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    migme-style

    1.0.2 • Public • Published

    migme JavaScript Style Guide

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

    Table of Contents

    1. Types
    2. References
    3. Objects
    4. Arrays
    5. Destructuring
    6. Strings
    7. Functions
    8. Arrow Functions
    9. Constructors
    10. Modules
    11. Iterators and Generators
    12. Properties
    13. Variables
    14. Hoisting
    15. Comparison Operators & Equality
    16. Blocks
    17. Comments
    18. Whitespace
    19. Commas
    20. Semicolons
    21. Type Casting & Coercion
    22. Naming Conventions
    23. Accessors
    24. Events
    25. jQuery
    26. ECMAScript 5 Compatibility
    27. ECMAScript 6 Styles
    28. Testing
    29. Performance
    30. Resources
    31. 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

    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

    // 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 (= 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

    ECMAScript 6 Styles

    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

    Other Style Guides

    Further Reading

    Books

    Blogs

    Podcasts

    License

    (The MIT License)

    Copyright (c) 2014-2016 Airbnb

    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.

    Install

    npm i migme-style

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    Version

    1.0.2

    License

    MIT

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    Collaborators

    • eddiemoore
    • migbuild