25th-style 中文文档教程

发布于 9年前 浏览 25 项目主页 更新于 3年前

25th-floor JavaScript Style Guide

一种最合理的 JavaScript 方法 基于令人惊叹的 AirBnB 风格指南。 谢谢你们!

其他风格指南

Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 Styles
  28. Testing
  29. Performance
  30. Resources
  31. In the Wild
  32. Translation
  33. The JavaScript Style Guide Guide
  34. Chat With Us About JavaScript
  35. Contributors
  36. License

Types

  • 1.1 Primitives:当你访问一个 primitives键入您直接处理它的值。

    • string
    • number
    • boolean
    • null
    • undefined
    const foo = 1;
    let bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9
    
  • 1.2 复杂:当您访问复杂类型时,您会处理对其值的引用。

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

⬆ 返回顶部

References

  • 2.1 对所有引用使用const; 避免使用 var。 eslint:prefer-constno-const-assign

    为什么? 这确保您不能重新分配您的引用,这可能会导致错误和难以理解的代码。

    // bad
    var a = 1;
    var b = 2;
    
    // good
    const a = 1;
    const b = 2;
    
  • 2.2 如果必须重新分配引用,请使用 let 而不是 var。 eslint:no-var jscs:disallowVar

    为什么? let 是块范围的,而不是像 var 那样的函数范围的。

    // bad
    var count = 1;
    if (true) {
        count += 1;
    }
    
    // good, use the let.
    let count = 1;
    if (true) {
        count += 1;
    }
    
  • 2.3 注意 letconst 都是块作用域的.

    // 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
    

⬆ 返回顶部

Objects

  • 3.1 使用文字语法创建对象。 eslint:no-new-object

    // bad
    const item = new Object();
    
    // good
    const item = {};
    
  • 3.2 如果您的代码将在脚本上下文中的浏览器中执行,请不要使用 保留字作为键。 它不会在 IE8 中工作。 更多信息。 可以在 ES6 模块和服务器端代码中使用它们。 jscs:disallowIdentifierNames

    // bad
    const superman = {
        default: { clark: 'kent' },
        private: true,
    };
    
    // good
    const superman = {
        defaults: { clark: 'kent' },
        hidden: true,
    };
    
  • 3.3 使用易读的同义词代替保留字。 jscs:disallowIdentifierNames

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

  • 3.4 在创建具有动态属性名称的对象时使用计算属性名称。

    为什么? 它们允许您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 使用对象方法简写。 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 使用属性值简写。 eslint:object-shorthand jscs:requireEnhancedObjectLiterals

    为什么? 书写和描述性较短。

    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
        lukeSkywalker: lukeSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
    };
    
  • 3.7 在对象声明的开头对速记属性进行分组。

    为什么? 更容易分辨哪些属性正在使用速记。

    const anakinSkywalker = 'Anakin Skywalker';
    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        lukeSkywalker,
        episodeThree: 3,
        mayTheFourth: 4,
        anakinSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
        anakinSkywalker,
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        episodeThree: 3,
        mayTheFourth: 4,
    };
    
  • 3.8 仅引用无效标识符的属性。 eslint:quote-props jscs:disallowQuotedKeysInObjects

为什么? 一般来说,我们认为它在主观上更容易阅读。 它改进了语法高亮,也更容易被许多 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,
  };

⬆ 返回顶部

Arrays

  • 4.1 使用文字语法创建数组。 eslint:no-array-constructor

    // bad
    const items = new Array();
    
    // good
    const items = [];
    
  • 4.2 使用 Array#push 而不是直接赋值来将项目添加到数组中。

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

  • 4.3 使用数组展开 .. . 复制数组。

    // 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 要将类数组对象转换为数组,请使用 Array#from。

    const foo = document.querySelectorAll('.foo');
    const nodes = Array.from(foo);
    
  • 4.5 在数组方法回调中使用 return 语句。 如果函数体由 8.2 之后的单个语句组成,则可以省略 return。 eslint:array-callback-return

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

⬆ 回到顶部

Destructuring

  • 5.1 在以下情况下使用对象解构访问和使用对象的多个属性。 jscs:requireObjectDestructuring

    为什么? 解构使您免于为这些属性创建临时引用。

    // 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 对多个返回值使用对象解构,而不是数组解构。

    为什么? 您可以随着时间的推移添加新属性或更改事物的顺序而不会中断调用站点。

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

⬆ 返回顶部

Strings

  • 6.1 对字符串使用单引号''。 eslint:quotes jscs:validateQuoteMarks

    // bad
    const name = "Capt. Janeway";
    
    // good
    const name = 'Capt. Janeway';
    
  • 6.2 导致该行的字符串超过 120 个字符应使用字符串连接跨多行书写。

  • 6.3 注意:如果过度使用,连接的长字符串可能会影响性能。 jsPerf & 讨论

    // 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 以编程方式构建字符串时,使用模板字符串而不是连接。 eslint:prefer-template template-curly-spacing jscs: requireTemplateStrings

    为什么? 模板字符串为您提供可读、简洁的语法以及适当的换行符和字符串插值功能。

    // 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(),它会打开太多漏洞。

⬆ 返回顶部

Functions

  • 7.1 使用函数声明而不是函数表达式。 jscs:requireFunctionDeclarations

    为什么? 函数声明被命名,因此它们更容易在调用堆栈中识别。 此外,提升了函数声明的整个主体,而仅提升了函数表达式的引用。 这条规则使得始终使用箭头函数代替函数表达式成为可能。

    // bad
    const foo = function () {
    };
    
    // good
    function foo() {
    }
    
  • 7.2 立即调用函数表达式:eslint: wrap-iife jscs:requireParenthesesAroundIIFE< /a>

    为什么? 一个立即调用的函数表达式是一个单一的单元——将它和它的调用括号一起包装在括号中,清楚地表达了这一点。 请注意,在一个到处都是模块的世界中,您几乎不需要 IIFE。

    // immediately-invoked function expression (IIFE)
    (function () {
      console.log('Welcome to the Internet. Please follow me.');
    }());
    
  • 7.3 永远不要在非函数块(if、while 等)中声明函数。 而是将函数分配给变量。 浏览器将允许您这样做,但它们对它的解释都不同,这是个坏消息。 eslint:no-loop-func

  • 7.4 注意:ECMA-262 将定义为语句列表。 函数声明不是语句。 阅读 ECMA-262 关于这个问题的说明

    // bad
    if (currentUser) {
        function test() {
            console.log('Nope.');
        }
    }
    
    // good
    let test;
    if (currentUser) {
        test = () => {
            console.log('Yup.');
        };
    }
    
  • 7.5 永远不要将参数命名为arguments。 这将优先于赋予每个函数范围的 arguments 对象。

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

  • 7.6 永远不要使用参数 , 选择使用 rest 语法 ... 代替。 prefer-rest-params

    为什么? ... 明确说明您要提取哪些参数。 另外,其余参数是一个真正的数组,而不是像 arguments 那样的 Array-like。

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

  • 7.7 使用默认参数语法而不是变异函数争论。

    // 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 使用默认参数避免副作用。

    为什么? 他们很难推理。

    var b = 1;
    // bad
    function count(a = b++) {
        console.log(a);
    }
    count();  // 1
    count();  // 2
    count(3); // 3
    count();  // 3
    
  • 7.9 始终将默认参数放在最后。

    // bad
    function handleThings(opts = {}, name) {
        // ...
    }
    
    // good
    function handleThings(name, opts = {}) {
        // ...
    }
    
  • 7.10 切勿使用 Function 构造函数创建新函数。

    为什么? 以这种方式创建一个函数会评估一个类似于 eval() 的字符串,这会打开漏洞。

    // bad
    var add = new Function('a', 'b', 'return a + b');
    
    // still bad
    var subtract = Function('a', 'b', 'return a - b');
    
  • 7.11 函数签名中的空格。

    为什么? 一致性很好,在添加或删除名称时不必添加或删除空格。

    // bad
    const f = function(){};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function () {};
    const y = function a() {};
    
  • 7.12 永远不要改变参数。 eslint:no-param-reassign

    为什么? 操作作为参数传入的对象可能会在原始调用者中产生不需要的变量副作用。

    // bad
    function f1(obj) {
        obj.key = 1;
    };
    
    // good
    function f2(obj) {
        const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    };
    
  • 7.13 永远不要重新分配参数。 eslint:no-param-reassign

    为什么? 重新分配参数可能会导致意外行为,尤其是在访问 arguments 对象时。 它还可能导致优化问题,尤其是在 V8 中。

    // 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) {
    }
    

⬆ 返回顶部

Arrow Functions

  • 8.1 当您必须使用函数表达式时(如传递匿名函数时),请使用箭头函数表示法。 eslint:prefer-arrow-callback, arrow-spacing jscs:requireArrowFunctions

    为什么? 它创建一个在 this 上下文中执行的函数版本,这通常是您想要的,并且是一种更简洁的语法。

    为什么不? 如果您有一个相当复杂的函数,您可以将该逻辑移出到它自己的函数声明中。

    // 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 如果函数体由单个表达式组成,则省略大括号并使用隐式返回。 否则,保留大括号并使用 return 语句。 eslint:arrow-parensarrow-body-style jscs:disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

    为什么? 语法糖。 当多个函数链接在一起时,它读起来很好。

    为什么不? 如果您打算返回一个对象。

    // good
    [1, 2, 3].map(number => `A string containing the ${number}.`);
    
    // bad
    [1, 2, 3].map(number => {
        const nextNumber = number + 1;
        `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number) => {
        const nextNumber = number + 1;
        return `A string containing the ${nextNumber}.`;
    });
    
  • 8.3 如果表达式跨越多行,请将其括在括号中以提高可读性。

    为什么? 它清楚地显示了函数的开始和结束位置。

    // 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 如果您的函数采用单个参数,请省略括号。 始终在多个参数周围包含括号。 eslint:arrow-parens jscs:disallowParenthesesAroundArrowParam

    为什么? 减少视觉混乱。

    // 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 避免将箭头函数语法 (=>) 与比较运算符 (<=>=)。 eslint:no-confusing-arrow

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

⬆ 回到顶部

Constructors

  • 9.1 始终使用 类。 避免直接操作 prototype

    为什么? class 语法更简洁,更容易推理。

    // 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 使用extends 进行继承。

    为什么? 它是一种在不破坏 instanceof 的情况下继承原型功能的内置方法。

    // 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 方法可以返回 this 以帮助进行方法链接。

    // 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 编写自定义的 toString() 方法是可以的,只要确保它能成功运行并且不会产生副作用即可。

    class Jedi {
        constructor(options = {}) {
            this.name = options.name || 'no name';
        }
    getName() {
        return this.name;
    }
    
    toString() {
        return `Jedi - ${this.getName()}`;
    }
    
    }
  • 9.5 如果没有指定,类有一个默认构造函数。 空构造函数或仅委托给父类的构造函数是不必要的。 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'; } }

⬆ 回到顶部

Modules

  • 10.1 始终使用模块(import/export) 在非标准模块系统上。 您始终可以转换为您喜欢的模块系统。

    为什么? 模块是未来,让我们现在就开始使用未来吧。

    // 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 不要使用通配符导入。

    为什么? 这可确保您有一个默认导出。

    // bad
    import * as AirbnbStyleGuide from './AirbnbStyleGuide';
    
    // good
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    
  • 10.3 并且不要直接从导入导出。

    为什么? 尽管单行代码很简洁,但使用一种清晰的导入方式和一种清晰的导出方式可以使事情保持一致。

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

⬆ 返回顶部

Iterators and Generators

  • 11.1 不要使用迭代器。 更喜欢 JavaScript 的高阶函数,如 map()reduce(),而不是像 for-of 这样的循环。 eslint:no-iterator

    为什么? 这强制执行了我们不变的规则。 处理返回值的纯函数比处理副作用更容易推理。

    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 现在不要使用生成器。

    为什么? 它们不能很好地转换为 ES5。

⬆ 返回顶部

Properties

  • 12.1 访问属性时使用点符号。 eslint:dot-notation jscs:requireDotNotation

    const luke = {
        jedi: true,
        age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;
    
  • 12.2 使用下标使用变量访问属性时的符号 []

    const luke = {
        jedi: true,
        age: 28,
    };
    
    function getProp(prop) {
        return luke[prop];
    }
    
    const isJedi = getProp('jedi');
    

⬆ 返回顶部

Variables

  • 13.1 始终使用const 来声明变量。 不这样做会导致全局变量。 我们希望避免污染全局命名空间。 星球队长警告过我们这一点。

    // bad
    superPower = new SuperPower();
    
    // good
    const superPower = new SuperPower();
    
  • 13.2 每个变量使用一个 const 声明。 eslint:one-var jscs:disallowMultipleVarDecl

    为什么? 以这种方式添加新的变量声明更容易,而且您永远不必担心将 ; 换成 , 或引入仅标点符号的差异。

    // 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 将所有的 const 分组,然后将所有的 lets。

    为什么? 这在稍后您可能需要根据先前分配的变量之一分配变量时很有用。

    // 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 在需要的地方分配变量,但是要放在合理的地方。

    为什么? letconst 是块作用域而不是函数作用域。

    // 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 尽可能使用赋值运算符简写。 eslint:operator-assignment

    为什么? 这提高了可读性,使表达式的实际作用更加清晰,并在长对象属性链的情况下消除了重复的表达式。

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

⬆ 返回顶部

Hoisting

  • 14.1 var 声明被提升到它们作用域的顶部,它们的赋值没有。 constlet 声明被称为 时间死区 (TDZ)。 了解为什么 typeof 不再安全 很重要。

    // 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 匿名函数表达式会提升它们的变量名,但不会提升函数赋值。

    function example() {
        console.log(anonymous); // => undefined
    anonymous(); // =&gt; TypeError anonymous is not a function
    
    var anonymous = function () {
        console.log('anonymous function expression');
    };
    
    }
  • 14.3 命名函数表达式提升变量名,而不是函数名或函数体。

    function example() {
        console.log(named); // => undefined
    named(); // =&gt; TypeError named is not a function
    
    superPower(); // =&gt; 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(); // =&gt; TypeError named is not a function
    
    var named = function named() {
        console.log('named');
    }
    
    }
  • 14.4 函数声明提升了它们的名称和函数体。

    function example() {
        superPower(); // => Flying
    function superPower() {
        console.log('Flying');
    }
    
    }
  • 有关详细信息,请参阅 JavaScript 作用域与提升 Ben Cherry 吊装。

⬆ 返回顶部

Comparison Operators & Equality

  • 15.1 ==!= 上使用 ===!==,比较除外使用 null(或 undefined)。 eslint:eqeqeq

    // bad
    if (name == 'John') {
        // ...stuff...
    }
    
    // good
    if (name === 'John') {
        // ...stuff...
    }
    
    // good
    if (name == null) {
        // ...stuff...
    }
    
  • 15.2 if 等条件语句使用 ToBoolean 抽象方法强制计算它们的表达式,并始终遵循以下简单规则:

    • 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 使用快捷方式。

    // bad
    if (name !== '') {
        // ...stuff...
    }
    
    // good
    if (name) {
        // ...stuff...
    }
    
    // bad
    if (collection.length > 0) {
        // ...stuff...
    }
    
    // good
    if (collection.length) {
        // ...stuff...
    }
    
  • 15.4 有关详细信息,请参阅 Truth Equality 和 JavaScript,作者 Angus Croll。

  • 15.5 casedefault 子句中使用大括号创建块包含词法声明的(例如 letconstfunctionclass)。

为什么? 词法声明在整个 switch 块中可见,但仅在分配时才初始化,这仅在达到其 case 时发生。 当多个 case 子句试图定义同一事物时,这会导致问题。

eslint 规则: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 三元组不应嵌套,通常是单行表达式。

    eslint 规则: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 避免不必要的三元语句。

    eslint 规则: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;
    

⬆ 返回顶部

Blocks

  • 16.1 在所有多行块中始终使用大括号。

    // bad
    if (test)
        return false;
    
    // good
    if (test) return false;
    
    // good
    if (test) {
        return false;
    }
    
    // bad
    if (test) {
        doThis();
    } else doThat();
    
    // good
    if (test) {
        doThis();
    } else {
        doThat();
    }
    
    // bad
    function foo() { return false; }
    
    // good
    function bar() {
        return false;
    }
    
  • 16.2 如果您使用带有 ifelse< 的多行代码块/code>, 把 else 和你的代码放在同一行 if 块的右大括号。 eslint:brace-style jscs:disallowNewlineBeforeBlockStatements

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

⬆ 返回顶部

Comments

  • 17.1 使用 /** ... */ 进行多行注释。 包括描述,为所有参数和返回值指定类型和值。

    // 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 使用 // 进行单行注释。 将单行评论放在评论主题上方的换行符上。 在注释之前放一个空行,除非它在块的第一行。

    // 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 在您的评论前加上FIXMETODO 可以帮助其他开发者快速了解您是在指出需要重新审视的问题,还是在建议需要实施的问题解决方案。 这些与常规评论不同,因为它们是可操作的。 这些操作是FIXME——需要解决这个问题TODO——需要实施

  • 17.4 使用// FIXME: 注释问题。

    class Calculator extends Abacus {
        constructor() {
            super();
        // FIXME: shouldn't use a global here
        total = 0;
    }
    
    }
  • 17.5 使用// TODO: 注释问题的解决方案。

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

⬆ 返回顶部

Whitespace

⬆ 返回顶部

Commas

  • 19.1 前导逗号:没有。 eslint:comma-stylejscs: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 附加尾随逗号:是的。 eslint:comma-dangle jscs: requireTrailingComma

    为什么? 这导致更清晰的 git 差异。 此外,像 Babel 这样的转译器将删除转译代码中的额外尾随逗号,这意味着您不必担心旧版浏览器中的尾随逗号问题 .

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

⬆ 返回顶部

Semicolons

  • 20.1 是的。 eslint:semi jscs : requireSemicolons

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

    阅读更多

⬆ 返回顶部

Type Casting & Coercion

  • 21.1 在语句的开头执行类型转换。

  • 21.2 字符串:

    // => this.reviewScore = 9;
    
    // bad
    const totalScore = this.reviewScore + '';
    
    // good
    const totalScore = String(this.reviewScore);
    
  • 21.3 数字:使用 Number 进行类型转换,使用 parseInt 始终使用基数来解析字符串。 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 如果出于某种原因你正在做一些疯狂的事情并且 parseInt 是你的瓶颈并且需要为 performance reasons,发表评论解释为什么以及你在做什么。

    // 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 注意:使用移位操作时要小心。 数字表示为 64 位值,但移位操作始终返回 32 位整数(来源)。 对于大于 32 位的整数值,位移位可能会导致意外行为。 讨论。 最大的有符号 32 位 Int 是 2,147,483,647:

    2147483647 >> 0 //=> 2147483647
    2147483648 >> 0 //=> -2147483648
    2147483649 >> 0 //=> -2147483647
    
  • 21.6 布尔值:

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

⬆ 返回顶部

Naming Conventions

  • 22.1 避免使用单字母名称。 用你的命名来描述。

    // bad
    function q() {
        // ...stuff...
    }
    
    // good
    function query() {
        // ..stuff..
    }
    
  • 22.2 在命名对象、函数和实例时使用驼峰式命名。 eslint:camelcase jscs:requireCamelCaseOrUpperCaseIdentifiers

    // bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c() {}
    
    // good
    const thisIsMyObject = {};
    function thisIsMyFunction() {}
    
  • 22.3 在命名构造函数或类。 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 使用一个命名私有属性时前导下划线 _。 eslint:no-underscore-dangle jscs:disallowDanglingUnderscores

    // bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    
    // good
    this._firstName = 'Panda';
    
  • 22.5 不要保存对 this 的引用。 使用箭头函数或 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 如果您的文件导出单个类,您的文件名应该正好是该类的名称。

    // file contents
    class CheckBox {
        // ...
    }
    export default CheckBox;
    
    // in some other file
    // bad
    import CheckBox from './checkBox';
    
    // bad
    import CheckBox from './check_box';
    
    // good
    import CheckBox from './CheckBox';
    
  • 22.7 export-default 函数时使用 camelCase。 您的文件名应与您的函数名称相同。

    function makeStyleGuide() {
    }
    
    export default makeStyleGuide;
    
  • 22.8 导出单例/函数库/裸对象时使用 PascalCase。

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

⬆ 返回顶部

Accessors

  • 23.1 Accessor functions for properties are not required.

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

    // bad
    dragon.age();
    
    // good
    dragon.getAge();
    
    // bad
    dragon.age(25);
    
    // good
    dragon.setAge(25);
    
  • 23.3 If the property is a boolean, use isVal() or hasVal().

    // bad
    if (!dragon.age()) {
        return false;
    }
    
    // good
    if (!dragon.hasAge()) {
        return false;
    }
    
  • 23.4 It's okay to create get() and set() functions, but be consistent.

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

⬆ back to top

Events

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

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

    prefer:

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

    ⬆ back to top

jQuery

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

    // bad
    const sidebar = $('.sidebar');
    
    // good
    const $sidebar = $('.sidebar');
    
    // good
    const $sidebarBtn = $('.sidebar-btn');
    
  • 25.2 Cache jQuery lookups.

    // bad
    function setSidebar() {
        $('.sidebar').hide();
    // ...stuff...
    
    $('.sidebar').css({
        'background-color': 'pink'
    });
    
    } // good function setSidebar() { const $sidebar = $('.sidebar'); $sidebar.hide();
    // ...stuff...
    
    $sidebar.css({
        'background-color': 'pink'
    });
    
    }
  • 25.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

  • 25.4 Use find with scoped jQuery object queries.

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

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ECMAScript 5 Compatibility

⬆ back to top

ECMAScript 6 Styles

  • 27.1 This is a collection of links to the various ES6 features.
  1. Arrow Functions
  2. Classes
  3. Object Shorthand
  4. Object Concise
  5. Object Computed Properties
  6. Template Strings
  7. Destructuring
  8. Default Parameters
  9. [Res

25th-floor JavaScript Style Guide

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

Other Style Guides

Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 Styles
  28. Testing
  29. Performance
  30. Resources
  31. In the Wild
  32. Translation
  33. The JavaScript Style Guide Guide
  34. Chat With Us About JavaScript
  35. Contributors
  36. License

Types

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

    • string
    • number
    • boolean
    • null
    • undefined
    const foo = 1;
    let bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9
    
  • 1.2 Complex: When you access a complex type you work on a reference to its value.

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

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References

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

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

    // bad
    var a = 1;
    var b = 2;
    
    // good
    const a = 1;
    const b = 2;
    
  • 2.2 If you must reassign references, use let instead of var. eslint: no-var jscs: disallowVar

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

    // bad
    var count = 1;
    if (true) {
        count += 1;
    }
    
    // good, use the let.
    let count = 1;
    if (true) {
        count += 1;
    }
    
  • 2.3 Note that both let and const are block-scoped.

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

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Objects

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

    // bad
    const item = new Object();
    
    // good
    const item = {};
    
  • 3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs: disallowIdentifierNames

    // bad
    const superman = {
        default: { clark: 'kent' },
        private: true,
    };
    
    // good
    const superman = {
        defaults: { clark: 'kent' },
        hidden: true,
    };
    
  • 3.3 Use readable synonyms in place of reserved words. jscs: disallowIdentifierNames

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

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

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

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

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

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

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

    Why? It is shorter to write and descriptive.

    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
        lukeSkywalker: lukeSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
    };
    
  • 3.7 Group your shorthand properties at the beginning of your object declaration.

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

    const anakinSkywalker = 'Anakin Skywalker';
    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        lukeSkywalker,
        episodeThree: 3,
        mayTheFourth: 4,
        anakinSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
        anakinSkywalker,
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        episodeThree: 3,
        mayTheFourth: 4,
    };
    
  • 3.8 Only quote properties that are invalid identifiers. eslint: quote-props jscs: disallowQuotedKeysInObjects

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

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

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

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Arrays

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

    // bad
    const items = new Array();
    
    // good
    const items = [];
    
  • 4.2 Use Array#push instead of direct assignment to add items to an array.

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

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

    // bad
    const len = items.length;
    const itemsCopy = [];
    let i;
    
    for (i = 0; i < len; i++) {
        itemsCopy[i] = items[i];
    }
    
    // good
    const itemsCopy = [...items];
    
  • 4.4 To convert an array-like object to an array, use Array#from.

    const foo = document.querySelectorAll('.foo');
    const nodes = Array.from(foo);
    
  • 4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint: array-callback-return

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

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Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object. jscs: requireObjectDestructuring

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

    // bad
    function getFullName(user) {
        const firstName = user.firstName;
        const lastName = user.lastName;
    return `${firstName} ${lastName}`;
    
    } // good function getFullName(user) { const { firstName, lastName } = user; return `${firstName} ${lastName}`; } // best function getFullName({ firstName, lastName }) { return `${firstName} ${lastName}`; }
  • 5.2 Use array destructuring. jscs: requireArrayDestructuring

    const arr = [1, 2, 3, 4];
    
    // bad
    const first = arr[0];
    const second = arr[1];
    
    // good
    const [first, second] = arr;
    
  • 5.3 Use object destructuring for multiple return values, not array destructuring.

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

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

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Strings

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

    // bad
    const name = "Capt. Janeway";
    
    // good
    const name = 'Capt. Janeway';
    
  • 6.2 Strings that cause the line to go over 120 characters should be written across multiple lines using string concatenation.

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

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

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

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

    // bad
    function sayHi(name) {
        return 'How are you, ' + name + '?';
    }
    
    // bad
    function sayHi(name) {
        return ['How are you, ', name, '?'].join();
    }
    
    // bad
    function sayHi(name) {
        return `How are you, ${ name }?`;
    }
    
    // good
    function sayHi(name) {
        return `How are you, ${name}?`;
    }
    
  • 6.5 Never use eval() on a string, it opens too many vulnerabilities.

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Functions

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

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

    // bad
    const foo = function () {
    };
    
    // good
    function foo() {
    }
    
  • 7.2 Immediately invoked function expressions: eslint: wrap-iife jscs: requireParenthesesAroundIIFE

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

    // immediately-invoked function expression (IIFE)
    (function () {
      console.log('Welcome to the Internet. Please follow me.');
    }());
    
  • 7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

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

    // bad
    if (currentUser) {
        function test() {
            console.log('Nope.');
        }
    }
    
    // good
    let test;
    if (currentUser) {
        test = () => {
            console.log('Yup.');
        };
    }
    
  • 7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

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

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

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

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

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

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

    Why? They are confusing to reason about.

    var b = 1;
    // bad
    function count(a = b++) {
        console.log(a);
    }
    count();  // 1
    count();  // 2
    count(3); // 3
    count();  // 3
    
  • 7.9 Always put default parameters last.

    // bad
    function handleThings(opts = {}, name) {
        // ...
    }
    
    // good
    function handleThings(name, opts = {}) {
        // ...
    }
    
  • 7.10 Never use the Function constructor to create a new function.

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

    // bad
    var add = new Function('a', 'b', 'return a + b');
    
    // still bad
    var subtract = Function('a', 'b', 'return a - b');
    
  • 7.11 Spacing in a function signature.

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

    // bad
    const f = function(){};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function () {};
    const y = function a() {};
    
  • 7.12 Never mutate parameters. eslint: no-param-reassign

    Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

    // bad
    function f1(obj) {
        obj.key = 1;
    };
    
    // good
    function f2(obj) {
        const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    };
    
  • 7.13 Never reassign parameters. eslint: no-param-reassign

    Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

    // bad
    function f1(a) {
        a = 1;
    }
    
    function f2(a) {
        if (!a) { a = 1; }
    }
    
    // good
    function f3(a) {
        const b = a || 1;
    }
    
    function f4(a = 1) {
    }
    

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Arrow Functions

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

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

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

    // bad
    [1, 2, 3].map(function (x) {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map(x => {
        const y = x + 1;
        return x * y;
    });
    
  • 8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

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

    Why not? If you plan on returning an object.

    // good
    [1, 2, 3].map(number => `A string containing the ${number}.`);
    
    // bad
    [1, 2, 3].map(number => {
        const nextNumber = number + 1;
        `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number) => {
        const nextNumber = number + 1;
        return `A string containing the ${nextNumber}.`;
    });
    
  • 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

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

    // bad
    [1, 2, 3].map(number => 'As time went by, the string containing the ' +
        `${number} became much longer. So we needed to break it over multiple ` +
        'lines.'
    );
    
    // good
    [1, 2, 3].map(number => (
        `As time went by, the string containing the ${number} became much ` +
        'longer. So we needed to break it over multiple lines.'
    ));
    
  • 8.4 If your function takes a single argument, omit the parentheses. Always include parentheses around multiple arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

    Why? Less visual clutter.

    // bad
    [1, 2, 3].map((x) => x * x);
    
    // good
    [1, 2, 3].map(x => x * x);
    
    // good
    [1, 2, 3].map(number => (
        `A long string with the ${number}. It’s so long that we’ve broken it ` +
        'over multiple lines!'
    ));
    
    // bad
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map(x => {
        const y = x + 1;
        return x * y;
    });
    
  • 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

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

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Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

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

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

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

    // bad
    const inherits = require('inherits');
    function PeekableQueue(contents) {
        Queue.apply(this, contents);
    }
    inherits(PeekableQueue, Queue);
    PeekableQueue.prototype.peek = function () {
        return this._queue[0];
    }
    
    // good
    class PeekableQueue extends Queue {
        peek() {
            return this._queue[0];
        }
    }
    
  • 9.3 Methods can return this to help with method chaining.

    // bad
    Jedi.prototype.jump = function () {
        this.jumping = true;
        return true;
    };
    
    Jedi.prototype.setHeight = function (height) {
        this.height = height;
    };
    
    const luke = new Jedi();
    luke.jump(); // => true
    luke.setHeight(20); // => undefined
    
    // good
    class Jedi {
        jump() {
            this.jumping = true;
            return this;
        }
    setHeight(height) {
        this.height = height;
        return this;
    }
    
    } const luke = new Jedi(); luke.jump() .setHeight(20);
  • 9.4 It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

    class Jedi {
        constructor(options = {}) {
            this.name = options.name || 'no name';
        }
    getName() {
        return this.name;
    }
    
    toString() {
        return `Jedi - ${this.getName()}`;
    }
    
    }
  • 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. no-useless-constructor

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

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Modules

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

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

    // bad
    const AirbnbStyleGuide = require('./AirbnbStyleGuide');
    module.exports = AirbnbStyleGuide.es6;
    
    // ok
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    export default AirbnbStyleGuide.es6;
    
    // best
    import { es6 } from './AirbnbStyleGuide';
    export default es6;
    
  • 10.2 Do not use wildcard imports.

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

    // bad
    import * as AirbnbStyleGuide from './AirbnbStyleGuide';
    
    // good
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    
  • 10.3 And do not export directly from an import.

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

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

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Iterators and Generators

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

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

    const numbers = [1, 2, 3, 4, 5];
    
    // bad
    let sum = 0;
    for (let num of numbers) {
        sum += num;
    }
    
    sum === 15;
    
    // good
    let sum = 0;
    numbers.forEach(num => sum += num);
    sum === 15;
    
    // best (use the functional force)
    const sum = numbers.reduce((total, num) => total + num, 0);
    sum === 15;
    
  • 11.2 Don't use generators for now.

    Why? They don't transpile well to ES5.

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Properties

  • 12.1 Use dot notation when accessing properties. eslint: dot-notation jscs: requireDotNotation

    const luke = {
        jedi: true,
        age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;
    
  • 12.2 Use subscript notation [] when accessing properties with a variable.

    const luke = {
        jedi: true,
        age: 28,
    };
    
    function getProp(prop) {
        return luke[prop];
    }
    
    const isJedi = getProp('jedi');
    

⬆ back to top

Variables

  • 13.1 Always use const to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.

    // bad
    superPower = new SuperPower();
    
    // good
    const superPower = new SuperPower();
    
  • 13.2 Use one const declaration per variable. eslint: one-var jscs: disallowMultipleVarDecl

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

    // bad
    const items = getItems(),
        goSportsTeam = true,
        dragonball = 'z';
    
    // bad
    // (compare to above, and try to spot the mistake)
    const items = getItems(),
        goSportsTeam = true;
        dragonball = 'z';
    
    // good
    const items = getItems();
    const goSportsTeam = true;
    const dragonball = 'z';
    
  • 13.3 Group all your consts and then group all your lets.

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

    // bad
    let i, len, dragonball,
        items = getItems(),
        goSportsTeam = true;
    
    // bad
    let i;
    const items = getItems();
    let dragonball;
    const goSportsTeam = true;
    let len;
    
    // good
    const goSportsTeam = true;
    const items = getItems();
    let dragonball;
    let i;
    let length;
    
  • 13.4 Assign variables where you need them, but place them in a reasonable place.

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

    // bad - unnecessary function call
    function checkName(hasName) {
        const name = getName();
    if (hasName === 'test') {
        return false;
    }
    
    if (name === 'test') {
        this.setName('');
        return false;
    }
    
    return name;
    
    } // good function checkName(hasName) { if (hasName === 'test') { return false; }
    const name = getName();
    
    if (name === 'test') {
        this.setName('');
        return false;
    }
    
    return name;
    
    }
  • 13.5 Use assignment operator shorthands where possible. eslint: operator-assignment

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

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

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Hoisting

  • 14.1 var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

    // we know this wouldn't work (assuming there
    // is no notDefined global variable)
    function example() {
        console.log(notDefined); // => throws a ReferenceError
    }
    
    // creating a variable declaration after you
    // reference the variable will work due to
    // variable hoisting. Note: the assignment
    // value of `true` is not hoisted.
    function example() {
        console.log(declaredButNotAssigned); // => undefined
        var declaredButNotAssigned = true;
    }
    
    // the interpreter is hoisting the variable
    // declaration to the top of the scope,
    // which means our example could be rewritten as:
    function example() {
        let declaredButNotAssigned;
        console.log(declaredButNotAssigned); // => undefined
        declaredButNotAssigned = true;
    }
    
    // using const and let
    function example() {
        console.log(declaredButNotAssigned); // => throws a ReferenceError
        console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
        const declaredButNotAssigned = true;
    }
    
  • 14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

    function example() {
        console.log(anonymous); // => undefined
    anonymous(); // =&gt; 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(); // =&gt; TypeError named is not a function
    
    superPower(); // =&gt; 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(); // =&gt; TypeError named is not a function
    
    var named = function named() {
        console.log('named');
    }
    
    }
  • 14.4 Function declarations hoist their name and the function body.

    function example() {
        superPower(); // => Flying
    function superPower() {
        console.log('Flying');
    }
    
    }
  • For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

⬆ back to top

Comparison Operators & Equality

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

    // bad
    if (name == 'John') {
        // ...stuff...
    }
    
    // good
    if (name === 'John') {
        // ...stuff...
    }
    
    // good
    if (name == null) {
        // ...stuff...
    }
    
  • 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

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

    // bad
    if (name !== '') {
        // ...stuff...
    }
    
    // good
    if (name) {
        // ...stuff...
    }
    
    // bad
    if (collection.length > 0) {
        // ...stuff...
    }
    
    // good
    if (collection.length) {
        // ...stuff...
    }
    
  • 15.4 For more information see Truth Equality and JavaScript by Angus Croll.

  • 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

eslint rules: no-case-declarations.

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

// good
switch (foo) {
    case 1: {
        let x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    case 3: {
        function f() {}
        break;
    }
    case 4:
        bar();
        break;
    default: {
        class C {}
    }
}
```
  • 15.6 Ternaries should not be nested and generally be single line expressions.

    eslint rules: no-nested-ternary.

    // bad
    const foo = maybe1 > maybe2
        ? "bar"
        : value1 > value2 ? "baz" : null;
    
    // better
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    const foo = maybe1 > maybe2
        ? 'bar'
        : maybeNull;
    
    // best
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
    
  • 15.7 Avoid unneeded ternary statements.

    eslint rules: no-unneeded-ternary.

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

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Blocks

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

    // bad
    if (test)
        return false;
    
    // good
    if (test) return false;
    
    // good
    if (test) {
        return false;
    }
    
    // bad
    if (test) {
        doThis();
    } else doThat();
    
    // good
    if (test) {
        doThis();
    } else {
        doThat();
    }
    
    // bad
    function foo() { return false; }
    
    // good
    function bar() {
        return false;
    }
    
  • 16.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

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

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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;
    }
    
    }

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Whitespace

  • 18.1 Use soft tabs set to 4 spaces. eslint: indent jscs: validateIndentation

    // bad
    function foo() {
    ∙∙const name;
    }
    
    // bad
    function bar() {
    ∙∙∙∙∙∙const name;
    }
    
    // good
    function baz() {
    ∙∙∙∙const name;
    }
    
  • 18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

    // bad
    function test(){
        console.log('test');
    }
    
    // good
    function test() {
        console.log('test');
    }
    
    // bad
    dog.set('attr',{
        age: '1 year',
        breed: 'Bernese Mountain Dog',
    });
    
    // good
    dog.set('attr', {
        age: '1 year',
        breed: 'Bernese Mountain Dog',
    });
    
  • 18.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: space-after-keywords, space-before-keywords jscs: requireSpaceAfterKeywords

    // bad
    if(isJedi) {
        fight ();
    }
    
    // good
    if (isJedi) {
        fight();
    }
    
    // bad
    function fight () {
        console.log ('Swooosh!');
    }
    
    // good
    function fight() {
        console.log('Swooosh!');
    }
    
  • 18.4 Set off operators with spaces. eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators

    // bad
    const x=y+5;
    
    // good
    const x = y + 5;
    
  • 18.5 End files with a single newline character.

    // bad
    (function (global) {
        // ...stuff...
    })(this);
    
    // bad
    (function (global) {
        // ...stuff...
    })(this);↵
    ↵
    
    // good
    (function (global) {
        // ...stuff...
    })(this);↵
    
  • 18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property dot-location

    // bad
    $('#items').find('.selected').highlight().end().find('.open').updateCount();
    
    // bad
    $('#items').
        find('.selected').
            highlight().
            end().
        find('.open').
            updateCount();
    
    // good
    $('#items')
        .find('.selected')
            .highlight()
            .end()
        .find('.open')
            .updateCount();
    
    // bad
    const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
        .attr('width', (radius + margin) * 2).append('svg:g')
        .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
        .call(tron.led);
    
    // good
    const leds = stage.selectAll('.led')
            .data(data)
        .enter().append('svg:svg')
            .classed('led', true)
            .attr('width', (radius + margin) * 2)
        .append('svg:g')
            .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
            .call(tron.led);
    
    // good
    const leds = stage.selectAll('.led').data(data);
    
  • 18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks

    // bad
    if (foo) {
        return bar;
    }
    return baz;
    
    // good
    if (foo) {
        return bar;
    }
    
    return baz;
    
    // bad
    const obj = {
        foo() {
        },
        bar() {
        },
    };
    return obj;
    
    // good
    const obj = {
        foo() {
        },
    bar() {
    },
    
    }; return obj; // bad const arr = [ function foo() { }, function bar() { }, ]; return arr; // good const arr = [ function foo() { },
    function bar() {
    },
    
    ]; return arr;
  • 18.8 Do not pad your blocks with blank lines. eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

    // bad
    function bar() {
    console.log(foo);
    
    } // also bad if (baz) {
    console.log(qux);
    
    } else { console.log(foo); } // good function bar() { console.log(foo); } // good if (baz) { console.log(qux); } else { console.log(foo); }
  • 18.9 Do not add spaces inside parentheses. eslint: space-in-parens jscs: disallowSpacesInsideParentheses

    // bad
    function bar( foo ) {
        return foo;
    }
    
    // good
    function bar(foo) {
        return foo;
    }
    
    // bad
    if ( foo ) {
        console.log(foo);
    }
    
    // good
    if (foo) {
        console.log(foo);
    }
    
  • 18.10 Do not add spaces inside brackets. eslint: array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets

    // bad
    const foo = [ 1, 2, 3 ];
    console.log(foo[ 0 ]);
    
    // good
    const foo = [1, 2, 3];
    console.log(foo[0]);
    
  • 18.11 Add spaces inside curly braces. eslint: object-curly-spacing jscs: disallowSpacesInsideObjectBrackets

    // bad
    const foo = {clark: 'kent'};
    
    // good
    const foo = { clark: 'kent' };
    
  • 18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len jscs: maximumLineLength

    Why? This ensures readability and maintainability.

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

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Commas

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

    // bad
    const story = [
        once
      , upon
      , aTime
    ];
    
    // good
    const story = [
        once,
        upon,
        aTime,
    ];
    
    // bad
    const hero = {
          firstName: 'Ada'
        , lastName: 'Lovelace'
        , birthYear: 1815
        , superPower: 'computers'
    };
    
    // good
    const hero = {
        firstName: 'Ada',
        lastName: 'Lovelace',
        birthYear: 1815,
        superPower: 'computers',
    };
    
  • 19.2 Additional trailing comma: Yup. eslint: comma-dangle jscs: requireTrailingComma

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

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

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Semicolons

  • 20.1 Yup. eslint: semi jscs: requireSemicolons

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

    Read more.

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Type Casting & Coercion

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

  • 21.2 Strings:

    // => this.reviewScore = 9;
    
    // bad
    const totalScore = this.reviewScore + '';
    
    // good
    const totalScore = String(this.reviewScore);
    
  • 21.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix

    const inputValue = '4';
    
    // bad
    const val = new Number(inputValue);
    
    // bad
    const val = +inputValue;
    
    // bad
    const val = inputValue >> 0;
    
    // bad
    const val = parseInt(inputValue);
    
    // good
    const val = Number(inputValue);
    
    // good
    const val = parseInt(inputValue, 10);
    
  • 21.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.

    // good
    /**
     * parseInt was the reason my code was slow.
     * Bitshifting the String to coerce it to a
     * Number made it a lot faster.
     */
    const val = inputValue >> 0;
    
  • 21.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:

    2147483647 >> 0 //=> 2147483647
    2147483648 >> 0 //=> -2147483648
    2147483649 >> 0 //=> -2147483647
    
  • 21.6 Booleans:

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

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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 when naming constructors or classes. eslint: new-cap jscs: requireCapitalizedConstructors

    // bad
    function user(options) {
        this.name = options.name;
    }
    
    const bad = new user({
        name: 'nope',
    });
    
    // good
    class User {
        constructor(options) {
            this.name = options.name;
        }
    }
    
    const good = new User({
        name: 'yup',
    });
    
  • 22.4 Use a leading underscore _ when naming private properties. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores

    // bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    
    // good
    this._firstName = 'Panda';
    
  • 22.5 Don't save references to this. Use arrow functions or Function#bind. jscs: disallowNodeTypes

    // bad
    function foo() {
        const self = this;
        return function () {
            console.log(self);
        };
    }
    
    // bad
    function foo() {
        const that = this;
        return function () {
            console.log(that);
        };
    }
    
    // good
    function foo() {
        return () => {
            console.log(this);
        };
    }
    
  • 22.6 If your file exports a single class, your filename should be exactly the name of the class.

    // file contents
    class CheckBox {
        // ...
    }
    export default CheckBox;
    
    // in some other file
    // bad
    import CheckBox from './checkBox';
    
    // bad
    import CheckBox from './check_box';
    
    // good
    import CheckBox from './CheckBox';
    
  • 22.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.

    function makeStyleGuide() {
    }
    
    export default makeStyleGuide;
    
  • 22.8 Use PascalCase when you export a singleton / function library / bare object.

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

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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
    dragon.age();
    
    // good
    dragon.getAge();
    
    // bad
    dragon.age(25);
    
    // good
    dragon.setAge(25);
    
  • 23.3 If the property is a boolean, use isVal() or hasVal().

    // bad
    if (!dragon.age()) {
        return false;
    }
    
    // good
    if (!dragon.hasAge()) {
        return false;
    }
    
  • 23.4 It's okay to create get() and set() functions, but be consistent.

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

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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. [Res
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