25th-floor JavaScript Style Guide

A mostly reasonable approach to JavaScript based on the amazing AirBnB style guide. Thanks guys!

Other Style Guides

• React
• CSS & Sass

Table of Contents

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

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

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

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

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

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

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

  
  function getKey(k) {
      return `a key named ${k}`;
  }
  
  // bad
  const obj = {
      id: 5,
      name: 'San Francisco',
  };
  obj[getKey('enabled')] = true;
  
  // good
  const obj = {
      id: 5,
      name: 'San Francisco',
      [getKey('enabled')]: true,
  };
  

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

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

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

  > Why? It is shorter to write and descriptive.

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

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

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

  const anakinSkywalker = 'Anakin Skywalker';
  const lukeSkywalker = 'Luke Skywalker';
  
  // bad
  const obj = {
      episodeOne: 1,
      twoJediWalkIntoACantina: 2,
      lukeSkywalker,
      episodeThree: 3,
      mayTheFourth: 4,
      anakinSkywalker,
  };
  
  // good
  const obj = {
      lukeSkywalker,
      anakinSkywalker,
      episodeOne: 1,
      twoJediWalkIntoACantina: 2,
      episodeThree: 3,
      mayTheFourth: 4,
  };
  

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

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

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

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

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

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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.

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

  // bad
  function processInput(input) {
      // then a miracle occurs
      return [left, right, top, bottom];
  }
  
  // the caller needs to think about the order of return data
  const [left, __, top] = processInput(input);
  
  // good
  function processInput(input) {
      // then a miracle occurs
      return { left, right, top, bottom };
  }
  
  // the caller selects only the data they need
  const { left, right } = processInput(input);
  

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Strings

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

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

• 6.2 Strings that cause the line to go over 120 characters should be written across multiple lines using string concatenation.

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

  // bad
  const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
  
  // bad
  const errorMessage = 'This is a super long error that was thrown because \
  of Batman. When you stop to think about how Batman had anything to do \
  with this, you would get nowhere \
  fast.';
  
  // good
  const errorMessage = 'This is a super long error that was thrown because ' +
      'of Batman. When you stop to think about how Batman had anything to do ' +
      'with this, you would get nowhere fast.';
  

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

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

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

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

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Functions

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

  > Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.

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

• 7.2 Immediately invoked function expressions: eslint: wrap-iife jscs: requireParenthesesAroundIIFE

  > Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

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

• 7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

• 7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.

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

• 7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

  // bad
  function nope(name, options, arguments) {
      // ...stuff...
  }
  
  // good
  function yup(name, options, args) {
      // ...stuff...
  }
  

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

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

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

• 7.7 Use default parameter syntax rather than mutating function arguments.

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

• 7.8 Avoid side effects with default parameters.

  > Why? They are confusing to reason about.

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

• 7.9 Always put default parameters last.

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

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

  > Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

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

• 7.11 Spacing in a function signature.

  > Why? Consistency is good, 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) {
  }
  

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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.

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

  // 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 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, omit the parentheses. Always include parentheses around multiple arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

  > Why? Less visual clutter.

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

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

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

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Constructors

• 9.1 Always use class. Avoid manipulating prototype directly.

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

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

• 9.2 Use extends for inheritance.

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

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

• 9.3 Methods can return this to help with method chaining.

  // bad
  Jedi.prototype.jump = function () {
      this.jumping = true;
      return true;
  };
  
  Jedi.prototype.setHeight = function (height) {
      this.height = height;
  };
  
  const luke = new Jedi();
  luke.jump(); // => true
  luke.setHeight(20); // => undefined
  
  // good
  class Jedi {
      jump() {
          this.jumping = true;
          return this;
      }
  
      setHeight(height) {
          this.height = height;
          return this;
      }
  }
  
  const luke = new Jedi();
  
  luke.jump()
      .setHeight(20);
  

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

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

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

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

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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;
  

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Iterators and Generators

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

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

  const numbers = [1, 2, 3, 4, 5];
  
  // bad
  let sum = 0;
  for (let num of numbers) {
      sum += num;
  }
  
  sum === 15;
  
  // good
  let sum = 0;
  numbers.forEach(num => sum += num);
  sum === 15;
  
  // best (use the functional force)
  const sum = numbers.reduce((total, num) => total + num, 0);
  sum === 15;
  

• 11.2 Don't use generators for now.

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

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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 subscript 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.

  // 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.

  // 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 Use assignment operator shorthands where possible. eslint: operator-assignment

  > Why? This improves readability, makes it more clear what the expression actually does and eliminates repeated expressions in case of long object property chains.

  // bad
  foo.bar.blubb = foo.bar.blubb + 1;
  
  // good
  foo.bar.blubb += 1;
  

⬆ back to top

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.

⬆ back to top

Comparison Operators & Equality

• 15.1 Use === and !== over == and !=, except for comparisons with null (or undefined). eslint: eqeqeq

  // bad
  if (name == 'John') {
      // ...stuff...
  }
  
  // good
  if (name === 'John') {
      // ...stuff...
  }
  
  // good
  if (name == null) {
      // ...stuff...
  }
  

• 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.

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

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

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

  eslint rules: no-nested-ternary.

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

• 15.7 Avoid unneeded ternary statements.

  eslint rules: no-unneeded-ternary.

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

⬆ back to top

Blocks

• 16.1 Use braces consistently with all multi-line blocks.

  // bad
  if (test)
      return false;
  
  // good
  if (test) return false;
  
  // good
  if (test) {
      return false;
  }
  
  // bad
  if (test) {
      doThis();
  } else doThat();
  
  // good
  if (test) {
      doThis();
  } else {
      doThat();
  }
  
  // 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();
  }
  

⬆ back to top

Comments

• 17.1 Use /** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

  // bad
  // make() returns a new element
  // based on the passed in tag name
  //
  // @param {String} tag
  // @return {Element} element
  function make(tag) {
  
      // ...stuff...
  
      return element;
  }
  
  // good
  /**
   * make() returns a new element
   * based on the passed in tag name
   *
   * @param {String} tag
   * @return {Element} element
   */
  function make(tag) {
  
      // ...stuff...
  
      return element;
  }
  

• 17.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it's on the first line of a block.

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

• 17.3 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME -- need to figure this out or TODO -- need to implement.

• 17.4 Use // FIXME: to annotate problems.

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

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

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

⬆ back to top

Whitespace

• 18.1 Use soft tabs set to 4 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: space-after-keywords, space-before-keywords jscs: requireSpaceAfterKeywords

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

• 18.4 Set off operators with spaces. eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators

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

• 18.5 End files with a single newline character.

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

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

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

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

  // 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: disallowSpacesInsideObjectBrackets

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

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

  > Why? This ensures readability and maintainability.

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

⬆ back to top

Commas

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

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

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

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

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

⬆ back to top

Semicolons

• 20.1 Yup. eslint: semi jscs: requireSemicolons

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

  Read more.

⬆ back to top

Type Casting & Coercion

• 21.1 Perform type coercion at the beginning of the statement.

• 21.2 Strings:

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

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

  const inputValue = '4';
  
  // bad
  const val = new Number(inputValue);
  
  // bad
  const val = +inputValue;
  
  // bad
  const val = inputValue >> 0;
  
  // bad
  const val = parseInt(inputValue);
  
  // good
  const val = Number(inputValue);
  
  // good
  const val = parseInt(inputValue, 10);
  

• 21.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.

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

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

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

• 21.6 Booleans:

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

⬆ back to top

Naming Conventions

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

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

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

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

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

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

• 22.4 Use a leading underscore _ when naming private properties. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores

  // bad
  this.__firstName__ = 'Panda';
  this.firstName_ = 'Panda';
  
  // good
  this._firstName = 'Panda';
  

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

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

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

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

• 22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.

  function makeStyleGuide() {
  }
  
  export default makeStyleGuide;
  

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

  const AirbnbStyleGuide = {
      es6: {
      }
  };
  
  export default AirbnbStyleGuide;
  

⬆ back to top

Accessors

• 23.1 Accessor functions for properties are not required.

• 23.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

  // bad
  dragon.age();
  
  // good
  dragon.getAge();
  
  // bad
  dragon.age(25);
  
  // good
  dragon.setAge(25);
  

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

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

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

  class Jedi {
      constructor(options = {}) {
          const lightsaber = options.lightsaber || 'blue';
          this.set('lightsaber', lightsaber);
      }
  
      set(key, val) {
          this[key] = val;
      }
  
      get(key) {
          return this[key];
      }
  }
  

⬆ back to top

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
  });
  

⬆ back to top

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. [Res