migme JavaScript Style Guide

migme's approach to JavaScript. Semi-based on Airbnb's Style Guide and Standard

Types
References
Objects
Arrays
Destructuring
Strings
Functions
Arrow Functions
Constructors
Modules
Iterators and Generators
Properties
Variables
Hoisting
Comparison Operators & Equality
Blocks
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Events
jQuery
ECMAScript 5 Compatibility
ECMAScript 6 Styles
Testing
Performance
Resources
License

Types

1.1 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 = 42
 
console.log(foo, bar) // => 1, 42
```

1.2 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] = 42
 
console.log(foo[0], bar[0]) // => 42, 42

References

2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

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

// bad
var a = 1
var b = 2
 
// good
const a = 1
const b = 2

2.2 If you must reassign references, use let instead of var. eslint: no-var jscs: disallowVar

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

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

2.3 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

3.1 Use the literal syntax for object creation. eslint: no-new-object

// bad
const item = new Object()
 
// good
const item = {}

3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs: disallowIdentifierNames

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

3.3 Use readable synonyms in place of reserved words. jscs: disallowIdentifierNames

// bad
const superman = {
  class: 'alien',
}
 
// bad
const superman = {
  klass: 'alien',
}
 
// good
const superman = {
  type: 'alien',
}

3.4 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,
}

3.5 Use object method shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

// bad
const atom = {
  value: 1,
 
  addValue: function (value) {
    return atom.value + value
  },
}
 
// good
const atom = {
  value: 1,
 
  addValue (value) {
    return atom.value + value
  },
}

3.6 Use property value shorthand. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

Why? It is shorter to write and descriptive.

const lukeSkywalker = 'Luke Skywalker'
 
// bad
const obj = {
  lukeSkywalker: lukeSkywalker,
}
 
// good
const obj = {
  lukeSkywalker,
}

3.7 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,
  twoJediWalkIntoACantina: 2,
  lukeSkywalker,
  episodeThree: 3,
  mayTheFourth: 4,
  anakinSkywalker,
}
 
// good
const obj = {
  lukeSkywalker,
  anakinSkywalker,
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4,
}

3.8 Only quote properties that are invalid identifiers. eslint: quote-props jscs: disallowQuotedKeysInObjects

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimised by many JS engines.

// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
}
 
// good
const good = {
  foo: 3,
  bar: 4,
  'data-blah': 5,
}

Arrays

4.1 Use the literal syntax for array creation. eslint: no-array-constructor

// bad
const items = new Array()
 
// good
const items = []

4.2 Use Array#push instead of direct assignment to add items to an array.

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

4.3 Use array spreads ... to copy arrays.

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

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

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

4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint: array-callback-return

// good
[1, 2, 3].map((x) => {
  const y = x + 1
  return x * y
})
 
// good
[1, 2, 3].map(x => x + 1)
 
// bad
const flat = {}
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item)
  flat[index] = memo.concat(item)
})
 
// good
const flat = {}
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item)
  flat[index] = flatten
  return flatten
})
 
// bad
inbox.filter((msg) => {
  const { subject, author } = msg
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee'
  } else {
    return false
  }
})
 
// good
inbox.filter((msg) => {
  const { subject, author } = msg
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee'
  }
 
  return false
})

Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring

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

// bad
function getFullName (user) {
  const firstName = user.firstName
  const lastName = user.lastName
 
  return `${firstName} ${lastName}`
}
 
// good
function getFullName (user) {
  const { firstName, lastName } = user
  return `${firstName} ${lastName}`
}
 
// best
function getFullName ({ firstName, lastName }) {
  return `${firstName} ${lastName}`
}

5.2 Use array destructuring. jscs: requireArrayDestructuring

const arr = [1, 2, 3, 4]
 
// bad
const first = arr[0]
const second = arr[1]
 
// good
const [first, second] = arr

5.3 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.

// 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

6.1 Use single quotes '' for strings. eslint: quotes jscs: validateQuoteMarks

// bad
const name = "Malcolm Reynolds"
 
// good
const name = 'Malcolm Reynolds'

6.2 Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.

6.3 Note: If overused, long strings with concatenation could impact performance. jsPerf & Discussion.

// 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.'

6.4 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings

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

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

6.5 Never use eval() on a string, it opens too many vulnerabilities.

Functions

7.1 Use function declarations instead of function expressions. jscs: requireFunctionDeclarations

Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.
```
// bad
const foo = function () {
}
 
// good
function foo () {
}
```
7.2 Immediately invoked function expressions: eslint: wrap-iife jscs: requireParenthesesAroundIIFE

Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
```
// immediately-invoked function expression (IIFE)
(function () {
  console.log('Welcome to the Internet. Please follow me.')
}())
```
7.3 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. eslint: no-loop-func

7.4 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.')
  }
}

7.5 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...
}

7.6 Never use arguments, opt to use rest syntax ... instead. prefer-rest-params

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

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

7.7 Use default parameter syntax rather than mutating function arguments.

// really bad
function handleThings (opts) {
  // No! We shouldn't mutate function arguments.
  // Double bad: if opts is falsy it'll be set to an object which may
  // be what you want but it can introduce subtle bugs.
  opts = opts || {}
  // ...
}
 
// still bad
function handleThings (opts) {
  if (opts === void 0) {
    opts = {}
  }
  // ...
}
 
// good
function handleThings (opts = {}) {
  // ...
}

7.8 Avoid side effects with default parameters.

Why? They are confusing to reason about.

var b = 1
// bad
function count (a = b++) {
  console.log(a)
}
count()  // 1
count()  // 2
count(3) // 3
count()  // 3

7.9 Always put default parameters last.

// bad
function handleThings (opts = {}, name) {
  // ...
}
 
// good
function handleThings (name, opts = {}) {
  // ...
}

7.10 Never use the Function constructor to create a new function.

Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
```
// bad
var add = new Function('a', 'b', 'return a + b')
 
// still bad
var subtract = Function('a', 'b', 'return a - b')
```

7.11 Spacing in a function signature.

Why? Consistency is good.

// bad
const f = function(){}
const g = function (){}
const h = function() {}
 
// good
const x = function () {}
const y = function a () {}

7.12 Never mutate parameters. eslint: no-param-reassign

Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

// bad
function f1 (obj) {
  obj.key = 1
}
 
// good
function f2 (obj) {
  const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1
}

7.13 Never reassign parameters. eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimisation issues, especially in V8.

// bad
function f1 (a) {
  a = 1
}
 
function f2 (a) {
  if (!a) { a = 1 }
}
 
// good
function f3 (a) {
  const b = a || 1
}
 
function f4 (a = 1) {
}

Arrow Functions

8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

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 (x) {
  const y = x + 1
  return x * y
})
 
// good
[1, 2, 3].map((x) => {
  const y = x + 1
  return x * y
})
```

8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

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

Why not? If you plan on returning an object.

// bad
[1, 2, 3].map(number => {
  const nextNumber = number + 1
  `A string containing the ${nextNumber}.`
})
 
// good
[1, 2, 3].map(number => `A string containing the ${number}.`)
 
// good
[1, 2, 3].map((number) => {
  const nextNumber = number + 1
  return `A string containing the ${nextNumber}.`
})

8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

Why? It shows clearly where the function starts and ends.

// bad
[1, 2, 3].map(number => 'As time went by, the string containing the ' +
  `${number} became much longer. So we needed to break it over multiple ` +
  'lines.'
)
 
// good
[1, 2, 3].map(number => (
  `As time went by, the string containing the ${number} became much ` +
  'longer. So we needed to break it over multiple lines.'
))

8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

Why? Less visual clutter.

// bad
[1, 2, 3].map((x) => x * x)
 
// good
[1, 2, 3].map(x => x * x)
 
// good
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we’ve broken it ` +
  'over multiple lines!'
))
 
// bad
[1, 2, 3].map(x => {
  const y = x + 1
  return x * y
})
 
// good
[1, 2, 3].map((x) => {
  const y = x + 1
  return x * y
})

8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

// bad
const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize
 
// bad
const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize
 
// good
const itemHeight = item => { return item.height > 256 ? item.largeSize : item.smallSize }

Constructors

9.1 Always use class. Avoid manipulating prototype directly.

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

// 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
  }
}

9.2 Use extends for inheritance.

Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

// 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]
  }
}

9.3 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)

9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

class Jedi {
  constructor (options = {}) {
    this.name = options.name || 'no name'
  }
 
  getName () {
    return this.name
  }
 
  toString () {
    return `Jedi - ${this.getName()}`
  }
}

9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. no-useless-constructor

// bad
class Jedi {
  constructor () {}
 
  getName () {
    return this.name
  }
}
 
// bad
class Rey extends Jedi {
  constructor (...args) {
    super (...args)
  }
}
 
// good
class Rey extends Jedi {
  constructor (...args) {
    super (...args)
    this.name = 'Rey'
  }
}

Modules

10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Why? Modules are the future, let's start using the future now.

// bad
const migmeStyleGuide = require('./migmeStyleGuide')
module.exports = migmeStyleGuide.es6
 
// ok
import migmeStyleGuide from './migmeStyleGuide'
export default migmeStyleGuide.es6
 
// best
import { es6 } from './migmeStyleGuide'
export default es6

10.2 Do not use wildcard imports.

Why? This makes sure you have a single default export.

// bad
import * as migmeStyleGuide from './migmeStyleGuide'
 
// good
import migmeStyleGuide from './migmeStyleGuide'

10.3 And do not export directly from an import.

Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

// bad
// filename es6.js
export { es6 as default } from './migmeStyleGuide'
 
// good
// filename es6.js
import { es6 } from './migmeStyleGuide'
export default es6

Iterators and Generators

11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of. eslint: no-iterator

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

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

11.2 Don't use generators for now.

Why? They don't transpile well to ES5.

Properties

12.1 Use dot notation when accessing properties. eslint: dot-notation jscs: requireDotNotation

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

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

const luke = {
  jedi: true,
  age: 28,
}
 
function getProp (prop) {
  return luke[prop]
}
 
const isJedi = getProp('jedi')

Variables

13.1 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()
```

13.2 Use one const declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl

Why? It's easier to add new variable declarations this way, and you never have to worry about adding 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'

13.3 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

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

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

// bad - unnecessary function call
function checkName (hasName) {
  const name = getName()
 
  if (hasName === 'test') {
    return false
  }
 
  if (name === 'test') {
    this.setName('')
    return false
  }
 
  return name
}
 
// good
function checkName (hasName) {
  if (hasName === 'test') {
    return false
  }
 
  const name = getName()
 
  if (name === 'test') {
    this.setName('')
    return false
  }
 
  return name
}

Hoisting

14.1 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
}

14.2 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')
  }
}

14.3 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')
  }
}

14.4 Function declarations hoist their name and the function body.

function example () {
  superPower() // => Flying
 
  function superPower() {
    console.log('Flying')
  }
}

Comparison Operators & Equality

15.1 Use === and !== over == and !=. eslint: eqeqeq
15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract 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 (even an empty one) is an object, objects will evaluate to true
}
```

15.3 Use shortcuts.

// bad
if (name !== '') {
  // ...stuff...
}
 
// good
if (name) {
  // ...stuff...
}
 
// bad
if (collection.length > 0) {
  // ...stuff...
}
 
// good
if (collection.length) {
  // ...stuff...
}

15.4 For more information see Truth Equality and JavaScript by Angus Croll.
15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

Why? Lexical declarations are visible in the entire switch block but only get initialised when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

eslint rules: no-case-declarations.

```javascript
// bad
switch (foo) {
  case 1:
    let x = 1
    break
  case 2:
    const y = 2
    break
  case 3:
    function f () {}
    break
  default:
    class C {}
}

// good
switch (foo) {
  case 1: {
    let x = 1
    break
  }
  case 2: {
    const y = 2
    break
  }
  case 3: {
    function f () {}
    break
  }
  case 4:
    bar()
    break
  default: {
    class C {}
  }
}
```

15.6 Ternaries should not be nested and generally be single line expressions.

eslint rules: no-nested-ternary.

// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null
 
// better
const maybeNull = value1 > value2 ? 'baz' : null
 
const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull
 
// best
const maybeNull = value1 > value2 ? 'baz' : null
 
const foo = maybe1 > maybe2 ? 'bar' : maybeNull

15.7 Avoid unneeded ternary statements.

eslint rules: no-unneeded-ternary.

// bad
const foo = a ? a : b
const bar = c ? true : false
const baz = c ? false : true
 
// good
const foo = a || b
const bar = !!c
const baz = !c

Blocks

16.1 Use braces with all multi-line blocks.

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

16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

// bad
if (test) {
  thing1()
  thing2()
}
else {
  thing3()
}
 
// good
if (test) {
  thing1()
  thing2()
} else {
  thing3()
}

Comments

17.1 Use /** ... */ for multi-line 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
}

17.2 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 unless it's on the first line of a block.

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

17.3 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.

17.4 Use // FIXME: to annotate problems.

class Calculator extends Abacus {
  constructor () {
    super()
 
    // FIXME: shouldn't use a global here
    total = 0
  }
}

17.5 Use // TODO: to annotate solutions to problems.

class Calculator extends Abacus {
  constructor () {
    super ()
 
    // TODO: total should be configurable by an options param
    this.total = 0
  }
}

Whitespace

18.1 Use soft tabs set to 2 spaces. eslint: indent jscs: validateIndentation

// bad
function foo () {
∙∙∙∙const name 
}
 
// bad
function bar () {
∙const name 
}
 
// good
function baz () {
∙∙const name 
}

18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

// 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',
})

18.3 Place 1 space before the opening parenthesis in control statements and function declarations (if, while, function, etc.). Place no space between the function name and argument list in function calls. eslint: keyword-spacing jscs: requireSpaceAfterKeywords
```
// bad
if(isJedi) {
  fight ()
}
 
// good
if (isJedi) {
  fight()
}
 
// bad
function fight() {
  console.log ('Swooosh!')
}
 
// good
function fight () {
  console.log('Swooosh!')
}
```
18.4 Set off operators with spaces. eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators
```
// bad
const x=y+5
 
// good
const x = y + 5
```

18.5 End files with a single newline character.

// bad
(function (global) {
  // ...stuff...
})(this)

// bad
(function (global) {
  // ...stuff...
})(this)↵
↵

// good
(function (global) {
  // ...stuff...
})(this)↵

18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasises that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

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

18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks

// bad
if (foo) {
  return bar
}
return baz
 
// good
if (foo) {
  return bar
}
 
return baz
 
// bad
const obj = {
  foo () {
  },
  bar () {
  },
}
return obj
 
// good
const obj = {
  foo () {
  },
 
  bar () {
  },
}
 
return obj
 
// bad
const arr = [
  function foo () {
  },
  function bar () {
  },
]
return arr
 
// good
const arr = [
  function foo () {
  },
 
  function bar () {
  },
]
 
return arr

18.8 Do not pad your blocks with blank lines. eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

// bad
function bar () {
 
  console.log(foo)
 
}
 
// also bad
if (baz) {
 
  console.log(qux)
} else {
  console.log(foo)
 
}
 
// good
function bar () {
  console.log(foo)
}
 
// good
if (baz) {
  console.log(qux)
} else {
  console.log(foo)
}

18.9 Do not add spaces inside parentheses. eslint: space-in-parens jscs: disallowSpacesInsideParentheses

// bad
function bar ( foo ) {
  return foo
}
 
// good
function bar (foo) {
  return foo
}
 
// bad
if ( foo ) {
  console.log(foo)
}
 
// good
if (foo) {
  console.log(foo)
}

18.10 Do not add spaces inside brackets. eslint: array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets

// bad
const foo = [ 1, 2, 3 ]
console.log(foo[ 0 ])
 
// good
const foo = [1, 2, 3]
console.log(foo[0])

18.11 Add spaces inside curly braces. eslint: object-curly-spacing jscs: requireSpacesInsideObjectBrackets

// bad
const foo = {clark: 'kent'}
 
// good
const foo = { clark: 'kent' }

18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len jscs: maximumLineLength

Why? This ensures readability and maintainability.

// bad
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!'
 
// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'))
 
// good
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
  'Whatever wizard constrains a helpful ally. The counterpart ascends!'
 
// good
$.ajax({
  method: 'POST',
  url: 'https://airbnb.com/',
  data: { name: 'John' },
})
  .done(() => console.log('Congratulations!'))
  .fail(() => console.log('You have failed this city.'))

Commas

19.1 Leading commas: Nope. eslint: comma-style jscs: requireCommaBeforeLineBreak

// bad
const story = [
    once
  , upon
  , aTime
]
 
// good
const story = [
  once,
  upon,
  aTime,
]
 
// bad
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
  , birthYear: 1815
  , superPower: 'computers'
}
 
// good
const hero = {
  firstName: 'Ada',
  lastName: 'Lovelace',
  birthYear: 1815,
  superPower: 'computers',
}

19.2 Additional trailing comma: Yup. eslint: comma-dangle jscs: requireTrailingComma

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the trailing comma problem in legacy browsers.

// bad - git diff without trailing comma
const hero = {
     firstName: 'Florence',
-    lastName: 'Nightingale'
+    lastName: 'Nightingale',
+    inventorOf: ['coxcomb graph', 'modern nursing']
}
 
// good - git diff with trailing comma
const hero = {
     firstName: 'Florence',
     lastName: 'Nightingale',
+    inventorOf: ['coxcomb chart', 'modern nursing'],
}
 
// bad
const hero = {
  firstName: 'Dana',
  lastName: 'Scully'
}
 
const heroes = [
  'Batman',
  'Superman'
]
 
// good
const hero = {
  firstName: 'Dana',
  lastName: 'Scully',
}
 
const heroes = [
  'Batman',
  'Superman',
]

Semicolons

20.1 Nope. It's fine. Really!. eslint: semi jscs: disallowSemicolons

// bad
(function () {
  const name = 'Skywalker';
  return name;
})();
 
// good
(() => {
  const name = 'Skywalker'
  return name
}())
 
// Even without semicolons, they are still allowed to disambiguate statements beginning with [, (, /, +, or -:
;(function () {
  const name = 'Skywalker'
  return name
}())

Type Casting & Coercion

21.1 Perform type coercion at the beginning of the statement.

21.2 Strings:

// => this.reviewScore = 9
 
// bad
const totalScore = this.reviewScore + ''
 
// good
const totalScore = String(this.reviewScore)

21.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix

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)
 
// good
const val = parseInt(inputValue, 10)

21.4 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.

// good
/**
 * parseInt was the reason my code was slow.
 * Bitshifting the String to coerce it to a
 * Number made it a lot faster.
 */
const val = inputValue >> 0

21.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0 //=> 2147483647
2147483648 >> 0 //=> -2147483648
2147483649 >> 0 //=> -2147483647
```

21.6 Booleans:

const age = 0
 
// bad
const hasAge = new Boolean(age)
 
// good
const hasAge = Boolean(age)
 
// good
const hasAge = !!age

Naming Conventions

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

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

22.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase jscs: requireCamelCaseOrUpperCaseIdentifiers

// bad
const OBJEcttsssss = {}
const this_is_my_object = {}
function c () {}
 
// good
const thisIsMyObject = {}
function thisIsMyFunction () {}

22.3 Use PascalCase when naming constructors or classes. eslint: new-cap jscs: requireCapitalizedConstructors

// bad
function user (options) {
  this.name = options.name
}
 
const bad = new user({
  name: 'nope',
})
 
// good
class User {
  constructor (options) {
    this.name = options.name
  }
}
 
const good = new User({
  name: 'yup',
})

22.4 Use a leading underscore _ when naming private properties. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores
```
// bad
this.__firstName__ = 'Panda'
this.firstName_ = 'Panda'
 
// good
this._firstName = 'Panda'
```

22.5 Don't save references to this. Use arrow functions or Function#bind. jscs: disallowNodeTypes

// bad
function foo () {
  const self = this
  return function () {
    console.log(self)
  }
}
 
// bad
function foo () {
  const that = this
  return function () {
    console.log(that)
  }
}
 
// good
function foo () {
  return () => {
    console.log(this)
  }
}

22.6 If your file exports a single class, your filename should be exactly the name of the class.

// file contents
class CheckBox {
  // ...
}
export default CheckBox
 
// in some other file
// bad
import CheckBox from './checkBox'
 
// bad
import CheckBox from './check_box'
 
// good
import CheckBox from './CheckBox'

22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
```
function makeStyleGuide () {
}
 
export default makeStyleGuide
```

22.8 Use PascalCase when you export a singleton / function library / bare object.

const MigmeStyleGuide = {
  es6: {
  }
}
 
export default MigmeStyleGuide

Accessors

23.1 Accessor functions for properties are not required.
23.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').
```
// bad
dragon.age()
 
// good
dragon.getAge()
 
// bad
dragon.age(25)
 
// good
dragon.setAge(25)
```

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

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

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

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

24.1 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', (e, listingId) => {
  // do something with listingId
})

prefer:

// good
$(this).trigger('listingUpdated', { listingId: listing.id })
 
...
 
$(this).on('listingUpdated', (e, data) => {
  // do something with data.listingId
})

jQuery

25.1 Prefix jQuery object variables with a $. jscs: requireDollarBeforejQueryAssignment

Why? It's a lot easier to read the code and determine which variables are assigned to jQuery objects.
```
// bad
const sidebar = $('.sidebar')
 
// good
const $sidebar = $('.sidebar')
 
// good
const $sidebarBtn = $('.sidebar-btn')
```

25.2 Cache jQuery lookups.

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

25.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

25.4 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()

ECMAScript 5 Compatibility

26.1 Refer to Kangax's ES5 compatibility table.

ECMAScript 6 Styles

27.1 This is a collection of links to the various ES6 features.

Testing

28.1 Yup.
```
function foo () {
  return true
}
```
28.2 No, but seriously:
Whichever testing framework you use, you should be writing tests!
Strive to write many small pure functions, and minimise where mutations occur.
Be cautious about stubs and mocks - they can make your tests more brittle.
We primarily use mocha at migme.
100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
We use Codecov to help analyse our test coverage.
Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

Performance

Resources

Learning ES6

Read This

Standard ECMA-262

Other Style Guides

Further Reading

Understanding JavaScript Closures - Angus Croll
Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
You Might Not Need jQuery - Zack Bloom & Adam Schwartz
ES6 Features - Luke Hoban
Frontend Guidelines - Benjamin De Cock

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
Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
Human JavaScript - Henrik Joreteg
Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
JSBooks - Julien Bouquillon
Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
Eloquent JavaScript - Marijn Haverbeke
You Don't Know JS: ES6 & Beyond - Kyle Simpson

Blogs

Podcasts

JavaScript Jabber

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.

migme-style

migme JavaScript Style Guide

Table of Contents

Types

References

Objects

Arrays

Destructuring

Strings

Functions

Arrow Functions

Constructors

Modules

Iterators and Generators

Properties

Variables

Hoisting

Comparison Operators & Equality

Blocks

Comments

Whitespace

Commas

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Events

jQuery

ECMAScript 5 Compatibility

ECMAScript 6 Styles

Testing

Performance

Resources

License

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