Table of Contents

1. Types
2. References
3. Objects
4. Arrays
5. Destructuring
6. Strings
7. Functions
8. Arrow Functions
9. Classes & 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+ (ES 2015+) Styles
28. Testing
29. Performance
30. Resources
31. In the Wild
32. Translation
33. The JavaScript Style Guide Guide
34. Chat With Us About JavaScript
35. Contributors
36. 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 = 9;

    console.log(foo, bar); // => 1, 9

- 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] = 9;

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

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References

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

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

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

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

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

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

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

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

- 2.3 Note that both let and const are block-scoped.

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

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Objects

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

    // bad
    const item = new Object();

    // good
    const item = {};

- 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 optimized by many JS engines.

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

// good
const good = {
foo: 3,
bar: 4,
‘data-blah’: 5,
};
“`

- 3.9 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

“`javascript
// bad
console.log(object.hasOwnProperty(key));

// good
console.log(Object.prototype.hasOwnProperty.call(object, key));

// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
/* or */
const has = require(‘has’);
…
console.log(has.call(object, key));
“`

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Arrays

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

    // bad
    const items = new Array();

    // good
    const items = [];

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

    const someStack = [];

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

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

- 4.3 Use array spreads ... to copy arrays.

    // bad
    const len = items.length;
    const itemsCopy = [];
    let i;

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

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

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

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

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

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

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

    // bad
    const flat = {};
    [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
      const flatten = memo.concat(item);
      flat[index] = flatten;
    });

    // 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;
    });

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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. jscs: disallowArrayDestructuringReturn

> 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, top } = processInput(input);

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Strings

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

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

    // bad - template literals should contain interpolation or newlines
    const name = `Capt. Janeway`;

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

- 6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

> Why? Broken strings are painful to work with and make code less searchable.

    // 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.

- 6.6 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

> Why? Backslashes harm readability, thus they should only be present when necessary.

    // bad
    const foo = '\'this\' \i\s \"quoted\"';

    // good
    const foo = '\'this\' is "quoted"';
    const foo = `'this' is "quoted"`;

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Functions

- 7.1 Use function declarations instead of function expressions. eslint: func-style 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](#arrow-functions) in place of function expressions. ([discussion](https://github.com/airbnb/javascript/issues/794))

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

    // good
    function foo() {
    }

- 7.2 Wrap immediately invoked function expressions in parentheses. 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. eslint: prefer-rest-params

> Why? `...` is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely 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. eslint: no-new-func

> 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. eslint: space-before-function-paren space-before-blocks

> Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

    // 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 optimization 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) {
    }

- 7.14 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

> Why? It's cleaner, you don't need to supply a context, and you can not easily compose `new` with `apply`.

    // bad
    const x = [1, 2, 3, 4, 5];
    console.log.apply(console, x);

    // good
    const x = [1, 2, 3, 4, 5];
    console.log(...x);

    // bad
    new (Function.prototype.bind.apply(Date, [null, 2016, 08, 05]));

    // good
    new Date(...[2016, 08, 05]);

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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.

    // 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}.`;
    });

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

- 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 => (item.height > 256 ? item.largeSize : item.smallSize);

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

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Classes & 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. eslint: 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';
      }
    }

- 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

> Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

    // bad
    class Foo {
      bar() { return 1; }
      bar() { return 2; }
    }

    // good
    class Foo {
      bar() { return 1; }
    }

    // good
    class Foo {
      bar() { return 2; }
    }

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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 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 Do not use wildcard imports.

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

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

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

- 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 './airbnbStyleGuide';

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

- 10.4 Only import from a path in one place.
eslint: no-duplicate-imports

Why? Having multiple lines that import from the same path can make code harder to maintain.

    // bad
    import foo from 'foo';
    // … some other imports … //
    import { named1, named2 } from 'foo';

    // good
    import foo, { named1, named2 } from 'foo';

    // good
    import foo, {
      named1,
      named2,
    } from 'foo';

- 10.5 Do not export mutable bindings.
eslint: import/no-mutable-exports

Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

    // bad
    let foo = 3;
    export { foo }

    // good
    const foo = 3;
    export { foo }

- 10.6 In modules with a single export, prefer default export over named export.
eslint: import/prefer-default-export

    // bad
    export function foo() {}

    // good
    export default function foo() {}

- 10.7 Put all imports above non-import statements.
eslint: import/imports-first

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

    // bad
    import foo from 'foo';
    foo.init();

    import bar from 'bar';

    // good
    import foo from 'foo';
    import bar from 'bar';

    foo.init();

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Iterators and Generators

- 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

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

> Use `map()` / `every()` / `filter()` / `find()` / `findIndex()` / `reduce()` / `some()` / ... to iterate over arrays, and `Object.keys()` / `Object.values()` / `Object.entries()` to produce arrays so you can iterate over objects.

    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.

- 11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: generator-star-spacing

> Why? `function` and `*` are part of the same conceptual keyword - `*` is not a modifier for `function`, `function*` is a unique construct, different from `function`.

    // bad
    function * foo() {
    }

    const bar = function * () {
    }

    const baz = function *() {
    }

    const quux = function*() {
    }

    function*foo() {
    }

    function *foo() {
    }

    // very bad
    function
    *
    foo() {
    }

    const wat = function
    *
    () {
    }

    // good
    function* foo() {
    }

    const foo = function* () {
    }

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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 bracket notation [] when accessing properties with a variable.

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

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

    const isJedi = getProp('jedi');

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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. eslint: no-undef prefer-const

    // 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 swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

    // 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;
    }

- 13.5 Don’t chain variable assignments.

> Why? Chaining variable assignments creates implicit global variables.

    // bad
    (function example() {
      // JavaScript interprets this as
      // let a = ( b = ( c = 1 ) );
      // The let keyword only applies to variable a; variables b and c become
      // global variables.
      let a = b = c = 1;
    }());

    console.log(a); // undefined
    console.log(b); // 1
    console.log(c); // 1

    // good
    (function example() {
      let a = 1;
      let b = a;
      let c = a;
    }());

    console.log(a); // undefined
    console.log(b); // undefined
    console.log(c); // undefined

    // the same applies for `const`

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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');
      }
    }

• For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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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 initialized 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.

    // 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`](http://eslint.org/docs/rules/no-nested-ternary.html).

    // 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`](http://eslint.org/docs/rules/no-unneeded-ternary.html).

    // 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;

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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();
    }

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Comments

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

    // 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
     */
    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;
      }
    }

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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 (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. 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. eslint: eol-last

    // 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
emphasizes 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). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len jscs: maximumLineLength

> Why? This ensures readability and maintainability.

    // bad
    const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

    // 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 = jsonData
      && jsonData.foo
      && jsonData.foo.bar
      && jsonData.foo.bar.baz
      && jsonData.foo.bar.baz.quux
      && jsonData.foo.bar.baz.quux.xyzzy;

    // good
    $.ajax({
      method: 'POST',
      url: 'https://airbnb.com/',
      data: { name: 'John' },
    })
      .done(() => console.log('Congratulations!'))
      .fail(() => console.log('You have failed this city.'));

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

    // bad - git diff without trailing comma
    const hero = {
         firstName: 'Florence',
    -    lastName: 'Nightingale'
    +    lastName: 'Nightingale',
    +    inventorOf: ['coxcomb chart', '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',
    ];

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Semicolons

- 20.1 Yup. eslint: semi jscs: requireSemicolons

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

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

    // good, but legacy (guards against the function becoming an argument when two files with IIFEs are concatenated)
    ;(() => {
      const name = 'Skywalker';
      return name;
    }());

[Read more](http://stackoverflow.com/questions/7365172/semicolon-before-self-invoking-function/7365214%237365214).

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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 + ''; // invokes this.reviewScore.valueOf()

    // bad
    const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string

    // 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);

    // best
    const hasAge = !!age;

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Naming Conventions

- 22.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

    // 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 only 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 Do not use trailing or leading underscores. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores

> Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won't count as breaking, or that tests aren't needed. tl;dr: if you want something to be “private”, it must not be observably present.

    // bad
    this.__firstName__ = 'Panda';
    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 A base filename should exactly match the name of its default export.

    // file 1 contents
    class CheckBox {
      // ...
    }
    export default CheckBox;

    // file 2 contents
    export default function fortyTwo() { return 42; }

    // file 3 contents
    export default function insideDirectory() {}

    // in some other file
    // bad
    import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
    import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
    import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

    // bad
    import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
    import forty_two from './forty_two'; // snake_case import/filename, camelCase export
    import inside_directory from './inside_directory'; // snake_case import, camelCase export
    import index from './inside_directory/index'; // requiring the index file explicitly
    import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

    // good
    import CheckBox from './CheckBox'; // PascalCase export/import/filename
    import fortyTwo from './fortyTwo'; // camelCase export/import/filename
    import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
    // ^ supports both insideDirectory.js and insideDirectory/index.js

- 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 constructor / class / singleton / function library / bare object.

    const AirbnbStyleGuide = {
      es6: {
      }
    };

    export default AirbnbStyleGuide;

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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
    class Dragon {
      get age() {
        // ...
      }

      set age(value) {
        // ...
      }
    }

    // good
    class Dragon {
      getAge() {
        // ...
      }

      setAge(value) {
        // ...
      }
    }

- 23.3 If the property/method 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];
      }
    }

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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
    });

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jQuery

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

    // 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();

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ECMAScript 5 Compatibility

- 26.1 Refer to Kangax’s ES5 compatibility table.

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ECMAScript 6 Styles

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

1. Arrow Functions
2. Classes
3. Object Shorthand
4. Object Concise
5. Object Computed Properties
6. Template Strings
7. Destructuring
8. Default Parameters
9. Rest
10. Array Spreads
11. Let and Const
12. Iterators and Generators
13. Modules

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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 minimize where mutations occur.
- Be cautious about stubs and mocks - they can make your tests more brittle.
- We primarily use mocha at Airbnb. tape is also used occasionally for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
- 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.

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Performance

• On Layout & Web Performance
• String vs Array Concat
• Try/Catch Cost In a Loop
• Bang Function
• jQuery Find vs Context, Selector
• innerHTML vs textContent for script text
• Long String Concatenation
• Are Javascript functions like map(), reduce(), and filter() optimized for traversing arrays?
• Loading…

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Resources

Learning ES6

• Draft ECMA 2015 (ES6) Spec
• ExploringJS
• ES6 Compatibility Table
• Comprehensive Overview of ES6 Features

Read This

• Standard ECMA-262

Tools

• Code Style Linters
  
  • ESlint - Airbnb Style .eslintrc
  • JSHint - Airbnb Style .jshintrc
  • JSCS - Airbnb Style Preset

Other Style Guides

• Google JavaScript Style Guide
• jQuery Core Style Guidelines
• Principles of Writing Consistent, Idiomatic JavaScript

Other Styles

• Naming this in nested functions - Christian Johansen
• Conditional Callbacks - Ross Allen
• Popular JavaScript Coding Conventions on Github - JeongHoon Byun
• Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

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

Books

• JavaScript: The Good Parts - Douglas Crockford
• JavaScript Patterns - Stoyan Stefanov
• Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
• High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
• Maintainable JavaScript - Nicholas C. Zakas
• JavaScript Web Applications - Alex MacCaw
• Pro JavaScript Techniques - John Resig
• Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
• Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
• Human JavaScript - Henrik Joreteg
• Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
• JSBooks - Julien Bouquillon
• Third Party JavaScript - Ben Vinegar and Anton Kovalyov
• Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
• Eloquent JavaScript - Marijn Haverbeke
• You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

• DailyJS
• JavaScript Weekly
• JavaScript, JavaScript…
• Bocoup Weblog
• Adequately Good
• NCZOnline
• Perfection Kills
• Ben Alman
• Dmitry Baranovskiy
• Dustin Diaz
• nettuts

Podcasts

• JavaScript Jabber

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