ES6模块化之export和import的用法

//------ lib.js ------
const sqrt = Math.sqrt;
function square(x) {
    return x * x;
}
function diag(x, y) {
    return sqrt(square(x) + square(y));
}

export {sqrt, square, diag}

//------ main.js ------
import { square, diag } from 'lib';				
console.log(square(11)); // 121
console.log(diag(4, 3)); // 5

把export直接加到声明前面就可以省略{} 

//------ lib.js ------
export const sqrt = Math.sqrt;
export function square(x) {
    return x * x;
}
export function diag(x, y) {
    return sqrt(square(x) + square(y));
}

//------ main.js ------
import { square, diag } from 'lib';				
console.log(square(11)); // 121
console.log(diag(4, 3)); // 5

无论怎样导出，引入的时候都需要{}。

别名引入（Aliasing named imports）

当从不同模块引入的变量名相同的时候

import {speak} from './cow.js'
import {speak} from './goat.js'	

这些写法显然会造成混乱

正确的方法是这样的

import {speak as cowSpeak} from './cow.js'
import {speak as goatSpeak} from './goat.js'

import {
  speak as cowSpeak,
  eat as cowEat,
  drink as cowDrink
} from './cow.js'

import {
  speak as goatSpeak,
  eat as goatEat,
  drink as goatDrink
} from './goat.js'

cowSpeak() // moo
cowEat() // cow eats
goatSpeak() // baa
goatDrink() // goat drinks

解决方案就是命名空间引入了

命名空间引入（Namespace imports）

import * as cow from './cow.js'
import * as goat from './goat.js'

cow.speak() // moo
goat.speak() // baa

十分的简洁优雅

默认导出（Default exports）

默认导出就不需要name了，但是一个js文件中只能有一个export default。

//------ myFunc.js ------
export default function () { ... };

//------ main1.js ------
import myFunc from 'myFunc';
myFunc();

其实这种导出方式可以看成是命名到处的变种，只不过把命名写成了default。

虽然export default只能有一个，但也可以导出多个方法。

export default {
  speak () {
    return 'moo'
  },
  eat () {
    return 'cow eats'
  },
  drink () {
    return 'cow drinks'
  }
}

import cow from './default-cow.js'
import goat from './default-goat.js'

cow.speak() // moo
goat.speak() // baa

如果我们在编写模块的时候使用的导出方法不统一，那么引入的时候就需要考虑不同模块引入的方式。这种麻烦可以通过自引用的方法消除。方法如下

编写两种引入方式通用的模块

import * as myself from './ambidextrous-cow.js' // import this file into itself

// this module's own namespace is its default export
export default myself

export function speak () {
  console.log('moo')
}

import cow from './ambidextrous-cow'
import * as alsocow from './ambidextrous-cow'

cow.speak() // moo
alsocow.speak() // moo

import * as myself from './ambidextrous-cow.js' 

export default Object.assign(speak, myself) 

export function speak () {
  console.log('moo')
}

import cow from './ambidextrous-cow'
import * as alsocow from './ambidextrous-cow'

cow() // moo
cow.speak() // moo
alsocow.speak() // moo