
let length

Assign variables where you need them, but place them in a reasonable place.

Why? let and const are block scoped and not function scoped.

// good
const up = function up () {
  test()
  console.log('doing stuff..')

  //..other stuff..

  const name = getName();

  if (name === 'test') {
    return false
  }

  return name
}

// bad
const up = function up () {
  const name = getName()

  if (!arguments.length) {
    return false
  }

  return true
}

// good
const up = function up () {
  if (!arguments.length) {
    return false
  }

  const name = getName()

  return true
}

Hoisting

var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

// 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() {
  let declaredButNotAssigned
  console.log(declaredButNotAssigned) // => undefined
  declaredButNotAssigned = true
}

// using const and let
function example() {
  console.log(declaredButNotAssigned) // => throws a ReferenceError
  console.log(typeof declaredButNotAssigned) // => throws a ReferenceError
  const 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

Comparison Operators & Equality [comparison-operators-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

Blocks

Single line blocks don't require braces, but should be easy to read. If your single line extends more than 80 characters, break into two lines, and indent the contents of the block.

// bad
if (test){
  return false
}

// bad
if (test)
  return false

// good
if (test) return false


// bad
if (test && test.arg === doSomethingWith(5) && puppies() === 'cute') ciderHouse = true

// good
if (test && test.arg === doSomethingWith(5) && puppies() === 'cute') {
  ciderHouse = true
}

// bad
function(){ return false }

// good
const fn = function fn (){
  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 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
}

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

  ```javascript
  // bad
  function(){
  ∙∙∙∙const name
  }

  // bad
  function(){
  ∙const name
  }

  // good
  function(){
  ∙∙const name
  }
  ```

- Place no spaces before the leading brace.

  ```javascript
  // 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'
  })
  ```

- Set off operators with spaces.

  ```javascript
  // bad
  const x=y+5

  // good
  const x = y + 5
  ```

- Place an empty newline at the end of the file.

  ```javascript
  // bad
  (function(global){
    // …stuff…
  })(this)
  ```

  ```javascript
  // good
  (function(global){
    // …stuff…
  })(this)

  ```

- Use indentation when making long method chains.

  ```javascript
  // bad
  $('#items').find('.selected').highlight().end().find('.open').updateCount()

  // good
  $('#items')
    .find('.selected')
      .highlight()
      .end()
    .find('.open')
      .updateCount()

  // bad
  const 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
  const 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)
  ```

- Leave a blank line after blocks and before the next statement

  ```javascript
  // bad
  if (foo) {
    return bar
  }
  return baz

  // good
  if (foo) {
    return bar
  }

  return baz

  // bad
  const obj = {
    foo: function() {
    }
    , bar: function() {
    }
  }
  return obj

  // good
  const obj = {
    foo: function() {
    }

    , bar: function() {
    }
  }

  return obj
  ```

### Commas

- Leading commas: **Hell yes.**
- [Read why comma first is better](https://gist.github.com/isaacs/357981/)

  ```javascript
  // bad
  const once,
      upon,
      aTime

  // good
  const once
    , upon
    , aTime

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

  // good
  const 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](http://es5.github.io/#D)):

> 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.

  ```javascript
  // bad
  const hero = {
    firstName: 'Kevin'
    , lastName: 'Flynn'
    ,
  }

  const heroes = [
    'Batman'
    , 'Superman'
    ,
  ]

  // good
  const hero = {
    firstName: 'Kevin'
    , lastName: 'Flynn'
  }

  const heroes = [
    'Batman'
    , 'Superman'
  ]
  ```



### Semicolons

- **NO.**
- ASI means that you almost never need semicolons. Leave them out and live free! JsHint will catch nearly any error you might make.

  ```javascript
  // bad
  (function(){
    const name = 'Skywalker';
    return name;
  })();

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

  // good
  const a

  ;(function(){
    const name = 'Skywalker'
    return name
  })()
  ```

### Type Casting & Coercion

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

  ```javascript
  //  => this.reviewScore = 9

  // bad
  const totalScore = this.reviewScore + ''

  // good
  const totalScore = '' + this.reviewScore

  // bad
  const totalScore = '' + this.reviewScore + ' total score'

  // good
  const totalScore = this.reviewScore + ' total score'
  ```

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

  ```javascript
  const inputValue = '4'

  // bad
  const val = new Number(inputValue)

  // bad
  const val = +inputValue

  // bad
  const val = inputValue >> 0

  // bad
  const val = parseInt(inputValue)

  // good
  const val = Number(inputValue)

  // better
  const 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](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.
- **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109)

  ```javascript
  // 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:

  ```javascript
  var age = 0

  // bad
  var hasAge = new Boolean(age)

  // good
  var hasAge = Boolean(age)

  // better
  var hasAge = !!age
  ```



### Naming Conventions

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

  ```javascript
  // bad
  function q(){
    // …stuff…
  }

  // good
  function query(){
    // …stuff…
  }
  ```

- Use camelCase when naming objects, functions, and instances

  ```javascript
  // bad
  const OBJEcttsssss = {}
  const this_is_my_object = {}
  function c(){}
  const u = new user({
    name: 'Bob Parr'
  })

  // good
  const thisIsMyObject = {}
  function thisIsMyFunction(){}
  const user = new User({
    name: 'Bob Parr'
  })
  ```

- Use PascalCase when naming constructors or classes

  ```javascript
  // bad
  function user(options){
    this.name = options.name
  }

  const bad = new user({
    name: 'nope'
  })

  // good
  function User(options){
    this.name = options.name
  }

  const good = new User({
    name: 'yup'
  })
  ```

- Use a leading underscore `_` when naming private properties

  ```javascript
  // bad
  this.__firstName__ = 'Panda'
  this.firstName_ = 'Panda'

  // good
  this._firstName = 'Panda'
  ```

- When saving a reference to `this` use `self`. Avoid using self unless absolutely necessary. Pefer `_.bind()`

  ```javascript
  // bad
  function(){
    const that = this
    return function(){
      console.log(that)
    }
  }

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

  // bad
  function(){
    const self = this
    self.thing = true
  }

  // good
  function(){
    const self = this
    this.thing = true
    return function(){
      console.log(self)
    }
  }
  ```

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

  ```javascript
  // bad
  const log = function(msg){
    console.log(msg)
  }

  // good
  const 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')

  ```javascript
  // bad
  dragon.age()

  // good
  dragon.getAge()

  // bad
  dragon.age(25)

  // good
  dragon.setAge(25)
  ```

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

  ```javascript
  // bad
  if (!dragon.age()){
    return false
  }

  // good
  if (!dragon.hasAge()){
    return false
  }
  ```

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

  ```javascript
  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]
  }
  ```

### Accessors

- Accessor functions for properties are not required.
- If you do make accessor functions use getVal() and setVal('hello').

  ```javascript
  // bad
  dragon.age()

  // good
  dragon.getAge()

  // bad
  dragon.age(25)

  // good
  dragon.setAge(25)
  ```

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

  ```javascript
  // bad
  if (!dragon.age()) {
    return false
  }

  // good
  if (!dragon.hasAge()) {
    return false
  }
  ```

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

  ```javascript
  class Jedi {
    constructor(options = {}) {
      const lightsaber = options.lightsaber || 'blue'
      this.set('lightsaber', lightsaber)
    }

    set(key, val) {
      this[key] = val
    }

    get(key) {
      return this[key]
    }
  }
  ```

### 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:

  ```javascript
  // bad
  $(this).trigger('listingUpdated', listing.id)

  ...

  $(this).on('listingUpdated', function(e, listingId) {
    // do something with listingId
  })
  ```

  prefer:

  ```javascript
  // good
  $(this).trigger('listingUpdated', { listingId : listing.id })

  ...

  $(this).on('listingUpdated', function(e, data) {
    // do something with data.listingId
  })
  ```


### jQuery

- Prefix jQuery object variables with a `$`.

  ```javascript
  // bad
  const sidebar = $('.sidebar')

  // good
  const $sidebar = $('.sidebar')
  ```

- Cache jQuery lookups.

  ```javascript
  // bad
  function setSidebar() {
    $('.sidebar').addClass('hidden')

    // ...stuff...

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

  // good
  function setSidebar() {
    const $sidebar = $('.sidebar')
    $sidebar.addClass('hidden')

    // ...stuff...

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

- For DOM queries use Cascading `$('.sidebar ul')` or parent > child `$('.sidebar > ul')`. [jsPerf](http://jsperf.com/jquery-find-vs-context-sel/16)
- Use `find` with scoped jQuery object queries.

  ```javascript
  // bad
  $('ul', '.sidebar').addClass('hidden')

  // bad
  $('.sidebar').find('ul').addClass('hidden')

  // good
  $('.sidebar ul').addClass('hidden')

  // good
  $('.sidebar > ul').addClass('hidden')

  // good
  $sidebar.find('ul').addClass('hidden')
  ```


## Resources


### Read This [read-this]

- [Annotated ECMAScript 5.1](http://es5.github.com/)

### Performance

- [On Layout & Web Performance](http://kellegous.com/j/2013/01/26/layout-performance/)
- [String vs Array Concat](http://jsperf.com/string-vs-array-concat/2)
- [Try/Catch Cost In a Loop](http://jsperf.com/try-catch-in-loop-cost)
- [Bang Function](http://jsperf.com/bang-function)
- [jQuery Find vs Context, Selector](http://jsperf.com/jquery-find-vs-context-sel/13)
- [innerHTML vs textContent for script text](http://jsperf.com/innerhtml-vs-textcontent-for-script-text)
- [Long String Concatenation](http://jsperf.com/ya-string-concat)

### Other Styleguides [other-styleguides]

- [Google JavaScript Style Guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml)
- [jQuery Core Style Guidelines](http://docs.jquery.com/JQuery_Core_Style_Guidelines)
- [Principles of Writing Consistent, Idiomatic JavaScript](https://github.com/rwldrn/idiomatic.js/)
- This style guide is based on [AirBnB's style guide](https://github.com/airbnb/javascript)

### Other Styles [other-styles]

- [Naming this in nested functions](https://gist.github.com/4135065) - Christian Johansen
- [Conditional Callbacks](https://github.com/airbnb/javascript/issues/52)
- [Popular JavaScript Coding Conventions on Github](http://sideeffect.kr/popularconvention/#javascript)

### Further Reading [further-readingj]

- [Understanding JavaScript Closures](http://javascriptweblog.wordpress.com/2010/10/25/understanding-javascript-closures/) - Angus Croll
- [Basic JavaScript for the impatient programmer](http://www.2ality.com/2013/06/basic-javascript.html) - Dr. Axel Rauschmayer

### Books

- [JavaScript: The Good Parts](http://www.amazon.com/JavaScript-Good-Parts-Douglas-Crockford/dp/0596517742) - Douglas Crockford
- [JavaScript Patterns](http://www.amazon.com/JavaScript-Patterns-Stoyan-Stefanov/dp/0596806752) - Stoyan Stefanov
- [Pro JavaScript Design Patterns](http://www.amazon.com/JavaScript-Design-Patterns-Recipes-Problem-Solution/dp/159059908X)  - Ross Harmes and Dustin Diaz
- [High Performance Web Sites: Essential Knowledge for Front-End Engineers](http://www.amazon.com/High-Performance-Web-Sites-Essential/dp/0596529309) - Steve Souders
- [Maintainable JavaScript](http://www.amazon.com/Maintainable-JavaScript-Nicholas-C-Zakas/dp/1449327680) - Nicholas C. Zakas
- [JavaScript Web Applications](http://www.amazon.com/JavaScript-Web-Applications-Alex-MacCaw/dp/144930351X) - Alex MacCaw
- [Pro JavaScript Techniques](http://www.amazon.com/Pro-JavaScript-Techniques-John-Resig/dp/1590597273) - John Resig
- [Smashing Node.js: JavaScript Everywhere](http://www.amazon.com/Smashing-Node-js-JavaScript-Everywhere-Magazine/dp/1119962595) - Guillermo Rauch
- [Secrets of the JavaScript Ninja](http://www.amazon.com/Secrets-JavaScript-Ninja-John-Resig/dp/193398869X) - John Resig and Bear Bibeault
- [Human JavaScript](http://humanjavascript.com/) - Henrik Joreteg
- [Superhero.js](http://superherojs.com/) - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
- [JSBooks](http://jsbooks.revolunet.com/)

### Blogs

- [DailyJS](http://dailyjs.com/)
- [JavaScript Weekly](http://javascriptweekly.com/)
- [JavaScript, JavaScript…](http://javascriptweblog.wordpress.com/)
- [Bocoup Weblog](http://weblog.bocoup.com/)
- [Adequately Good](http://www.adequatelygood.com/)
- [NCZOnline](http://www.nczonline.net/)
- [Perfection Kills](http://perfectionkills.com/)
- [Ben Alman](http://benalman.com/)
- [Dmitry Baranovskiy](http://dmitry.baranovskiy.com/)
- [Dustin Diaz](http://dustindiaz.com/)
- [nettuts](http://net.tutsplus.com/?s=javascript)


## License

Based on the style guides from [Airbnb](https://github.com/airbnb/javascript) and [bevacqua/js](https://github.com/bevacqua/js)

(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.

**[[⬆]](#TOC)**

Name		Name	Last commit message	Last commit date
Latest commit History 17 Commits
.editorconfig		.editorconfig
.eslintrc		.eslintrc
.gitignore		.gitignore
.jscsrc		.jscsrc
.jshintrc		.jshintrc
.npmignore		.npmignore
.npmrc		.npmrc
.travis.yml		.travis.yml
CHANGELOG.md		CHANGELOG.md
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
README.md		README.md
index.js		index.js
package.json		package.json
scripts.sh		scripts.sh

License

joeybaker/styleguide-js

Folders and files

Latest commit

History

Repository files navigation

JavaScript Style Guide

Best Practices

Semicolons;

Style Checking

Linting

A few tips when using JSHint

Events

Modules

jQuery

ECMAScript 5 Compatibility [es5]

Testing

console statements [console statements]

Comments

Bad

Good

Variable Naming

Bad

Good

Polyfills

Everyday Tricks

Performance

Syntax

Types

References

Objects

Arrays

Destructuring

Strings

Functions

Constructors

Iterators and Generators

Properties

Variables

Hoisting

Comparison Operators & Equality [comparison-operators-equality]

Blocks

Comments

About

Resources

License

Stars

Watchers

Forks

Languages

Semicolons`;`

`console` statements [console statements]