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Airbnb JavaScript Style Guide() {

用更合理的方式写 JavaScript

译注:
本篇为 ES2015/ES6 版 Airbnb JavaScript 风格规范。你可能需要搭配 Babel 来使用。
我也翻译了 Airbnb JavaScript 风格规范的 ES5 版,适用于国内常见 Web 前端开发环境。

  1. 类型
  2. 引用
  3. 对象
  4. 数组
  5. 解构
  6. 字符串
  7. 函数
  8. Arrow Functions
  9. Classes & Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 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

  • 1.1 原始值:存取直接作用于它自身。

    • string
    • number
    • boolean
    • null
    • undefined
    const foo = 1;
    let bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9

  • 1.2 复杂类型:存取时作用于它自身值的引用。

    • object
    • array
    • function
    const foo = [1, 2];
    const bar = foo;
    
    bar[0] = 9;
    
    console.log(foo[0], bar[0]); // => 9, 9

⬆ 回到顶部

  • 2.1 使用 const 进行引用,避免使用 var。eslint: prefer-const, no-const-assign

    为什么?这将确保你不能对引用重新赋值,从而不会导致出错或让代码难以理解。

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

  • 2.2 如果你必须对引用重新赋值,使用 let 代替 var。eslint: no-var jscs: disallowVar

    为什么?相较于函数作用域的 varlet 的作用域在区块内。

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

  • 2.3 要注意 letconst 都是块级作用域。

    // const 和 let 只存在于它们被定义的区块内。
    {
      let a = 1;
      const b = 1;
    }
    console.log(a); // ReferenceError
    console.log(b); // ReferenceError

⬆ 回到顶部

  • 3.1 使用直接量创建对象。eslint: no-new-object

    // bad
    const item = new Object();
    
    // good
    const item = {};

  • 3.2 如果你的代码将要在浏览器中执行,不要使用保留字作为键名。它们在 IE8 下不会工作。更多信息。但可以在 ES6 模块中或者服务器端代码使用它们。jscs: disallowIdentifierNames

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

  • 3.3 使用可读性强的同义词替换需要使用的保留字。jscs: disallowIdentifierNames

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

  • 3.4 创建含有动态属性名的对象时,使用可被计算的属性名。

    为什么?这样你可以在一个地方定义一个对象的所有属性。

    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 使用对象方法的简写方式。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 使用属性值的简写方式。eslint: object-shorthand jscs: requireEnhancedObjectLiterals

    为什么?这样写起来更短且更有描述性。

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

  • 3.7 把简写的属性值归类放在对象声明的开头。

    为什么?这样更容易表述哪些属性值在使用简写方式。

    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,
    };

为什么?通常我们认为这样写主观上更容易阅读。它改善了语法高亮并且更容易被多数 JS 引擎优化。

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

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

  • 3.9 不要直接调用 Object.prototype 里的方法,比如 hasOwnPropertypropertyIsEnumerable 以及 isPrototypeOf

为什么?这些方法可能会被有问题的对象属性所遮蔽。考虑 { hasOwnProperty: false },或者对象本身是一个空对象(Object.create(null))。

// bad
console.log(object.hasOwnProperty(key));

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

// best
const has = Object.prototype.hasOwnProperty; // 在模块作用域缓存查找结果
/* or */
const has = require('has');

console.log(has.call(object, key));

⬆ 回到顶部

  • 4.1 使用直接量创建数组。eslint: no-array-constructor

    // bad
    const items = new Array();
    
    // good
    const items = [];

  • 4.2 向数组增加元素时使用 Array#push 替代直接赋值。

    const someStack = [];
    
    // bad
    someStack[someStack.length] = 'abracadabra';
    
    // good
    someStack.push('abracadabra');

  • 4.3 使用数组展开 ... 来拷贝数组。

    // 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 使用 Array.from 将类数组对象转换成数组。

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

  • 4.5 在数组方法回调中使用返回语句。如果函数体遵守 8.2 的写法只包含一段语句则可以省略返回。eslint: array-callback-return

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

⬆ 回到顶部

  • 5.1 在访问和使用一个对象的多个属性时使用对象解构。jscs: requireObjectDestructuring

    为什么?解构可以让你免于创建这些属性的临时引用。

    // 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 使用数组解构。jscs: requireArrayDestructuring

    const arr = [1, 2, 3, 4];
    
    // bad
    const first = arr[0];
    const second = arr[1];
    
    // good
    const [first, second] = arr;

  • 5.3 处理多个返回值时使用对象解构而不是数组解构。jscs: disallowArrayDestructuringReturn

    为什么?你可以随着时间的推移不断向里增加新属性或在不破坏调用位置的前提下改变次序。

    // bad
    function processInput(input) {
      // then a miracle occurs
      return [left, right, top, bottom];
    }
    
    // 调用时需要考虑返回数据的顺序
    const [left, __, top] = processInput(input);
    
    // good
    function processInput(input) {
      // then a miracle occurs
      return { left, right, top, bottom };
    }
    
    // 只选择需要的数据调用
    const { left, top } = processInput(input);

⬆ 回到顶部

  • 6.1 使用单引号 '' 包裹字符串。eslint: quotes jscs: validateQuoteMarks

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

  • 6.2 超过 100 个字符的字符串应该使用连接符写成多行。

  • 6.3 注:若过度使用,通过连接符连接的长字符串会影响性能。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 程序化生成的字符串时使用模板字符串替代串联。eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings

    为什么?模板字符串提供了可读并简洁的语法以及正确的换行符和字符串插值特性。

    // 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 永远不要在字符串上使用 eval(),它开启了太多漏洞。

  • 6.6 不要在字符串中没理由的使用转义符。eslint: no-useless-escape

    为什么?反斜线破坏可读性,所以它们只应当在必要时存在。

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

⬆ 回到顶部

  • 7.1 使用函数声明替代函数表达式。jscs: requireFunctionDeclarations

    为什么?函数声明是被命名的,所以他们在调用栈中更容易被识别。此外,函数声明体自身会被提升(hoisted),而函数表达式只有它的引用会被提升。这条规则使得永远使用箭头函数替代函数表达式成为可能。

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

  • 7.2 使用圆括号包括立即调用函数表达式。eslint: wrap-iife jscs: requireParenthesesAroundIIFE

    为什么?一个立即调用函数表达式是一个独立单元 —— 用圆括号将它自身和调用括号包裹在一起来明确表达这一点。注意在一个模块的世界,你几乎永远都不需要立即调用函数。

    // 立即调用函数表达式(IIFE)
    (function () {
      console.log('Welcome to the Internet. Please follow me.');
    }());

  • 7.3 永远不要在非函数代码块(if、while 等)中声明一个函数。通过把函数赋值给一个变量来替代它。虽然浏览器允许这么做,但它们的解析表现不一致。eslint: no-loop-func

  • 7.4 注: ECMA-262 把 定义为一组语句。函数声明不是语句。阅读对 ECMA-262 这个问题的说明

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

  • 7.5 永远不要把参数命名为 arguments。这将取代函数作用域内的 arguments 对象。

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

  • 7.6 永远不要使用 arguments,使用剩余参数语法 ... 来替代。eslint: prefer-rest-params

    为什么?... 明确说明了你想传入的参数。此外,剩余参数是真正的数组,而不像 arguments 仅仅是类数组。

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

  • 7.7 使用默认参数值语法而不是转变函数参数。

    // really bad
    function handleThings(opts) {
      // 错!我们不应该变动函数参数。
      // 再错:如果参数 opts 为否值,它将被设为一个你想要对象,但这将带来隐性 bug。
      opts = opts || {};
      // ...
    }
    
    // still bad
    function handleThings(opts) {
      if (opts === void 0) {
        opts = {};
      }
      // ...
    }
    
    // good
    function handleThings(opts = {}) {
      // ...
    }

  • 7.8 避免采用默认参数时的副作用。

    为什么?它们难以理解。

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

  • 7.9 永远把默认参数放到最后。

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

  • 7.10 永远不要使用函数构造器创建新函数。

    为什么?通过这种方式创建的函数在处理字符串时会像 eval() 一样开启漏洞。

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

  • 7.11 在函数标识中添加空格。

    为什么?保持一致性是好的,而且你不需要在增加或删除函数名是增减空格。

    // bad
    const f = function(){};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function () {};
    const y = function a() {};

  • 7.12 永远不要转变参数。eslint: no-param-reassign

    为什么?操作以参数传入的对象会在原始调用处引起不期望的副作用。

    // bad
    function f1(obj) {
      obj.key = 1;
    };
    
    // good
    function f2(obj) {
      const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    };

  • 7.13 永远不要给参数重新赋值。eslint: no-param-reassign

    为什么?给参数重新赋值会导致预料外的行为,尤其是访问 arguments 对象时。同时它会引起优化问题,尤其在 V8 里。

    // 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) {
    }

⬆ 回到顶部

Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

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

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

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

  • 8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

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

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

  • 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

    Why? It shows clearly where the function starts and ends.

    // bad
    [1, 2, 3].map(number => 'As time went by, the string containing the ' +
      `${number} became much longer. So we needed to break it over multiple ` +
      'lines.'
    );
    
    // good
    [1, 2, 3].map(number => (
      `As time went by, the string containing the ${number} became much ` +
      'longer. So we needed to break it over multiple lines.'
    ));

  • 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

    Why? Less visual clutter.

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

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

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

⬆ 回到顶部

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 10000 -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; }
    }

⬆ 回到顶部

Modules

  • 10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

    Why? Modules are the future, let's start using the future now.

    // bad
    const AirbnbStyleGuide = require('./AirbnbStyleGuide');
    module.exports = AirbnbStyleGuide.es6;
    
    // ok
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    export default AirbnbStyleGuide.es6;
    
    // best
    import { es6 } from './AirbnbStyleGuide';
    export default es6;

  • 10.2 Do not use wildcard imports.

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

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

  • 10.3 And do not export directly from an import.

    Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

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

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

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

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

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

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

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

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

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

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

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

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

⬆ 回到顶部

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.

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

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

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

⬆ 回到顶部

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');

⬆ 回到顶部

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. 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;
    }

⬆ 回到顶部

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.

⬆ 回到顶部

Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=. eslint: eqeqeq

  • 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

    • Objects evaluate to true
    • Undefined evaluates to false
    • Null evaluates to false
    • Booleans evaluate to the value of the boolean
    • Numbers evaluate to false if +0, -0, or NaN, otherwise true
    • Strings evaluate to false if an empty string '', otherwise true
    if ([0] && []) {
      // true
      // an array (even an empty one) is an object, objects will evaluate to true
    }

  • 15.3 Use shortcuts.

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

  • 15.5 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;

⬆ 回到顶部

Blocks

  • 16.1 Use braces with all multi-line blocks.

    // bad
    if (test)
      return false;
    
    // good
    if (test) return false;
    
    // good
    if (test) {
      return false;
    }
    
    // bad
    function foo() { return false; }
    
    // good
    function bar() {
      return false;
    }

  • 16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

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

⬆ 回到顶部

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;
      }
    }

⬆ 回到顶部

Whitespace

  • 18.1 Use soft tabs set to 2 spaces. eslint: indent jscs: validateIndentation

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

  • 18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

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

  • 18.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing jscs: requireSpaceAfterKeywords

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

  • 18.5 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

    // 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);
    }

  • 18.12 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.'));

⬆ 回到顶部

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 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',
    ];

⬆ 回到顶部

Semicolons

  • 20.1 Yup. eslint: semi jscs: requireSemicolons

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

    Read more.

⬆ 回到顶部

Type Casting & Coercion

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

  • 21.2 Strings:

    // => this.reviewScore 
    F438
    = 9;
    
    // bad
    const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
    
    // bad
    const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string
    
    // good
    const totalScore = String(this.reviewScore);

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

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

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

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

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

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

  • 21.6 Booleans:

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

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Naming Conventions

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

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

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

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

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

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

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

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

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

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

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

  • 22.6 A base filename should exactly match the name of its default export.

    // file 1 contents
    class CheckBox {
      // ...
    }
    export default CheckBox;
    
    // file 2 contents
    export default function fortyTwo() { return 42; }
    
    // file 3 contents
    export default function insideDirectory() {}
    
    // in some other file
    // bad
    import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
    import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
    import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
    
    // bad
    import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
    import forty_two from './forty_two'; // snake_case import/filename, camelCase export
    import inside_directory from './inside_directory'; // snake_case import, camelCase export
    import index from './inside_directory/index'; // requiring the index file explicitly
    import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
    
    // good
    import CheckBox from './CheckBox'; // PascalCase export/import/filename
    import fortyTwo from './fortyTwo'; // camelCase export/import/filename
    import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
    // ^ supports both insideDirectory.js and insideDirectory/index.js

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

    function makeStyleGuide() {
    }
    
    export default makeStyleGuide;

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

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

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Accessors

  • 23.1 Accessor functions for properties are not required.

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

    // bad
    class Dragon {
      get age() {
        // ...
      }
    
      set age(value) {
        // ...
      }
    }
    
    // good
    class Dragon {
      getAge() {
        // ...
      }
    
      setAge(value) {
        // ...
      }
    }

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

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

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

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

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Events

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

    // bad
    $(this).trigger('listingUpdated', listing.id);
    
    ...
    
    $(this).on('listingUpdated', (e, listingId) => {
      // do something with listingId
    });

    prefer:

    // good
    $(this).trigger('listingUpdated', { listingId: listing.id });
    
    ...
    
    $(this).on('listingUpdated', (e, data) => {
      // do something with data.listingId
    });

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jQuery

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

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

  • 25.2 Cache jQuery lookups.

    // bad
    function setSidebar() {
      $('.sidebar').hide();
    
      // ...stuff...
    
      $('.sidebar').css({
        'background-color': 'pink'
      });
    }
    
    // good
    function setSidebar() {
      const $sidebar = $('.sidebar');
      $sidebar.hide();
    
      // ...stuff...
    
      $sidebar.css({
        'background-color': 'pink'
      });
    }

  • 25.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

  • 25.4 Use find with scoped jQuery object queries.

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

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ECMAScript 5 Compatibility

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ECMAScript 6 Styles

  • 27.1 This is a collection of links to the various ES6 features.
  1. Arrow Functions
  2. Classes
  3. Object Shorthand
  4. Object Concise
  5. Object Computed Properties
  6. Template Strings
  7. Destructuring
  8. Default Parameters
  9. Rest
  10. Array Spreads
  11. Let and Const
  12. Iterators and Generators
  13. Modules

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Testing

  • 28.1 Yup.

    function foo() {
      return true;
    }

  • 28.2 No, but seriously:
  • Whichever testing framework you use, you should be writing tests!
  • Strive to write many small pure functions, and minimize where mutations occur.
  • Be cautious about stubs and mocks - they can make your tests more brittle.
  • We primarily use mocha at Airbnb. tape is also used occasionally for small, separate modules.
  • 100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
  • Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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Resources

Learning ES6

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

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

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Translation

This style guide is also available in other languages:

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

(The MIT License)

Copyright (c) 2014-2016 Airbnb

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Amendments

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

};

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