bigchaindb/docs/contributing/source/cross-project-policies/js-style-guide.md

# BigchainDB JavaScript Style Guide

For consistent JavaScript across BigchainDB-related repos.

[![Build Status](https://travis-ci.org/ascribe/javascript.svg?branch=master)](https://travis-ci.org/ascribe/javascript)
[![Greenkeeper badge](https://badges.greenkeeper.io/ascribe/javascript.svg)](https://greenkeeper.io/)
[![js ascribe](https://img.shields.io/badge/js-ascribe-39BA91.svg)](https://github.com/ascribe/javascript)

## Introduction

At ascribe we write a lot of JavaScript and value quality code. Since all of us liked [Airbnb's JavaScript Style Guide](https://github.com/airbnb/javascript), we figured that we can just fork it and change it to our needs.

- [JavaScript Style Guide (this document)](#table-of-contents)
- [React Style Guide](react/)

## Usage

Use the provided ESlint packages under `packages/` and refer to their documentation for detailed usage:

- [![npm](https://img.shields.io/npm/v/eslint-config-ascribe.svg)](https://www.npmjs.com/package/eslint-config-ascribe) [eslint-config-ascribe](packages/eslint-config-ascribe)
- [![npm](https://img.shields.io/npm/v/eslint-config-ascribe-react.svg)](https://www.npmjs.com/package/eslint-config-ascribe-react) [eslint-config-ascribe-react](packages/eslint-config-ascribe-react)

## Table of Contents

  1. [Types](#types)
  1. [References](#references)
  1. [Objects](#objects)
  1. [Arrays](#arrays)
  1. [Destructuring](#destructuring)
  1. [Strings](#strings)
  1. [Functions](#functions)
  1. [Arrow Functions](#arrow-functions)
  1. [Constructors](#constructors)
  1. [Modules](#modules)
  1. [Iterators and Generators](#iterators-and-generators)
  1. [Properties](#properties)
  1. [Variables](#variables)
  1. [Hoisting](#hoisting)
  1. [Comparison Operators & Equality](#comparison-operators--equality)
  1. [Blocks](#blocks)
  1. [Comments](#comments)
  1. [Whitespace](#whitespace)
  1. [Commas](#commas)
  1. [Semicolons](#semicolons)
  1. [Type Casting & Coercion](#type-casting--coercion)
  1. [Naming Conventions](#naming-conventions)
  1. [Accessors](#accessors)
  1. [Events](#events)
  1. [jQuery](#jquery)
  1. [ECMAScript 5 Compatibility](#ecmascript-5-compatibility)
  1. [ECMAScript 6 Styles](#ecmascript-6-styles)
  1. [Testing](#testing)
  1. [Performance](#performance)
  1. [Resources](#resources)
  1. [In the Wild](#in-the-wild)
  1. [Contributors](#contributors)
  1. [License](#license)

## Types

  - [1.1](#1.1) <a name='1.1'></a> **Primitives**: When you access a primitive type you work directly on its value.

    + `string`
    + `number`
    + `boolean`
    + `null`
    + `undefined`

    ```javascript
    const foo = 1;
    let bar = foo;

    bar = 9;

    console.log(foo, bar); // => 1, 9
    ```
  - [1.2](#1.2) <a name='1.2'></a> **Complex**: When you access a complex type you work on a reference to its value.

    + `object`
    + `array`
    + `function`

    ```javascript
    const foo = [1, 2];
    const bar = foo;

    bar[0] = 9;

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

**[⬆ back to top](#table-of-contents)**

## References

  - [2.1](#2.1) <a name='2.1'></a> Use `const` for all of your references; avoid using `var`.

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

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

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

  - [2.2](#2.2) <a name='2.2'></a> If you must mutate references, use `let` instead of `var`.

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

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

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

  - [2.3](#2.3) <a name='2.3'></a> Note that both `let` and `const` are block-scoped.

    ```javascript
    // 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
    ```

**[⬆ back to top](#table-of-contents)**

## Objects

  - [3.1](#3.1) <a name='3.1'></a> Use the literal syntax for object creation.

    ```javascript
    // bad
    const item = new Object();

    // good
    const item = {};
    ```

  - [3.2](#3.2) <a name='3.2'></a> If your code will be executed in browsers in script context, don't use [reserved words](http://es5.github.io/#x7.6.1) as keys. It won't work in IE8. [More info](https://github.com/airbnb/javascript/issues/61). It’s OK to use them in ES6 modules and server-side code.

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

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

  - [3.3](#3.3) <a name='3.3'></a> Use readable synonyms in place of reserved words.

    ```javascript
    // bad
    const superman = {
        class: 'alien',
    };

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

    // good
    const superman = {
        type: 'alien',
    };
    ```

  <a name="es6-computed-properties"></a>
  - [3.4](#3.4) <a name='3.4'></a> 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.

    ```javascript

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

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

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

  <a name="es6-object-shorthand"></a>
  - [3.5](#3.5) <a name='3.5'></a> Use object method shorthand.

    ```javascript
    // bad
    const atom = {
        value: 1,

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

    // good
    const atom = {
        value: 1,

        addValue(value) {
            return atom.value + value;
        },
    };
    ```

  <a name="es6-object-concise"></a>
  - [3.6](#3.6) <a name='3.6'></a> Use property value shorthand.

    > Why? It is shorter to write and descriptive.

    ```javascript
    const lukeSkywalker = 'Luke Skywalker';

    // bad
    const obj = {
        lukeSkywalker: lukeSkywalker,
    };

    // good
    const obj = {
        lukeSkywalker,
    };
    ```

  - [3.7](#3.7) <a name='3.7'></a> Group your shorthand properties at the beginning of your object declaration.

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

    ```javascript
    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](#3.8) <a name='3.8'></a> Prefer quoting only properties that are invalid identifiers, but always ensure that all properties are consistently quoted.

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

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

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

    // better
    const better = {
        foo: 3,
        bar: 4,
        dataBlah: 5
    };
    ```

**[⬆ back to top](#table-of-contents)**

## Arrays

  - [4.1](#4.1) <a name='4.1'></a> Use the literal syntax for array creation.

    ```javascript
    // bad
    const items = new Array();

    // good
    const items = [];
    ```

  - [4.2](#4.2) <a name='4.2'></a> Use Array#push instead of direct assignment to add items to an array.

    ```javascript
    const someStack = [];

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

    // good
    someStack.push('abracadabra');
    ```

  <a name="es6-array-spreads"></a>
  - [4.3](#4.3) <a name='4.3'></a> Use array spreads `...` to copy arrays.

    ```javascript
    // 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](#4.4) <a name='4.4'></a> To convert an array-like object to an array, use Array#from.

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

**[⬆ back to top](#table-of-contents)**

## Destructuring

  - [5.1](#5.1) <a name='5.1'></a> Use object destructuring when accessing and using multiple properties of an object.

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

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

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

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

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

  - [5.2](#5.2) <a name='5.2'></a> When destructuring requires multiple lines, follow formatting rules for [objects](#3.1):

    ```javascript
    // bad
    const { first: {
                nested
            },
            second } = obj;

    // bad
    const {
        first: {
            nested
        },
        second } = obj;

    // good
    const {
        first: {
            nested
        },
        second
    } = obj;
    ```

  - [5.3](#5.3) <a name='5.3'></a> Use array destructuring.

    ```javascript
    const arr = [1, 2, 3, 4];

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

    // good
    const [first, second] = arr;
    ```

  - [5.4](#5.4) <a name='5.4'></a> Use object destructuring for multiple return values, not array destructuring.

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

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

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

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

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

  - [5.5](#5.5) <a name='5.5'></a> You can use destructuring and an object spread operator to filter out specific properties while keeping the other properties in a new object.

    ```javascript
    // bad
    const val = obj.value;
    delete obj.value;

    // good
    const { value: val, ...otherObj } = obj;
    // otherObj will hold all other properties of obj except for value
    ```

**[⬆ back to top](#table-of-contents)**

## Strings

  - [6.1](#6.1) <a name='6.1'></a> Use single quotes `''` for strings.

    ```javascript
    // bad
    const name = "Capt. Janeway";

    // good
    const name = 'Capt. Janeway';
    ```

  - [6.2](#6.2) <a name='6.2'></a> When using (single- or double) quotes in a string, use the other literal (`''` or `""`).

    ```javascript
    // bad
    const name = "What a \"nice\" day!";

    // bad
    const name = 'Let\'s go to Rosi\'s!';

    // good
    const name = 'What a "nice" day!';

    // good
    const name = "Let's go to Rosi's!";
    ```

  - [6.3](#6.3) <a name='6.3'></a> Strings longer than 100 characters should be written across multiple lines using string concatenation.

    ```javascript
    // 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](#6.4) <a name='6.4'></a> Note: If overused, long strings with concatenation could impact performance. [jsPerf](http://jsperf.com/ya-string-concat) & [Discussion](https://github.com/airbnb/javascript/issues/40).

  <a name="es6-template-literals"></a>
  - [6.5](#6.5) <a name='6.5'></a> When programmatically building up strings, use template strings instead of concatenation.

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

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

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

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

  - [6.6](#6.6) <a name='6.6'></a> **NEVER** use eval() on a string, it opens too many vulnerabilities.

**[⬆ back to top](#table-of-contents)**

## Functions

  - [7.1](#7.1) <a name='7.1'></a> Use function declarations instead of function expressions.

    > 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](#arrow-functions) in place of function expressions.

    ```javascript
    // bad
    const foo = function () {
    };

    // good
    function foo() {
    }
    ```

  - [7.2](#7.2) <a name='7.2'></a> Immediately-invoked function expressions should use arrow functions as opposed to traditional functions:

    ```javascript
    // immediately-invoked function expression (IIFE)
    (() => {
        console.log('Welcome to the Internet. Please follow me.');
    })();
    ```

  - [7.3](#7.3) <a name='7.3'></a> **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.
  - [7.4](#7.4) <a name='7.4'></a> **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](http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf#page=97).

    ```javascript
    // bad
    if (currentUser) {
        function test() {
            console.log('Nope.');
        }
    }

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

  - [7.5](#7.5) <a name='7.5'></a> **NEVER** name a parameter `arguments`. This will take precedence over the `arguments` object that is given to every function scope.

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

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

  <a name="es6-rest"></a>
  - [7.6](#7.6) <a name='7.6'></a> **NEVER** use `arguments`, opt to use rest syntax `...` instead.

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

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

    // good
    function concatenateAll(...args) {
        return args.join('');
    }
    ```

  <a name="es6-default-parameters"></a>
  - [7.7](#7.7) <a name='7.7'></a> Use default parameter syntax rather than mutating function arguments.

    ```javascript
    // 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](#7.8) <a name='7.8'></a> Avoid side effects with default parameters.

    > Why? They are confusing to reason about.

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

  - [7.9](#7.9) <a name='7.9'></a> Always put default parameters last.

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

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

  - [7.10](#7.10) <a name='7.10'></a> **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.

    ```javascript
    // bad
    var add = new Function('a', 'b', 'return a + b');

    // still bad
    var subtract = Function('a', 'b', 'return a - b');
    ```

**[⬆ back to top](#table-of-contents)**

## Arrow Functions

  - [8.1](#8.1) <a name='8.1'></a> When you must use function expressions (as when passing an anonymous function), use arrow function notation.

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

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

    ```javascript
    // 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](#8.2) <a name='8.2'></a> If the function body consists of a single expression, feel free to omit the braces and use the implicit return. Otherwise use a `return` statement.

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

    > Why not? If you plan on returning an object.

    ```javascript
    // good
    [1, 2, 3].map(number => `A string containing the ${number}.`);

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

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

  - [8.3](#8.3) <a name='8.3'></a> 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.

    ```javascript
    // 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](#8.4) <a name='8.4'></a> If your function only takes a single argument, feel free to omit the parentheses.

    > Why? Less visual clutter.

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

    // good
    [1, 2, 3].reduce((y, x) => x + y);
    ```

**[⬆ back to top](#table-of-contents)**

## Constructors

  - [9.1](#9.1) <a name='9.1'></a> Always use `class`. Avoid manipulating `prototype` directly.

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

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


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

  - [9.2](#9.2) <a name='9.2'></a> Use `extends` for inheritance.

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

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

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

  - [9.3](#9.3) <a name='9.3'></a> Methods can return `this` to help with method chaining.

    ```javascript
    // 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](#9.4) <a name='9.4'></a> It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

    ```javascript
    class Jedi {
        constructor({ name = 'no name' } = {}) {
            this.name = name;
        }

        getName() {
            return this.name;
        }

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

**[⬆ back to top](#table-of-contents)**


## Modules

  - [10.1](#10.1) <a name='10.1'></a> 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.

    ```javascript
    // bad
    const AirbnbStyleGuide = require('./AirbnbStyleGuide');
    module.exports = AirbnbStyleGuide.es6;

    // ok
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    export default AirbnbStyleGuide.es6;

    // best
    import { es6 } from './AirbnbStyleGuide';
    export default es6;
    ```

  - [10.2](#10.2) <a name='10.2'></a> Do not use wildcard imports.

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

    ```javascript
    // bad
    import * as AirbnbStyleGuide from './AirbnbStyleGuide';

    // good
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    ```

  - [10.3](#10.3) <a name='10.3'></a>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.

    ```javascript
    // bad
    // filename es6.js
    export { es6 as default } from './airbnbStyleGuide';

    // good
    // filename es6.js
    import { es6 } from './AirbnbStyleGuide';
    export default es6;
    ```

**[⬆ back to top](#table-of-contents)**

## Iterators and Generators

  - [11.1](#11.1) <a name='11.1'></a> Prefer [JavaScript's higher-order functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) like `map()` and `reduce()` instead of loops like `for-of` unless there is a substantial performance disadvantage by doing so.

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

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

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

    sum === 15;

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

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

  - [11.2](#11.2) <a name='11.2'></a> Only use `for-in` if you know exactly what you're doing. If unsure, prefer the options given in [11.1](#11.1).

  - [11.3](#11.3) <a name='11.3'></a> Don't use generators for now.

    > Why? They don't transpile well to ES5.

**[⬆ back to top](#table-of-contents)**


## Properties

  - [12.1](#12.1) <a name='12.1'></a> Use dot notation when accessing properties.

    ```javascript
    const luke = {
        jedi: true,
        age: 28,
    };

    // bad
    const isJedi = luke['jedi'];

    // good
    const isJedi = luke.jedi;
    ```

  - [12.2](#12.2) <a name='12.2'></a> Use subscript notation `[]` when accessing properties with a variable.

    ```javascript
    const luke = {
        jedi: true,
        age: 28,
    };

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

    const isJedi = getProp('jedi');
    ```

**[⬆ back to top](#table-of-contents)**


## Variables

  - [13.1](#13.1) <a name='13.1'></a> Always use `const` or `let` 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.

    ```javascript
    // bad
    superPower = new SuperPower();

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

  - [13.2](#13.2) <a name='13.2'></a> Use one `const` or `let` declaration per variable.

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

    ```javascript
    // 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](#13.3) <a name='13.3'></a> Group all your `const`s and then group all your `let`s.

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

    ```javascript
    // 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](#13.4) <a name='13.4'></a> Assign variables where you need them, but place them in a reasonable place.

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

    ```javascript
    // good
    function() {
        test();
        console.log('doing stuff..');

        //..other stuff..

        const name = getName();

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

        return name;
    }

    // bad - unnecessary function call
    function(hasName) {
        const name = getName();

        if (!hasName) {
            return false;
        }

        this.setFirstName(name);

        return true;
    }

    // good
    function(hasName) {
        if (!hasName) {
            return false;
        }

        const name = getName();
        this.setFirstName(name);

        return true;
    }
    ```

    > Note that referencing a variable declared by `let` or `const` before they are set results in a reference error, including typeof (see [Why `typeof` is no longer "safe"](http://es-discourse.com/t/why-typeof-is-no-longer-safe/15))

    ```javascript
    if (condition) {
        console.log(typeof value);     // ReferenceError!
        let value = "blue";
    }
    ```

  - [13.5](#13.5) <a name='13.5'></a> Avoid declaring unused variables, however the cases where it can be convenient (such as filtering some properties out of an object or destructuring an array, for example), prefix the variable name with `ignored`:

    ```javascript
    // bad
    const {
        first, // ignored
        second, // ignored
        third
    } = winners;

    // good
    const {
        first: ignoredFirst, // ignored
        second: ignoredSecond, // ignored
        third
    } = winners;
    ```

    Note that our ESLint configuration is set up to error on any unused variable unless it is prefixed by `ignored`. An exception to this is argument names; any arguments listed before the first one used is OK:

    ```javascript
    // bad -- `second` is unused
    function (first, second) {
        return first;
    }

    // good -- `first` is listed before the used `second` argument
    function (first, second) {
        return second;
    }
    ```

**[⬆ back to top](#table-of-contents)**

## Hoisting

  - [14.1](#14.1) <a name='14.1'></a> `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)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Temporal_dead_zone_and_errors_with_let). It's important to know why [typeof is no longer safe](http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).

    ```javascript
    // 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](#14.2) <a name='14.2'></a> Anonymous function expressions hoist their variable name, but not the function assignment.

    ```javascript
    function example() {
        console.log(anonymous); // => undefined

        anonymous(); // => TypeError anonymous is not a function

        var anonymous = function() {
            console.log('anonymous function expression');
        };
    }
    ```

  - [14.3](#14.3) <a name='14.3'></a> Named function expressions hoist the variable name, not the function name or the function body.

    ```javascript
    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](#14.4) <a name='14.4'></a> Function declarations hoist their name and the function body.

    ```javascript
    function example() {
        superPower(); // => Flying

        function superPower() {
            console.log('Flying');
        }
    }
    ```

  - [14.5](#14.5) <a name='14.5'></a> ES6 `import`s are hoisted to the beginning of their module while modules imported through `require`s (ie. CommonJS modules) are not.

    ```javascript
    // This works
    foo();

    import { foo } from 'my_module';

    // This will import 'imported_module' before 'required_module'
    require('required_module');

    import 'imported_module';
    ```

  - For more information refer to [JavaScript Scoping & Hoisting](http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting) by [Ben Cherry](http://www.adequatelygood.com/).

**[⬆ back to top](#table-of-contents)**


## Comparison Operators & Equality

  - [15.1](#15.1) <a name='15.1'></a> Use `===` and `!==` over `==` and `!=`. Avoid `==` and `!=` because they are 'loose' equality comparisons, only evaluating equality after coercing both values following confusing and difficult to remember rules ([see MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness)).
  - [15.2](#15.2) <a name='15.2'></a> 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**

    ```javascript
    if ([]) {
        // true
        // An array is an object, objects evaluate to true
    }
    ```

  - [15.3](#15.3) <a name='15.3'></a> Use shortcuts.

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

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

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

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

  - [15.4](#15.4) <a name='15.4'></a> For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.

  - [15.5](#15.5) <a name='15.5'></a> 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 initialized when assigned, which only happens when its `case` is reached. This causes problems when multiple `case` clauses attempt to define the same thing.

    ```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](#15.6) <a name='15.6'></a> Indent one full level for case statements.

    ```javascript
    // bad
    switch (foo) {
    case 1:
        break;
    default:
        break;
    }

    // bad
    switch (foo) {
      case 1:
        break;
      default:
        break;
    }

    // good
    switch (foo) {
        case 1:
            break;
        default:
            break;
    }
    ```

  - [15.7](#15.7) <a name='15.7'></a> Ternaries should not be nested and generally be single line expressions.

    ```javascript
    // 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;
    ```

  - [15.8](#15.8) <a name='15.8'></a> Avoid unneeded ternary statements.

    ```javascript
    // 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;
    ```

  - [15.9](#15.9) <a name='15.9'></a> Use any of the following styles for multi-line ternary statements:

    ```javascript
    // good
    const foo = thisisasuperlongexpression ? value
                                           : otherValue;

    // good
    const foo = thisisasuperlongexpression
        ? value : otherValue;

    // good
    const foo = thisisasuperlongexpression
        ? value
        : otherValue;
    ```

**[⬆ back to top](#table-of-contents)**


## Blocks

  - [16.1](#16.1) <a name='16.1'></a> Use braces with all multi-line blocks.

    ```javascript
    // bad
    if (test)
        return false;

    // good
    if (test) return false;

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

    // bad
    function() { return false; }

    // good
    function() {
        return false;
    }
    ```

  - [16.2](#16.2) <a name='16.2'></a> If you're using multi-line blocks with `if` and `else`, put `else` on the same line as your `if` block's closing brace.

    ```javascript
    // bad
    if (test) {
        thing1();
        thing2();
    }
    else {
        thing3();
    }

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


**[⬆ back to top](#table-of-contents)**


## Comments

  - [17.1](#17.1) <a name='17.1'></a> Use `/** ... */` for multi-line comments. Include a description, specify types and values for all parameters and return values by using [JSDoc](http://www.2ality.com/2011/08/jsdoc-intro.html).

    ```javascript
    // 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](#17.2) <a name='17.2'></a> 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.

    ```javascript
    // 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;
    }
    ```

  - [17.3](#17.3) <a name='17.3'></a> Always put a single space between where your comment starts (ie. `/*`, `/**`, or `//`) and the comment.

  - [17.4](#17.4) <a name='17.4'></a> 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`.

  - [17.5](#17.5) <a name='17.5'></a> Use `// FIXME:` to annotate problems.

    ```javascript
    class Calculator extends Abacus {
        constructor() {
            super();

            // FIXME: shouldn't use a global here
            total = 0;
        }
    }
    ```

  - [17.6](#17.6) <a name='17.6'></a> Use `// TODO:` to annotate solutions to problems.

    ```javascript
    class Calculator extends Abacus {
        constructor() {
            super();

            // TODO: total should be configurable by an options param
            this.total = 0;
        }
    }
    ```

**[⬆ back to top](#table-of-contents)**


## Whitespace

  - [18.1](#18.1) <a name='18.1'></a> Use soft tabs set to 4 spaces.

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

    // bad
    function() {
    ∙const name;
    }

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

  - [18.2](#18.2) <a name='18.2'></a> Place 1 space before the leading brace.

    ```javascript
    // bad
    function test(){
        console.log('test');
    }

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

    // bad
    dog.set('attr',{
        age: '1 year',
        breed: 'Bernese Mountain Dog',
    });

    // good
    dog.set('attr', {
        age: '1 year',
        breed: 'Bernese Mountain Dog',
    });
    ```

  - [18.3](#18.3) <a name='18.3'></a> Place 1 space before the opening parenthesis in control statements (`if`, `while` etc.) and anonymous function declarations. Place no space before the argument list in function calls and named declarations.

    ```javascript
    // bad
    if(isJedi) {
        fight ();
    }

    // good
    if (isJedi) {
        fight();
    }

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

    // good -- easier to tell this is a function decarlation rather than function call
    function () {
        console.log('Anonymous');
    }

    // bad
    function fight () {
        console.log ('Swooosh!');
    }

    // good
    function fight() {
        console.log('Swooosh!');
    }
    ```

  - [18.4](#18.4) <a name='18.4'></a> Set off operators with spaces.

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

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

  - [18.5](#18.5) <a name='18.5'></a> End files with a single newline character.

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

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

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

  - [18.6](#18.6) <a name='18.6'></a> Use indentation when making long method chains. Use a leading dot, which
    emphasizes that the line is a method call, not a new statement.

    ```javascript
    // 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 request = fetch('/users').then(...).catch(...).finally(...);

    // good
    const request = fetch('/users')
        .then(...)
        .catch(...)
        .finally(...);
    ```

  - [18.7](#18.7) <a name='18.7'></a> Leave a blank line after blocks and before the next statement.

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

    // good
    if (foo) {
        return bar;
    }

    return baz;

    // bad
    const obj = {
        foo() {
        },
        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](#18.8) <a name='18.8'></a> Break long logical operations into multiple lines, leaving operators at the end of the line and intenting the later lines to the first line's first operand.

    ```javascript
    // bad
    if (aReallyReallyLongExpr && anotherSuperLongExpr && wowSoManyExpr && longExprToCheckTheWorldIsOk) {
        ...
    }

    // good
    if (aReallyReallyLongExpr &&
        anotherSuperLongExpr &&
        wowSoManyExpr &&
        longExprToCheckTheWorldIsOk) {
        ...
    }

    // good
    while (aReallyReallyLongExpr &&
           anotherSuperLongExpr &&
           wowSoManyExpr &&
           longExprToCheckTheWorldIsOk) {
        ...
    }
    ```

  - [18.9](#18.9) <a name='18.9'></a> Do not pad your blocks with blank lines.

    ```javascript
    // 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);
    }
    ```

  - [18.10](#18.10) <a name='18.10'></a> Do not add spaces inside parentheses.

    ```javascript
    // bad
    function bar( foo ) {
        return foo;
    }

    // good
    function bar(foo) {
        return foo;
    }

    // bad
    if ( foo ) {
        console.log(foo);
    }

    // good
    if (foo) {
        console.log(foo);
    }
    ```

  - [18.11](#18.11) <a name='18.11'></a> Do not add spaces inside brackets.

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

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

  - [18.12](#18.12) <a name='18.12'></a> Add spaces inside curly braces.

    ```javascript
    // bad
    const foo = {clark: 'kent'};

    // good
    const foo = { clark: 'kent' };
    ```

  - [18.13](#18.13) <a name='18.13'></a> Avoid having lines of code that are longer than 100 characters (including whitespace).

    > Why? This ensures readability and maintainability.

    ```javascript
    // 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.'));
    ```

    In some cases, you can go slightly over the limit (urls, code that's *just* slightly over), but
    our ESLint configuration is set up to warn on code lines that are over 105 characters.


  - [18.14](#18.14) <a name='18.14'></a> When a function call needs to be broken up into multiple lines, put arguments on a separate line, indented four spaces:

    ```javascript
    // bad
    const foo = funcCall(this, is, a, really,
                         reallllyyyyyyy, long,
                         function, call);

    // good
    const foo = funcCall(
        this, is, a, really,
        reallllyyyyyyy, long,
        function,c all
    );


**[⬆ back to top](#table-of-contents)**


## Commas

  - [19.1](#19.1) <a name='19.1'></a> Leading commas: **Nope.**

    ```javascript
    // 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](#19.2) <a name='19.2'></a> Additional trailing comma: **Yup.**

    > 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](es5/README.md#commas) in legacy browsers.

    ```javascript
    // 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',
    ];
    ```

**[⬆ back to top](#table-of-contents)**


## Semicolons

  - [20.1](#20.1) <a name='20.1'></a> **Nope.**

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

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

**[⬆ back to top](#table-of-contents)**


## Type Casting & Coercion

  - [21.1](#21.1) <a name='21.1'></a> Perform type coercion at the beginning of the statement.
  - [21.2](#21.2) <a name='21.2'></a> Strings:

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

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

    // good
    const totalScore = String(this.reviewScore);
    ```

  - [21.3](#21.3) <a name='21.3'></a> Numbers: Use `Number` for type casting and `parseInt` always with a radix.

    ```javascript
    const inputValue = '4';

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

    // bad
    const val = +inputValue;

    // bad
    const val = inputValue >> 0;

    // bad
    const val = parseInt(inputValue);

    // good
    const val = Number(inputValue);

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

  - [21.4](#21.4) <a name='21.4'></a> If for whatever reason you are doing something wild and `parseInt` is your bottleneck and need to use Bitshift for [performance reasons](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.

    ```javascript
    // 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](#21.5) <a name='21.5'></a> **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. [Discussion](https://github.com/airbnb/javascript/issues/109). Largest signed 32-bit Int is 2,147,483,647:

    ```javascript
    2147483647 >> 0 //=> 2147483647
    2147483648 >> 0 //=> -2147483648
    2147483649 >> 0 //=> -2147483647
    ```

  - [21.6](#21.6) <a name='21.6'></a> Booleans:

    ```javascript
    const age = 0;

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

    // good
    const hasAge = Boolean(age);

    // good
    const hasAge = !!age;
    ```

**[⬆ back to top](#table-of-contents)**


## Naming Conventions

  - [22.1](#22.1) <a name='22.1'></a> Avoid single letter names. Be descriptive with your naming.

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

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

  - [22.2](#22.2) <a name='22.2'></a> Use camelCase when naming objects, functions, and instances.

    ```javascript
    // bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c() {}

    // good
    const thisIsMyObject = {};
    function thisIsMyFunction() {}
    ```

  - [22.3](#22.3) <a name='22.3'></a> Use PascalCase when naming constructors or classes.

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

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

    // good
    class UserPascalCase {
        constructor(options) {
            this.name = options.name;
        }
    }

    const good = new UserPascalCase({
        name: 'yup',
    });
    ```

  - [22.4](#22.4) <a name='22.4'></a> Use a leading underscore `_` when naming private properties.

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

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

  - [22.5](#22.5) <a name='22.5'></a> Don't save references to `this`. Use arrow functions or Function#bind.

    ```javascript
    // 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);
        };
    }

    // good
    function foo() {
        return (function() {
            console.log(this);
        }).bind(this);
    }
    ```

  - [22.6](#22.6) <a name='22.6'></a> If your file exports a single class, your filename should be exactly the name of the class, converted from PascalCase to snake_case.

    ```javascript
    // file contents
    class CheckBox {
        // ...
    }
    export default CheckBox;

    // in some other file
    // bad
    import CheckBox from './checkBox';

    // bad
    import CheckBox from './CheckBox';

    // good
    import CheckBox from './check_box';
    ```

  - [22.7](#22.7) <a name='22.7'></a> Use camelCase when you export-default a function. Your filename should be identical to your function's name.

    ```javascript
    function makeStyleGuide() {
    }

    export default makeStyleGuide;
    ```

  - [22.8](#22.8) <a name='22.8'></a> Use PascalCase when you export a singleton / function library / bare object.

    ```javascript
    const AirbnbStyleGuide = {
        es6: {
        }
    };

    export default AirbnbStyleGuide;
    ```


**[⬆ back to top](#table-of-contents)**


## Accessors

  - [23.1](#23.1) <a name='23.1'></a> Accessor functions for properties are not required.
  - [23.2](#23.2) <a name='23.2'></a> If you do make accessor functions use getVal() and setVal('hello').

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

    // good
    dragon.getAge();

    // bad
    dragon.age(25);

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

  - [23.3](#23.3) <a name='23.3'></a> If the property is a `boolean`, use `isVal()` or `hasVal()`.

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

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

  - [23.4](#23.4) <a name='23.4'></a> It's okay to create get() and set() functions, but be consistent.

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

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

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

**[⬆ back to top](#table-of-contents)**


## Events

  - [24.1](#24.1) <a name='24.1'></a> When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass a hash instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

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

    ...

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

    prefer:

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

    ...

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

  **[⬆ back to top](#table-of-contents)**


## jQuery

  - [25.1](#25.1) <a name='25.1'></a> Prefix jQuery object variables with a `$`.

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

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

    // good
    const $sidebarBtn = $('.sidebar-btn');
    ```

  - [25.2](#25.2) <a name='25.2'></a> Cache jQuery lookups.

    ```javascript
    // 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](#25.3) <a name='25.3'></a> For DOM queries use Cascading `$('.sidebar ul')` or parent > child `$('.sidebar > ul')`. [jsPerf](http://jsperf.com/jquery-find-vs-context-sel/16)
  - [25.4](#25.4) <a name='25.4'></a> Use `find` with scoped jQuery object queries.

    ```javascript
    // bad
    $('ul', '.sidebar').hide();

    // bad
    $('.sidebar').find('ul').hide();

    // good
    $('.sidebar ul').hide();

    // good
    $('.sidebar > ul').hide();

    // good
    $sidebar.find('ul').hide();
    ```

**[⬆ back to top](#table-of-contents)**


## ECMAScript 5 Compatibility

  - [26.1](#26.1) <a name='26.1'></a> Refer to [Kangax](https://twitter.com/kangax/)'s ES5 [compatibility table](http://kangax.github.com/es5-compat-table/).

**[⬆ back to top](#table-of-contents)**


## ECMAScript 6 Styles

  - [27.1](#27.1) <a name='27.1'></a> This is a collection of links to the various es6 features.

    1. [Arrow Functions](#arrow-functions)
    1. [Classes](#constructors)
    1. [Object Shorthand](#es6-object-shorthand)
    1. [Object Concise](#es6-object-concise)
    1. [Object Computed Properties](#es6-computed-properties)
    1. [Template Strings](#es6-template-literals)
    1. [Destructuring](#destructuring)
    1. [Default Parameters](#es6-default-parameters)
    1. [Rest](#es6-rest)
    1. [Array Spreads](#es6-array-spreads)
    1. [Let and Const](#references)
    1. [Iterators and Generators](#iterators-and-generators)
    1. [Modules](#modules)

  - [27.2](#27.2) <a name='27.2'></a> Khan Academy has a nice section in their [Javascript styleguide](https://github.com/Khan/style-guides/blob/master/style/javascript.md) that discusses various ways to [accomplish tasks in ES6 rather than using underscore/lodash](https://github.com/Khan/style-guides/blob/master/style/javascript.md#dont-use-underscore).

**[⬆ back to top](#table-of-contents)**


## Testing

  - [28.1](#28.1) <a name='28.1'></a> **Yup.**

    ```javascript
    function() {
      return true;
    }
    ```

**[⬆ back to top](#table-of-contents)**


## Performance

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

**[⬆ back to top](#table-of-contents)**


## Resources

**Learning ES6**

  - [Draft ECMA 2015 (ES6) Spec](https://people.mozilla.org/~jorendorff/es6-draft.html)
  - [ExploringJS](http://exploringjs.com/)
  - [ES6 Compatibility Table](https://kangax.github.io/compat-table/es6/)
  - [Comprehensive Overview of ES6 Features](http://es6-features.org/)

**Read This**

  - [Standard ECMA-262](http://www.ecma-international.org/ecma-262/6.0/index.html)

**Tools**

  - Code Style Linters
    + [ESlint](http://eslint.org/) - [Airbnb Style .eslintrc](https://github.com/airbnb/javascript/blob/master/linters/.eslintrc)
    + [JSHint](http://www.jshint.com/) - [Airbnb Style .jshintrc](https://github.com/airbnb/javascript/blob/master/linters/jshintrc)
    + [JSCS](https://github.com/jscs-dev/node-jscs) - [Airbnb Style Preset](https://github.com/jscs-dev/node-jscs/blob/master/presets/airbnb.json)

**Other Style Guides**

  - [Google JavaScript Style Guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml)
  - [jQuery Core Style Guidelines](http://docs.jquery.com/JQuery_Core_Style_Guidelines)
  - [Principles of Writing Consistent, Idiomatic JavaScript](https://github.com/rwldrn/idiomatic.js/)

**Other Styles**

  - [Naming this in nested functions](https://gist.github.com/4135065) - Christian Johansen
  - [Conditional Callbacks](https://github.com/airbnb/javascript/issues/52) - Ross Allen
  - [Popular JavaScript Coding Conventions on Github](http://sideeffect.kr/popularconvention/#javascript) - JeongHoon Byun
  - [Multiple var statements in JavaScript, not superfluous](http://benalman.com/news/2012/05/multiple-var-statements-javascript/) - Ben Alman

**Further Reading**

  - [Understanding JavaScript Closures](http://javascriptweblog.wordpress.com/2010/10/25/understanding-javascript-closures/) - Angus Croll
  - [Basic JavaScript for the impatient programmer](http://www.2ality.com/2013/06/basic-javascript.html) - Dr. Axel Rauschmayer
  - [You Might Not Need jQuery](http://youmightnotneedjquery.com/) - Zack Bloom & Adam Schwartz
  - [ES6 Features](https://github.com/lukehoban/es6features) - Luke Hoban
  - [Frontend Guidelines](https://github.com/bendc/frontend-guidelines) - Benjamin De Cock

**Books**

  - [JavaScript: The Good Parts](http://www.amazon.com/JavaScript-Good-Parts-Douglas-Crockford/dp/0596517742) - Douglas Crockford
  - [JavaScript Patterns](http://www.amazon.com/JavaScript-Patterns-Stoyan-Stefanov/dp/0596806752) - Stoyan Stefanov
  - [Pro JavaScript Design Patterns](http://www.amazon.com/JavaScript-Design-Patterns-Recipes-Problem-Solution/dp/159059908X)  - Ross Harmes and Dustin Diaz
  - [High Performance Web Sites: Essential Knowledge for Front-End Engineers](http://www.amazon.com/High-Performance-Web-Sites-Essential/dp/0596529309) - Steve Souders
  - [Maintainable JavaScript](http://www.amazon.com/Maintainable-JavaScript-Nicholas-C-Zakas/dp/1449327680) - Nicholas C. Zakas
  - [JavaScript Web Applications](http://www.amazon.com/JavaScript-Web-Applications-Alex-MacCaw/dp/144930351X) - Alex MacCaw
  - [Pro JavaScript Techniques](http://www.amazon.com/Pro-JavaScript-Techniques-John-Resig/dp/1590597273) - John Resig
  - [Smashing Node.js: JavaScript Everywhere](http://www.amazon.com/Smashing-Node-js-JavaScript-Everywhere-Magazine/dp/1119962595) - Guillermo Rauch
  - [Secrets of the JavaScript Ninja](http://www.amazon.com/Secrets-JavaScript-Ninja-John-Resig/dp/193398869X) - John Resig and Bear Bibeault
  - [Human JavaScript](http://humanjavascript.com/) - Henrik Joreteg
  - [Superhero.js](http://superherojs.com/) - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
  - [JSBooks](http://jsbooks.revolunet.com/) - Julien Bouquillon
  - [Third Party JavaScript](http://manning.com/vinegar/) - Ben Vinegar and Anton Kovalyov
  - [Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript](http://amzn.com/0321812182) - David Herman
  - [Eloquent JavaScript](http://eloquentjavascript.net/) - Marijn Haverbeke
  - [You Don't Know JS: ES6 & Beyond](http://shop.oreilly.com/product/0636920033769.do) - Kyle Simpson

**Blogs**

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

**Podcasts**

  - [JavaScript Jabber](http://devchat.tv/js-jabber/)


**[⬆ back to top](#table-of-contents)**


## In the Wild

  This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.

  - **Aan Zee**: [AanZee/javascript](https://github.com/AanZee/javascript)
  - **Adult Swim**: [adult-swim/javascript](https://github.com/adult-swim/javascript)
  - **Airbnb**: [airbnb/javascript](https://github.com/airbnb/javascript)
  - **Apartmint**: [apartmint/javascript](https://github.com/apartmint/javascript)
  - **Ascribe**: You're reading it!
  - **Avalara**: [avalara/javascript](https://github.com/avalara/javascript)
  - **Billabong**: [billabong/javascript](https://github.com/billabong/javascript)
  - **Blendle**: [blendle/javascript](https://github.com/blendle/javascript)
  - **ComparaOnline**: [comparaonline/javascript](https://github.com/comparaonline/javascript)
  - **Compass Learning**: [compasslearning/javascript-style-guide](https://github.com/compasslearning/javascript-style-guide)
  - **DailyMotion**: [dailymotion/javascript](https://github.com/dailymotion/javascript)
  - **Digitpaint** [digitpaint/javascript](https://github.com/digitpaint/javascript)
  - **Ecosia**: [ecosia/javascript](https://github.com/ecosia/javascript)
  - **Evernote**: [evernote/javascript-style-guide](https://github.com/evernote/javascript-style-guide)
  - **ExactTarget**: [ExactTarget/javascript](https://github.com/ExactTarget/javascript)
  - **Expensify** [Expensify/Style-Guide](https://github.com/Expensify/Style-Guide/blob/master/javascript.md)
  - **Flexberry**: [Flexberry/javascript-style-guide](https://github.com/Flexberry/javascript-style-guide)
  - **Gawker Media**: [gawkermedia/javascript](https://github.com/gawkermedia/javascript)
  - **General Electric**: [GeneralElectric/javascript](https://github.com/GeneralElectric/javascript)
  - **GoodData**: [gooddata/gdc-js-style](https://github.com/gooddata/gdc-js-style)
  - **Grooveshark**: [grooveshark/javascript](https://github.com/grooveshark/javascript)
  - **How About We**: [howaboutwe/javascript](https://github.com/howaboutwe/javascript)
  - **Huballin**: [huballin/javascript](https://github.com/huballin/javascript)
  - **HubSpot**: [HubSpot/javascript](https://github.com/HubSpot/javascript)
  - **Hyper**: [hyperoslo/javascript-playbook](https://github.com/hyperoslo/javascript-playbook/blob/master/style.md)
  - **InfoJobs**: [InfoJobs/JavaScript-Style-Guide](https://github.com/InfoJobs/JavaScript-Style-Guide)
  - **Intent Media**: [intentmedia/javascript](https://github.com/intentmedia/javascript)
  - **Jam3**: [Jam3/Javascript-Code-Conventions](https://github.com/Jam3/Javascript-Code-Conventions)
  - **JSSolutions**: [JSSolutions/javascript](https://github.com/JSSolutions/javascript)
  - **Kinetica Solutions**: [kinetica/javascript](https://github.com/kinetica/javascript)
  - **Mighty Spring**: [mightyspring/javascript](https://github.com/mightyspring/javascript)
  - **MinnPost**: [MinnPost/javascript](https://github.com/MinnPost/javascript)
  - **MitocGroup**: [MitocGroup/javascript](https://github.com/MitocGroup/javascript)
  - **ModCloth**: [modcloth/javascript](https://github.com/modcloth/javascript)
  - **Money Advice Service**: [moneyadviceservice/javascript](https://github.com/moneyadviceservice/javascript)
  - **Muber**: [muber/javascript](https://github.com/muber/javascript)
  - **National Geographic**: [natgeo/javascript](https://github.com/natgeo/javascript)
  - **National Park Service**: [nationalparkservice/javascript](https://github.com/nationalparkservice/javascript)
  - **Nimbl3**: [nimbl3/javascript](https://github.com/nimbl3/javascript)
  - **Orion Health**: [orionhealth/javascript](https://github.com/orionhealth/javascript)
  - **Peerby**: [Peerby/javascript](https://github.com/Peerby/javascript)
  - **Razorfish**: [razorfish/javascript-style-guide](https://github.com/razorfish/javascript-style-guide)
  - **reddit**: [reddit/styleguide/javascript](https://github.com/reddit/styleguide/tree/master/javascript)
  - **REI**: [reidev/js-style-guide](https://github.com/reidev/js-style-guide)
  - **Ripple**: [ripple/javascript-style-guide](https://github.com/ripple/javascript-style-guide)
  - **SeekingAlpha**: [seekingalpha/javascript-style-guide](https://github.com/seekingalpha/javascript-style-guide)
  - **Shutterfly**: [shutterfly/javascript](https://github.com/shutterfly/javascript)
  - **Springload**: [springload/javascript](https://github.com/springload/javascript)
  - **StudentSphere**: [studentsphere/javascript](https://github.com/studentsphere/javascript)
  - **Target**: [target/javascript](https://github.com/target/javascript)
  - **TheLadders**: [TheLadders/javascript](https://github.com/TheLadders/javascript)
  - **T4R Technology**: [T4R-Technology/javascript](https://github.com/T4R-Technology/javascript)
  - **VoxFeed**: [VoxFeed/javascript-style-guide](https://github.com/VoxFeed/javascript-style-guide)
  - **Weggo**: [Weggo/javascript](https://github.com/Weggo/javascript)
  - **Zillow**: [zillow/javascript](https://github.com/zillow/javascript)
  - **ZocDoc**: [ZocDoc/javascript](https://github.com/ZocDoc/javascript)

**[⬆ back to top](#table-of-contents)**


## Contributors

  - [View Contributors](https://github.com/airbnb/javascript/graphs/contributors)


## License

(The MIT License)

Copyright (c) 2014 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.

**[⬆ back to top](#table-of-contents)**


## Amendments

We encourage you to fork this guide and change the rules to fit your team's style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.