Airbnb JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Note: this guide assumes you are using Babel, and requires that you use babel-preset-airbnb or the equivalent. It also assumes you are installing shims/polyfills in your app, with airbnb-browser-shims or the equivalent.

This guide is available in other languages too. See Translation

Other Style Guides

• ES5 (Deprecated)
• React
• CSS-in-JavaScript
• CSS & Sass
• Ruby

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. Control Statements
18. Comments
19. Whitespace
20. Commas
21. Semicolons
22. Type Casting & Coercion
23. Naming Conventions
24. Accessors
25. Events
26. jQuery
27. ECMAScript 5 Compatibility
28. ECMAScript 6+ (ES 2015+) Styles
29. Standard Library
30. Testing
31. Performance
32. Resources
33. In the Wild
34. Translation
35. The JavaScript Style Guide Guide
36. Chat With Us About JavaScript
37. Contributors
38. License
39. Amendments

Types

• 1.1 Primitives: When you access a primitive type you work directly on its value.

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol

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

  • Symbols cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

• 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 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,
  };
  

• 3.3 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;
    },
  };
  

• 3.4 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,
  };
  

• 3.5 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,
  };
  

• 3.6 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,
  };
  

• 3.7 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)).

  // 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 */
  import has from 'has'; // https://www.npmjs.com/package/has
  // ...
  console.log(has.call(object, key));
  

• 3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

  // very bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
  delete copy.a; // so does this
  
  // bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
  
  // good
  const original = { a: 1, b: 2 };
  const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
  
  const { a, ...noA } = copy; // noA => { b: 2, c: 3 }
  

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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 += 1) {
    itemsCopy[i] = items[i];
  }
  
  // good
  const itemsCopy = [...items];
  

• 4.4 To convert an array-like object to an array, use spreads ... instead of Array.from.

  const foo = document.querySelectorAll('.foo');
  
  // good
  const nodes = Array.from(foo);
  
  // best
  const nodes = [...foo];
  

• 4.5 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

  // bad
  const baz = [...foo].map(bar);
  
  // good
  const baz = Array.from(foo, bar);
  

• 4.6 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, 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 - no returned value means `acc` becomes undefined after the first iteration
  [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    acc[index] = flatten;
  });
  
  // good
  [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    acc[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;
  });
  

• 4.7 Use line breaks after open and before close array brackets if an array has multiple lines

  // bad
  const arr = [
    [0, 1], [2, 3], [4, 5],
  ];
  
  const objectInArray = [{
    id: 1,
  }, {
    id: 2,
  }];
  
  const numberInArray = [
    1, 2,
  ];
  
  // good
  const arr = [[0, 1], [2, 3], [4, 5]];
  
  const objectInArray = [
    {
      id: 1,
    },
    {
      id: 2,
    },
  ];
  
  const numberInArray = [
    1,
    2,
  ];
  

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Destructuring

• 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring 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. eslint: prefer-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.';
  

• 6.3 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}?`;
  }
  

• 6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

• 6.5 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 = `my name is '${name}'`;
  

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Functions

• 7.1 Use named function expressions instead of function declarations. eslint: func-style jscs: disallowFunctionDeclarations

  Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. (Discussion)

  // bad
  function foo() {
    // ...
  }
  
  // bad
  const foo = function () {
    // ...
  };
  
  // good
  // lexical name distinguished from the variable-referenced invocation(s)
  const short = function longUniqueMoreDescriptiveLexicalFoo() {
    // ...
  };
  

• 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.

  // 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 foo(name, options, arguments) {
    // ...
  }
  
  // good
  function foo(name, options, args) {
    // ...
  }
  

• 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, 8, 5]));
  
  // good
  new Date(...[2016, 8, 5]);
  

• 7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

  // bad
  function foo(bar,
               baz,
               quux) {
    // ...
  }
  
  // good
  function foo(
    bar,
    baz,
    quux,
  ) {
    // ...
  }
  
  // bad
  console.log(foo,
    bar,
    baz);
  
  // good
  console.log(
    foo,
    bar,
    baz,
  );
  

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Arrow Functions

• 8.1 When you must use an anonymous function (as when passing an inline callback), 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 named function expression.

  // 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 statement returning an expression without side effects, 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,
  }));
  
  // No implicit return with side effects
  function foo(callback) {
    const val = callback();
    if (val === true) {
      // Do something if callback returns true
    }
  }
  
  let bool = false;
  
  // bad
  foo(() => bool = true);
  
  // good
  foo(() => {
    bool = true;
  });
  

• 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
  ['get', 'post', 'put'].map(httpMethod => Object.prototype.hasOwnProperty.call(
      httpMagicObjectWithAVeryLongName,
      httpMethod,
    )
  );
  
  // good
  ['get', 'post', 'put'].map(httpMethod => (
    Object.prototype.hasOwnProperty.call(
      httpMagicObjectWithAVeryLongName,
      httpMethod,
    )
  ));
  

• 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint or do not include disallowParenthesesAroundArrowParam for jscs. 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 don’t want it to take up space on the .map line!`
  ));
  
  // 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; }
  }
  

⬆ back to top

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

  Why? To encourage more files that only ever export one thing, which is better for readability and maintainability.

  // bad
  export function foo() {}
  
  // good
  export default function foo() {}
  

• 10.7 Put all imports above non-import statements. eslint: import/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();
  

• 10.8 Multiline imports should be indented just like multiline array and object literals.

  Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

  // bad
  import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';
  
  // good
  import {
    longNameA,
    longNameB,
    longNameC,
    longNameD,
    longNameE,
  } from 'path';
  

• 10.9 Disallow Webpack loader syntax in module import statements. eslint: import/no-webpack-loader-syntax

  Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.

  // bad
  import fooSass from 'css!sass!foo.scss';
  import barCss from 'style!css!bar.css';
  
  // good
  import fooSass from 'foo.scss';
  import barCss from 'bar.css';
  

⬆ back to top

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;
  
  // bad
  const increasedByOne = [];
  for (let i = 0; i < numbers.length; i++) {
    increasedByOne.push(numbers[i] + 1);
  }
  
  // good
  const increasedByOne = [];
  numbers.forEach((num) => {
    increasedByOne.push(num + 1);
  });
  
  // best (keeping it functional)
  const increasedByOne = numbers.map(num => num + 1);
  

• 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() {
    // ...
  }
  
  // bad
  const bar = function * () {
    // ...
  };
  
  // bad
  const baz = function *() {
    // ...
  };
  
  // bad
  const quux = function*() {
    // ...
  };
  
  // bad
  function*foo() {
    // ...
  }
  
  // bad
  function *foo() {
    // ...
  }
  
  // very bad
  function
  *
  foo() {
    // ...
  }
  
  // very bad
  const wat = function
  *
  () {
    // ...
  };
  
  // good
  function* foo() {
    // ...
  }
  
  // good
  const foo = function* () {
    // ...
  };
  

⬆ back to top

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');
  

• 12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.

  // bad
  const binary = Math.pow(2, 10);
  
  // good
  const binary = 2 ** 10;
  

⬆ back to top

Variables

• 13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const

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

• 13.2 Use one const or let 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. eslint: no-multi-assign

  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); // throws ReferenceError
  console.log(b); // 1
  console.log(c); // 1
  
  // good
  (function example() {
    let a = 1;
    let b = a;
    let c = a;
  }());
  
  console.log(a); // throws ReferenceError
  console.log(b); // throws ReferenceError
  console.log(c); // throws ReferenceError
  
  // the same applies for `const`
  

• 13.6 Avoid using unary increments and decrements (++, –). eslint no-plusplus

  Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

  // bad
  
  const array = [1, 2, 3];
  let num = 1;
  num++;
  --num;
  
  let sum = 0;
  let truthyCount = 0;
  for (let i = 0; i < array.length; i++) {
    let value = array[i];
    sum += value;
    if (value) {
      truthyCount++;
    }
  }
  
  // good
  
  const array = [1, 2, 3];
  let num = 1;
  num += 1;
  num -= 1;
  
  const sum = array.reduce((a, b) => a + b, 0);
  const truthyCount = array.filter(Boolean).length;
  

• 13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

  Why? Linebreaks surrounding = can obfuscate the value of an assignment.

  // bad
  const foo =
    superLongLongLongLongLongLongLongLongFunctionName();
  
  // bad
  const foo
    = 'superLongLongLongLongLongLongLongLongString';
  
  // good
  const foo = (
    superLongLongLongLongLongLongLongLongFunctionName()
  );
  
  // good
  const foo = 'superLongLongLongLongLongLongLongLongString';
  

⬆ back to top

Hoisting

• 14.1 var declarations get hoisted to the top of their closest enclosing function 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 !=. 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 for booleans, but explicit comparisons for strings and numbers.

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

• 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). eslint: no-case-declarations

  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.

  // 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: no-nested-ternary

  // bad
  const foo = maybe1 > maybe2
    ? "bar"
    : value1 > value2 ? "baz" : null;
  
  // split into 2 separated ternary expressions
  const maybeNull = value1 > value2 ? 'baz' : null;
  
  // better
  const foo = maybe1 > maybe2
    ? 'bar'
    : maybeNull;
  
  // best
  const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
  

• 15.7 Avoid unneeded ternary statements. eslint: 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;
  

• 15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (+, -, *, & /) since their precedence is broadly understood. eslint: no-mixed-operators

  Why? This improves readability and clarifies the developer’s intention.

  // bad
  const foo = a && b < 0 || c > 0 || d + 1 === 0;
  
  // bad
  const bar = a ** b - 5 % d;
  
  // bad
  // one may be confused into thinking (a || b) && c
  if (a || b && c) {
    return d;
  }
  
  // good
  const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
  
  // good
  const bar = (a ** b) - (5 % d);
  
  // good
  if (a || (b && c)) {
    return d;
  }
  
  // good
  const bar = a + b / c * d;
  

⬆ back to top

Blocks

• 16.1 Use braces with all multi-line blocks. eslint: nonblock-statement-body-position

  // 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();
  }
  

• 16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

  // bad
  function foo() {
    if (x) {
      return x;
    } else {
      return y;
    }
  }
  
  // bad
  function cats() {
    if (x) {
      return x;
    } else if (y) {
      return y;
    }
  }
  
  // bad
  function dogs() {
    if (x) {
      return x;
    } else {
      if (y) {
        return y;
      }
    }
  }
  
  // good
  function foo() {
    if (x) {
      return x;
    }
  
    return y;
  }
  
  // good
  function cats() {
    if (x) {
      return x;
    }
  
    if (y) {
      return y;
    }
  }
  
  //good
  function dogs(x) {
    if (x) {
      if (z) {
        return y;
      }
    } else {
      return z;
    }
  }
  

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Control Statements

• 17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

  Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

  // bad
  if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
    thing1();
  }
  
  // bad
  if (foo === 123 &&
    bar === 'abc') {
    thing1();
  }
  
  // bad
  if (foo === 123
    && bar === 'abc') {
    thing1();
  }
  
  // bad
  if (
    foo === 123 &&
    bar === 'abc'
  ) {
    thing1();
  }
  
  // good
  if (
    foo === 123
    && bar === 'abc'
  ) {
    thing1();
  }
  
  // good
  if (
    (foo === 123 || bar === 'abc')
    && doesItLookGoodWhenItBecomesThatLong()
    && isThisReallyHappening()
  ) {
    thing1();
  }
  
  // good
  if (foo === 123 && bar === 'abc') {
    thing1();
  }
  

• 17.2 Don’t use selection operators in place of control statements.

  // bad
  !isRunning && startRunning();
  
  // good
  if (!isRunning) {
    startRunning();
  }
  

⬆ back to top

Comments

• 18.1 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) {
  
    // ...
  
    return element;
  }
  
  // good
  /**
   * make() returns a new element
   * based on the passed-in tag name
   */
  function make(tag) {
  
    // ...
  
    return element;
  }
  

• 18.2 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;
  }
  

• 18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

  // bad
  //is current tab
  const active = true;
  
  // good
  // is current tab
  const active = true;
  
  // bad
  /**
   *make() returns a new element
   *based on the passed-in tag name
   */
  function make(tag) {
  
    // ...
  
    return element;
  }
  
  // good
  /**
   * make() returns a new element
   * based on the passed-in tag name
   */
  function make(tag) {
  
    // ...
  
    return element;
  }
  

• 18.4 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.

• 18.5 Use // FIXME: to annotate problems.

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

• 18.6 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

• 19.1 Use soft tabs (space character) set to 2 spaces. eslint: indent jscs: validateIndentation

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

• 19.2 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',
  });
  

• 19.3 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!');
  }
  

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

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

• 19.5 End files with a single newline character. eslint: eol-last

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

  // bad
  import { es6 } from './AirbnbStyleGuide';
    // ...
  export default es6;↵
  ↵
  

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

• 19.6 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);
  

• 19.7 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;
  

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

  // bad
  function bar() {
  
    console.log(foo);
  
  }
  
  // bad
  if (baz) {
  
    console.log(qux);
  } else {
    console.log(foo);
  
  }
  
  // bad
  class Foo {
  
    constructor(bar) {
      this.bar = bar;
    }
  }
  
  // good
  function bar() {
    console.log(foo);
  }
  
  // good
  if (baz) {
    console.log(qux);
  } else {
    console.log(foo);
  }
  

• 19.9 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);
  }
  

• 19.10 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]);
  

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

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

• 19.12 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

• 20.1 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',
  };
  

• 20.2 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 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',
  ];
  
  // bad
  function createHero(
    firstName,
    lastName,
    inventorOf
  ) {
    // does nothing
  }
  
  // good
  function createHero(
    firstName,
    lastName,
    inventorOf,
  ) {
    // does nothing
  }
  
  // good (note that a comma must not appear after a "rest" element)
  function createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  ) {
    // does nothing
  }
  
  // bad
  createHero(
    firstName,
    lastName,
    inventorOf
  );
  
  // good
  createHero(
    firstName,
    lastName,
    inventorOf,
  );
  
  // good (note that a comma must not appear after a "rest" element)
  createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  );
  

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Semicolons

• 21.1 Yup. eslint: semi jscs: requireSemicolons

  Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

  // bad - raises exception
  const luke = {}
  const leia = {}
  [luke, leia].forEach(jedi => jedi.father = 'vader')
  
  // bad - raises exception
  const reaction = "No! That's impossible!"
  (async function meanwhileOnTheFalcon() {
    // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
    // ...
  }())
  
  // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
  function foo() {
    return
      'search your feelings, you know it to be foo'
  }
  
  // good
  const luke = {};
  const leia = {};
  [luke, leia].forEach((jedi) => {
    jedi.father = 'vader';
  });
  
  // good
  const reaction = "No! That's impossible!";
  (async function meanwhileOnTheFalcon() {
    // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
    // ...
  }());
  
  // good
  function foo() {
    return 'search your feelings, you know it to be foo';
  }
  

  Read more.

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Type Casting & Coercion

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

• 22.2 Strings: eslint: no-new-wrappers

  // => this.reviewScore = 9;
  
  // bad
  const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
  
  // 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);
  

• 22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

  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);