类型Types
基本数据类型
- string
- number
- boolean
- null
- undefined
- symbol
const foo = 1;
let bar = foo;
bar = 9;
console.log(foo, bar); // => 1, 9
- Symbols不能真正的被polyfilled，因此当目标浏览器或者环境本地不支持时，不应当使用Symbols.
复杂数据类型
- object
- array
- function
const foo = [1, 2];
const bar = foo;
bar[0] = 9;
console.log(foo[0], bar[0]); // => 9, 9
引用References
- 尽量使用const，避免使用var。eslint: prefer-const,no-const-assign
// bad
var a = 1;
var b = 2;
// good
const a = 1;
const b = 2;
- 如果你必须对变量重新赋值，使用let. eslint: no-var
why? let属于块作用域，var是函数作用域
// bad
var count = 1;
if (true) {
count += 1;
}
// good, use the let.
let count = 1;
if (true) {
count += 1;
}
- 记住let和const都属于块作用域
// 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
对象Objects
- 使用字面量方式创建对象。eslint: no-new-object
// bad
const item = new Object();
// good
const item = {};
- 在创建对象时，定义对象的所有属性
Why?这样对象所有的属性都在同一处定义
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,
};
- 使用简写方式定义对象方法。eslint: object-shorthand
// bad
const atom = {
value: 1,
addValue: function (value) {
return atom.value + value;
},
};
// good
const atom = {
value: 1,
addValue(value) {
return atom.value + value;
},
};
- 使用简写方式定义对象属性。eslint: object-shorthand
why?书写更简洁
const lukeSkywalker = 'Luke Skywalker';
// bad
const obj = {
lukeSkywalker: lukeSkywalker,
};
// good
const obj = {
lukeSkywalker,
};
- 把简写的属性放在对象定义的开头
why?这样更容易判断哪些属性使用了简写
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,
};
- 只对不合法的标志符使用引号
why?通常这样代码更可读，同时语法高亮，且js引擎更容易优化。
// bad
const bad = {
'foo': 3,
'bar': 4,
'data-blah': 5,
};
// good
const good = {
foo: 3,
bar: 4,
'data-blah': 5,
};
- 不要直接调用Object.prototype，同理： hasOwnProperty, propertyIsEnumerable, isPrototypeOf
why?这些方法可能被吞没，比如用Object.create(null)方式创建的对象
// bad
console.log(object.hasOwnProperty(key));
// good
console.log(Object.prototype.hasOwnProperty.call(object, key));
// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
/* or */
import has from 'has';
// ...
console.log(has.call(object, key));
- 使用对象的spread操作符而不是Object.assign方法来浅拷贝对象。使用rest操作符获得一个去除某些属性新的对象。
// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this
// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
const { a, ...noA } = copy; // noA => { b: 2, c: 3 }
数组Arrays
- 使用字面量定义数组 eslint: no-array-constructor
// bad
const items = new Array();
// good
const items = [];
- 使用Array#push方法添加元素
const someStack = [];
// bad
someStack[someStack.length] = 'abracadabra';
// good
someStack.push('abracadabra');
- 使用...操作拷贝数组
// bad
const len = items.length;
const itemsCopy = [];
let i;
for (i = 0; i < len; i += 1) {
itemsCopy[i] = items[i];
}
// good
const itemsCopy = [...items];
- 转换array-like对象为array时，使用...而不是Array.from
const foo = document.querySelectorAll('.foo');
// good
const nodes = Array.from(foo);
// best
const nodes = [...foo];
- 对数组进行map时，使用Array.from替代...。因为前者的方式能避免创建中间数组
// bad
const baz = [...foo].map(bar);
// good
const baz = Array.from(foo, bar);
- 在数组遍历处理的回调函数中，使用返回语句。当回调函数中函数体只有单条语句且不会产生副作用时，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 - no returned value means `memo` becomes undefined after the first iteration
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
const flatten = memo.concat(item);
memo[index] = flatten;
});
// good
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
const flatten = memo.concat(item);
memo[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;
});
- 当数组有多行时，在开始和结束符号均换行
// bad
const arr = [
[0, 1], [2, 3], [4, 5],
];
const objectInArray = [{
id: 1,
}, {
id: 2,
}];
const numberInArray = [
1, 2,
];
// good
const arr = [[0, 1], [2, 3], [4, 5]];
const objectInArray = [
{
id: 1,
},
{
id: 2,
},
];
const numberInArray = [
1,
2,
];
解构Destructuring
- 当访问对象的多个属性时，使用解构方式 eslint: prefer-destructuring
why?解构可以减少临时变量的定义
// 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}`;
}
- 使用数组解构 eslint: prefer-destructuring
const arr = [1, 2, 3, 4];
// bad
const first = arr[0];
const second = arr[1];
// good
const [first, second] = arr;
- 当返回多个值时，使用对象解构方式，而不是数组解构
why?当添加字段或者顺序发生变化时，不依赖于位置
// bad
function processInput(input) {
// then a miracle occurs
return [left, right, top, bottom];
}
// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);
// good
function processInput(input) {
// then a miracle occurs
return { left, right, top, bottom };
}
// the caller selects only the data they need
const { left, top } = processInput(input);
字符串Strings
- 字符串使用单引号' ' eslint: quotes
// bad
const name = "Capt. Janeway";
// bad - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;
// good
const name = 'Capt. Janeway';
- 字符串超过100个字符时，不应该跨行
断开的字符串不易搜索，且不方便
// 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.';
- 拼接字符串时，使用模板字符串 eslint: prefer-template template-curly-spacing
why?模板字符串可读性更强，语法更简洁
// 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, ${name}?`;
}
- 永远不要使用eval()字符串 eslint: no-eval
- 不要多余的转义 eslint: no-useless-escape
why？影响可读性
// bad
const foo = '\'this\' \i\s \"quoted\"';
// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
函数Functions
- ?使用命名函数表达式，而不是函数声明 eslint: func-style
函数声明的方式存在提升，即：无论在哪里声明，效果等同于在函数顶部声明，只要在同一个作用域范围，就视为已经声明，哪怕在声明前就使用，也不会报错。
如果使用函数声明方式定义函数，会影响可读性和可维护性。当函数足够大或者复杂时，对阅读其余代码造成困扰。
// bad
function foo() {
// ...
}
// bad
const foo = function () {
// ...
};
// good
// lexical name distinguished from the variable-referenced invocation(s)
const short = function longUniqueMoreDescriptiveLexicalFoo() {
// ...
};
- 要求 IIFE 使用括号括起来 eslint: wrap-iife
// immediately-invoked function expression (IIFE)
(function () {
console.log('Welcome to the Internet. Please follow me.');
}());
？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
？ 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.');
};
}
- 不要命名参数为arguments。
// bad
function foo(name, options, arguments) {
// ...
}
// good
function foo(name, options, args) {
// ...
}
- 选择缺省参数的方式，避免修改参数
// 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 = {}) {
// ...
}
- 避免缺省参数带来的副作用
var b = 1;
// bad
function count(a = b++) {
console.log(a);
}
count(); // 1
count(); // 2
count(3); // 3
count(); // 3
- 将缺省参数放置最后
// bad
function handleThings(opts = {}, name) {
// ...
}
// good
function handleThings(name, opts = {}) {
// ...
}
- 不要使用Function构造函数创建函数 eslint: no-new-func
// bad
var add = new Function('a', 'b', 'return a + b');
// still bad
var subtract = Function('a', 'b', 'return a - b');
// bad
const f = function(){};
const g = function (){};
const h = function() {};
// good
const x = function () {};
const y = function a() {};
- 不要修改参数eslint: no-param-reassign
why?修改作为参数传入的对象，容易引发不预料的副作用
// bad
function f1(obj) {
obj.key = 1;
}
// good
function f2(obj) {
const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
}
- 不要对参数重新赋值 eslint: no-param-reassign
why?对参数重新赋值会引发不可预料的行为，特别是访问arguments对象时。同时，会引发V8优化问题
// 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) {
// ...
}
- 使用...操作符调用变长函数 eslint: prefer-spread
why?更简洁
// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);
// good
const x = [1, 2, 3, 4, 5];
console.log(...x);
// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));
// good
new Date(...[2016, 8, 5]);
- 函数签名或者函数调用占多行时，每一个参数占一行，并且最后一个参数带逗号结束
// bad
function foo(bar,
baz,
quux) {
// ...
}
// good
function foo(
bar,
baz,
quux,
) {
// ...
}
// bad
console.log(foo,
bar,
baz);
// good
console.log(
foo,
bar,
baz,
);
箭头函数Arrow Functions
- 如果必须使用匿名函数，或者inline 回调函数，使用箭头函数。eslint: prefer-arrow-callback, arrow-spacing
why?语法更简洁，并且this更符合预期
如果函数逻辑相当复杂，应当使用命名函数
// 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;
});
- 如果函数体只有一条语句，且该语句不会产生副作用。使用简写方式，隐式返回；或者使用完整写法，显式return。
eslint: arrow-parens, arrow-body-style
// bad
[1, 2, 3].map(number => {
const nextNumber = number + 1;
`A string containing the ${nextNumber}.`;
});
// good
[1, 2, 3].map(number => `A string containing the ${number}.`);
// good
[1, 2, 3].map((number) => {
const nextNumber = number + 1;
return `A string containing the ${nextNumber}.`;
});
// good
[1, 2, 3].map((number, index) => ({
[index]: number,
}));
// No implicit return with side effects
function foo(callback) {
const val = callback();
if (val === true) {
// Do something if callback returns true
}
}
let bool = false;
// bad
foo(() => bool = true);
// good
foo(() => {
bool = true;
});
- 当表达式占多行时，使用括号括起来增强可读性
why?函数开头和结束更明确
// bad
['get', 'post', 'put'].map(httpMethod => Object.prototype.hasOwnProperty.call(
httpMagicObjectWithAVeryLongName,
httpMethod,
)
);
// good
['get', 'post', 'put'].map(httpMethod => (
Object.prototype.hasOwnProperty.call(
httpMagicObjectWithAVeryLongName,
httpMethod,
)
));
- 如果函数只有一个参数，省略括号，省略花括号。否则，一直使用完整写法，保持一致性。eslint: arrow-parens
// bad
[1, 2, 3].map((x) => x * x);
// good
[1, 2, 3].map(x => x * x);
// good
[1, 2, 3].map(number => (
`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));
// bad
[1, 2, 3].map(x => {
const y = x + 1;
return x * y;
});
// good
[1, 2, 3].map((x) => {
const y = x + 1;
return x * y;
});
- 使用无歧义的=>语法，与<=,>=区分开。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;
类和构造函数 Classes & Constructors
- 使用class，避免直接操作prototype
why?class 语法更简洁，更易懂
// 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;
}
}
- 使用extends 继承
why?这是内置支持的继承原型方法，同时不影响instanceof结果
// 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];
}
}
- 通过return this帮助链式方法调用
// 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);
- 自定义toString方法
class Jedi {
constructor(options = {}) {
this.name = options.name || 'no name';
}
getName() {
return this.name;
}
toString() {
return `Jedi - ${this.getName()}`;
}
}
- class有缺省构造函数，因此没必要定义空的构造函数。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';
}
}
- 避免重复的类成员定义 eslint: no-dupe-class-members
why?后者会静默覆盖前者
// bad
class Foo {
bar() { return 1; }
bar() { return 2; }
}
// good
class Foo {
bar() { return 1; }
}
// good
class Foo {
bar() { return 2; }
}
模块Modules
- 使用modules（import/export)
why?modules是未来的趋势
// 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;
- 不要使用*import
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';
// good
import AirbnbStyleGuide from './AirbnbStyleGuide';
- 不要从import直接export
why?尽管单行的方式更简洁，但是一行import一行export代码一致性更好
// bad
// filename es6.js
export { es6 as default } from './AirbnbStyleGuide';
// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;
- 只从一个地方import eslint: no-duplicate-imports
why?维护性更好
// 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';
- 不要export变值 eslint: import/no-mutable-exports
// bad
let foo = 3;
export { foo };
// good
const foo = 3;
export { foo };
- 模块只有单个export时，倾向使用export default eslint: import/prefer-default-export
why?可读性和可维护性更强
// bad
export function foo() {}
// good
export default function foo() {}
- 把import语句置顶 eslint: import/first
why?因为import行为会提升，因此保持它们放在最上面，防止产生令人奇怪的行为
// bad
import foo from 'foo';
foo.init();
import bar from 'bar';
// good
import foo from 'foo';
import bar from 'bar';
foo.init();
- 多行import，像多行数组和多行对象字面量一样写
why?缩进一致，尾行逗号结束
// bad
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';
// good
import {
longNameA,
longNameB,
longNameC,
longNameD,
longNameE,
} from 'path';
- 在import语句时不要使用webpack loader语法。eslint: import/no-webpack-loader-syntax
// bad
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';
// good
import fooSass from 'foo.scss';
import barCss from 'bar.css';
迭代器与生成器 Iterators and Generators
- 不要使用迭代器。更倾向于使用JavaScript的高阶函数代替循环（如for-in 或者for-of)eslint: no-iterator no-restricted-syntax
why?有利于函数式编程
使用map() / every() / filter() / find() / findIndex() / reduce() / some() / ... 迭代数组, Object.keys() / Object.values() / Object.entries() 生成数组，这样做到可以迭代任何对象
const numbers = [1, 2, 3, 4, 5];
// bad
let sum = 0;
for (let num of numbers) {
sum += num;
}
sum === 15;
// good
let sum = 0;
numbers.forEach((num) => {
sum += num;
});
sum === 15;
// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;
// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
increasedByOne.push(numbers[i] + 1);
}
// good
const increasedByOne = [];
numbers.forEach((num) => {
increasedByOne.push(num + 1);
});
// best (keeping it functional)
const increasedByOne = numbers.map(num => num + 1);
- 暂时不要使用生成器
why?es5编译得不太好
- 如果你必须使用生成器，确保函数签名空格方式正确。eslint: generator-star-spacing
why? function和都是相同概念级别的关键字的一部分。function是唯一的标识符，区别于function。
// bad
function * foo() {
// ...
}
// bad
const bar = function * () {
// ...
};
// bad
const baz = function *() {
// ...
};
// bad
const quux = function*() {
// ...
};
// bad
function*foo() {
// ...
}
// bad
function *foo() {
// ...
}
// very bad
function
*
foo() {
// ...
}
// very bad
const wat = function
*
() {
// ...
};
// good
function* foo() {
// ...
}
// good
const foo = function* () {
// ...
};
属性Properties
- 不要使用引号方式访问属性 eslint: dot-notation
const luke = {
jedi: true,
age: 28,
};
// bad
const isJedi = luke['jedi'];
// good
const isJedi = luke.jedi;
- 通过变量访问对象属性时，使用[]
const luke = {
jedi: true,
age: 28,
};
function getProp(prop) {
return luke[prop];
}
const isJedi = getProp('jedi');
- 使用**操作符进行指数计算 eslint: no-restricted-properties.
// bad
const binary = Math.pow(2, 10);
// good
const binary = 2 ** 10;
变量
- 总是使用const或者let定义变量。避免污染全局命名空间。eslint: no-undef
prefer-const
// bad
superPower = new SuperPower();
// good
const superPower = new SuperPower();
- 一个const或者let定义一个变量 eslint: one-var
why? 更方便添加变量定义，方便debug
// 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';
- const放在一起，let放在一起
why?更容易区分
// 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;
- 需要时再定义变量，并且放在合理的地方
why?let和const属于块作用域
// 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;
}
- 不要嵌套变量赋值
why?嵌套变量赋值隐式创建全局变量
// bad
(function example() {
// JavaScript interprets this as
// let a = ( b = ( c = 1 ) );
// The let keyword only applies to variable a; variables b and c become
// global variables.
let a = b = c = 1;
}());
console.log(a); // throws ReferenceError
console.log(b); // 1
console.log(c); // 1
// good
(function example() {
let a = 1;
let b = a;
let c = a;
}());
console.log(a); // throws ReferenceError
console.log(b); // throws ReferenceError
console.log(c); // throws ReferenceError
// the same applies for `const`
- 禁用一元操作符 ++ 和 -- eslint no-plusplus
why?因为一元操作符 ++ 和 -- 会自动添加分号，不同的空白可能会改变源代码的语义。
// bad
const array = [1, 2, 3];
let num = 1;
num++;
--num;
let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
let value = array[i];
sum += value;
if (value) {
truthyCount++;
}
}
// good
const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;
const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
提升Hoisting
- var声明变量会提升到作用域顶部，赋值操作不会提升。const和let没有类似的提升行为，因此要明白typeof操作不再安全。
// 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;
}
- 匿名函数表达式会提升变量，但是不会提升函数赋值
function example() {
console.log(anonymous); // => undefined
anonymous(); // => TypeError anonymous is not a function
var anonymous = function () {
console.log('anonymous function expression');
};
}
- 命名函数表达式会提升变量，但是不是函数名或者函数体
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');
};
}
- 函数定义提升函数名称和函数体
function example() {
superPower(); // => Flying
function superPower() {
console.log('Flying');
}
}
比较操作符和等于操作Comparison Operators & Equality
- 使用===、!== eslint: eqeqeq
- 条件语句比如if，判断表达式值时，会强制执行ToBoolean，遵循以下规则：
- Object:true
- Undefined:false
- Null:false
- Booleans 取决于本身的值
- Number：+0,-0,NaN为false,其余为true
- String：‘’空字符串为false，其余为true
if ([0] && []) {
// true
// an array (even an empty one) is an object, objects will evaluate to true
}
- 使用简写方式判断，但是对strings和numbers比较使用显式判断
// bad
if (isValid === true) {
// ...
}
// good
if (isValid) {
// ...
}
// bad
if (name) {
// ...
}
// good
if (name !== '') {
// ...
}
// bad
if (collection.length) {
// ...
}
// good
if (collection.length > 0) {
// ...
}
- 使用{}在case、default子句中定义变量、函数和类 eslint: no-case-declarations
// bad
switch (foo) {
case 1:
let x = 1;
break;
case 2:
const y = 2;
break;
case 3:
function f() {
// ...
}
break;
default:
class C {}
}
// good
switch (foo) {
case 1: {
let x = 1;
break;
}
case 2: {
const y = 2;
break;
}
case 3: {
function f() {
// ...
}
break;
}
case 4:
bar();
break;
default: {
class C {}
}
}
- 三元操作放在一行 eslint: no-nested-ternary
// bad
const foo = maybe1 > maybe2
? "bar"
: value1 > value2 ? "baz" : null;
// split into 2 separated ternary expressions
const maybeNull = value1 > value2 ? 'baz' : null;
// better
const foo = maybe1 > maybe2
? 'bar'
: maybeNull;
// best
const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
- 避免不必要的三元操作 eslint: no-unneeded-ternary
// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;
// good
const foo = a || b;
const bar = !!c;
const baz = !c;
- 禁止混合使用不同的操作符 eslint: no-mixed-operators
// bad
const foo = a && b < 0 || c > 0 || d + 1 === 0;
// bad
const bar = a ** b - 5 % d;
// bad
if (a || b && c) {
return d;
}
// good
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
// good
const bar = (a ** b) - (5 % d);
// good
if ((a || b) && c) {
return d;
}
// good
const bar = a + b / c * d;
块Blocks
- ?使用{}
// 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;
}
- 把else与if的结束放在一行 eslint: brace-style
// bad
if (test) {
thing1();
thing2();
}
else {
thing3();
}
// good
if (test) {
thing1();
thing2();
} else {
thing3();
}
- 禁止 if 语句中 return 语句之后有 else 块 eslint: no-else-return
// bad
function foo() {
if (x) {
return x;
} else {
return y;
}
}
// bad
function cats() {
if (x) {
return x;
} else if (y) {
return y;
}
}
// bad
function dogs() {
if (x) {
return x;
} else {
if (y) {
return y;
}
}
}
// good
function foo() {
if (x) {
return x;
}
return y;
}
// good
function cats() {
if (x) {
return x;
}
if (y) {
return y;
}
}
//good
function dogs(x) {
if (x) {
if (z) {
return y;
}
}
return z;
}
控制语句 Control Statements
- 对于控制语句中的条件表达式太长的情况，应当换行处理，把操作符放在每行的开头。
// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
thing1();
}
// bad
if (foo === 123 &&
bar === 'abc') {
thing1();
}
// bad
if (foo === 123
&& bar === 'abc') {
thing1();
}
// bad
if (
foo === 123 &&
bar === 'abc'
) {
thing1();
}
// good
if (
foo === 123
&& bar === 'abc'
) {
thing1();
}
// good
if (
(foo === 123 || bar === "abc")
&& doesItLookGoodWhenItBecomesThatLong()
&& isThisReallyHappening()
) {
thing1();
}
// good
if (foo === 123 && bar === 'abc') {
thing1();
}
注释
- 使用/** ... */多行注释
// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {
// ...
return element;
}
// good
/**
* make() returns a new element
* based on the passed-in tag name
*/
function make(tag) {
// ...
return element;
}
- 单行注释使用//。//放在代码的上面一行，同时，如果不是区块的开头，注释上方空行处理。
// 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;
}
- 所有注释空格开头，增强易读性 eslint: spaced-comment
// bad
//is current tab
const active = true;
// good
// is current tab
const active = true;
// bad
/**
*make() returns a new element
*based on the passed-in tag name
*/
function make(tag) {
// ...
return element;
}
// good
/**
* make() returns a new element
* based on the passed-in tag name
*/
function make(tag) {
// ...
return element;
}
- 使用FIXME或者TODO帮助开发者明白问题。
- 使用 // FIXME 注释问题
class Calculator extends Abacus {
constructor() {
super();
// FIXME: shouldn’t use a global here
total = 0;
}
}
- 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;
}
}
空白Whitespace eslint: indent
- 使用2个空格
// bad
function foo() {
∙∙∙∙let name;
}
// bad
function bar() {
∙let name;
}
// good
function baz() {
∙∙let name;
}
- 在 { 之前空格 eslint: space-before-blocks
// 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',
});
- 在条件判断语句的 ( 之前空格。 eslint: keyword-spacing
// bad
if(isJedi) {
fight ();
}
// good
if (isJedi) {
fight();
}
// bad
function fight () {
console.log ('Swooosh!');
}
// good
function fight() {
console.log('Swooosh!');
}
- 要求操作符周围有空格 eslint: space-infix-ops
// bad
const x=y+5;
// good
const x = y + 5;
- 文件结束时换一行 . eslint: eol-last
// bad
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;
// bad
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;↵
↵
// good
import { es6 } from './AirbnbStyleGuide';
// ...
export default es6;↵
- 要求方法链中每个调用都有一个换行符 eslint: newline-per-chained-call
// 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);
- 在每个代码块的结束位置和下一语句的开头空行
// 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;
- 不要使用空行填充代码块 eslint: padded-blocks
// bad
function bar() {
console.log(foo);
}
// bad
if (baz) {
console.log(qux);
} else {
console.log(foo);
}
// bad
class Foo {
constructor(bar) {
this.bar = bar;
}
}
// good
function bar() {
console.log(foo);
}
// good
if (baz) {
console.log(qux);
} else {
console.log(foo);
}
- 禁止在 () 内有空格 eslint: space-in-parens
// bad
function bar( foo ) {
return foo;
}
// good
function bar(foo) {
return foo;
}
// bad
if ( foo ) {
console.log(foo);
}
// good
if (foo) {
console.log(foo);
}
- 禁止在 [] 内使用空格 eslint: array-bracket-spacing
// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);
// good
const foo = [1, 2, 3];
console.log(foo[0]);
- 在 {} 中使用空格 eslint: object-curly-spacing
// bad
const foo = {clark: 'kent'};
// good
const foo = { clark: 'kent' };
- 强制一行的最大长度100 eslint: max-len
// bad
const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
// good
const foo = jsonData
&& jsonData.foo
&& jsonData.foo.bar
&& jsonData.foo.bar.baz
&& jsonData.foo.bar.baz.quux
&& jsonData.foo.bar.baz.quux.xyzzy;
// good
$.ajax({
method: 'POST',
url: 'https://airbnb.com/',
data: { name: 'John' },
})
.done(() => console.log('Congratulations!'))
.fail(() => console.log('You have failed this city.'));
- 逗号风格：结束位置 eslint: comma-style
// 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',
};
- 末尾逗号 eslint: comma-dangle
why?减少无意义的git diffs
// bad - git diff without trailing comma
const hero = {
firstName: 'Florence',
- lastName: 'Nightingale'
+ lastName: 'Nightingale',
+ inventorOf: ['coxcomb chart', 'modern nursing']
};
// good - git diff with trailing comma
const hero = {
firstName: 'Florence',
lastName: 'Nightingale',
+ inventorOf: ['coxcomb chart', 'modern nursing'],
};
// bad
const hero = {
firstName: 'Dana',
lastName: 'Scully'
};
const heroes = [
'Batman',
'Superman'
];
// good
const hero = {
firstName: 'Dana',
lastName: 'Scully',
};
const heroes = [
'Batman',
'Superman',
];
// bad
function createHero(
firstName,
lastName,
inventorOf
) {
// does nothing
}
// good
function createHero(
firstName,
lastName,
inventorOf,
) {
// does nothing
}
// good (note that a comma must not appear after a "rest" element)
function createHero(
firstName,
lastName,
inventorOf,
...heroArgs
) {
// does nothing
}
// bad
createHero(
firstName,
lastName,
inventorOf
);
// good
createHero(
firstName,
lastName,
inventorOf,
);
// good (note that a comma must not appear after a "rest" element)
createHero(
firstName,
lastName,
inventorOf,
...heroArgs
);
分号 Semicolons
- eslint: semi
// 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;
})());
类型转换和强制类型转换
- ？Perform type coercion at the beginning of the statement.
- Strings:
// => this.reviewScore = 9;
// bad
const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
// bad
const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string
// good
const totalScore = String(this.reviewScore);
- Numbers:使用Number和parseInt 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);
- 不管因为什么原因你必须使用位移操作，增加注释说明
// 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;
- 小心位移操作。数值能表示64位数，但是位移操作只能返回32位整数。
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
- Boolean
const age = 0;
// bad
const hasAge = new Boolean(age);
// good
const hasAge = Boolean(age);
// best
const hasAge = !!age;
命名约定Naming Conventions
- 不要使用一个字母命名 eslint: id-length
// bad
function q() {
// ...
}
// good
function query() {
// ...
}
- 使用驼峰命名对象、函数和实例 eslint: camelcase
// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}
// good
const thisIsMyObject = {};
function thisIsMyFunction() {}
- 使用PascalCase命名构造函数或者Class eslint: new-cap
// 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',
});
- 不要使用 _ 开头或者结束 eslint: no-underscore-dangle
why?JavaScript没有私有属性或者私有方法的概念。使用下划线容易造成误解。
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';
// good
this.firstName = 'Panda';
- 不要保存this的引用。使用箭头函数或者 Function#bind
// 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);
};
}
- 文件名称与export default 相符
// 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
- 当export default一个函数时，使用驼峰命名。文件名也必须一致
function makeStyleGuide() {
// ...
}
export default makeStyleGuide;
- 当export constructor / class / singleton / function library / bare object，使用PascalCase命名方式
const AirbnbStyleGuide = {
es6: {
},
};
export default AirbnbStyleGuide;
- 缩略词必须首字母大写，或者全部小写
// bad
import SmsContainer from './containers/SmsContainer';
// bad
const HttpRequests = [
// ...
];
// good
import SMSContainer from './containers/SMSContainer';
// good
const HTTPRequests = [
// ...
];
// also good
const httpRequests = [
// ...
];
// best
import TextMessageContainer from './containers/TextMessageContainer';
// best
const requests = [
// ...
];
访问器Accessors
- 不需要属性的访问器函数
- 不要使用getters/setters,会产生不可预期的副作用，而且难以维护和测试。
// bad
class Dragon {
get age() {
// ...
}
set age(value) {
// ...
}
}
// good
class Dragon {
getAge() {
// ...
}
setAge(value) {
// ...
}
}
- 如果属性或者方法是boolean，使用 isVal() 或者 hasVal()
// bad
if (!dragon.age()) {
return false;
}
// good
if (!dragon.hasAge()) {
return false;
}
- 创建 get() 和 set() 方法没有问题，但是保持一致性
class Jedi {
constructor(options = {}) {
const lightsaber = options.lightsaber || 'blue';
this.set('lightsaber', lightsaber);
}
set(key, val) {
this[key] = val;
}
get(key) {
return this[key];
}
}
事件Events
- 对于event的handler传递数据时，使用 { key:value... } 方式。这样，当需要传递更多信息时，不需要更改每个handler签名
// bad
$(this).trigger('listingUpdated', listing.id);
// ...
$(this).on('listingUpdated', (e, listingId) => {
// do something with listingId
});
// good
$(this).trigger('listingUpdated', { listingId: listing.id });
// ...
$(this).on('listingUpdated', (e, data) => {
// do something with data.listingId
});
jQuery
- jQuery对象变量使用 $ 区分
// bad
const sidebar = $('.sidebar');
// good
const $sidebar = $('.sidebar');
// good
const $sidebarBtn = $('.sidebar-btn');
- 缓存jQuery lookups
// bad
function setSidebar() {
$('.sidebar').hide();
// ...
$('.sidebar').css({
'background-color': 'pink',
});
}
// good
function setSidebar() {
const $sidebar = $('.sidebar');
$sidebar.hide();
// ...
$sidebar.css({
'background-color': 'pink',
});
}
- Dom查找，使用级联 $('.sidebar ul') 或者父子 $('.sidebar > ul')
- 指定范围进行find
// bad
$('ul', '.sidebar').hide();
// bad
$('.sidebar').find('ul').hide();
// good
$('.sidebar ul').hide();
// good
$('.sidebar > ul').hide();
// good
$sidebar.find('ul').hide();
