# 模块

## 写法差异

ES6 中常见导出和引人写法如下:

```javascript
// 导出
export default a          // 编译成 CommonJS: exports.default = a (ES 编译成的 CommonJS 形式)

// 引人
import a from './xxx'     // 编译成 CommonJS: require('./xxx').default

// 导出
export { a }              // 编译成 CommonJS: exports.a = a

// 引人
import { a } from './xxx' // 编译成 CommonJS: require('./xxx').a
```

CommonJS 中常见导出和引人写法如下:

```javascript
// 导出
module.exports = a

// 引人
const a = require('./xxx')
```

## **加载机制的差异**

在 CommonJS 的模块中, require 是动态(运行时)加载的；在 ES6 的模块中, import 是静态(编译时)加载的。

> ES6 [import() 动态加载的提案](https://github.com/tc39/proposal-dynamic-import) 目前已到第 3 阶段。

## **获取模块的差异**

在 CommonJS 中, 获取的是一个值/模块的拷贝, 见如下例子:

```javascript
// test.js
let a = 1

function add() {
  a = 2
}

module.exports = { a, add }

// test2.js
const { a, add } = require('./test')

console.log(a)
add()
console.log(a)

// 执行 node test2.js, 输出结果为 1 1
```

在 ES6 中, 获取的是一个值/模块的引用, 见如下例子:

```javascript
// test.js
let a = 1

function add() {
  a = 2
}

export { a, add }

// index.html
import { a, add } from './index.js'

console.log(a)

add()

console.log(a)

// 在浏览器中输出结果为 1 2
```


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