Javascript模块化编程（二）：AMD规范

逃离我推掉我的手 2022-08-01 03:06 226阅读 0赞

转自：http://www.ruanyifeng.com/blog/2012/10/asynchronous\_module\_definition.html

这个系列的[第一部分][Link 1]介绍了Javascript模块的基本写法，今天介绍如何规范地使用模块。

![bg2012103001.jpg][]

（接[上文][Link 1]）

七、模块的规范

先想一想，为什么模块很重要？

因为有了模块，我们就可以更方便地使用别人的代码，想要什么功能，就加载什么模块。

但是，这样做有一个前提，那就是大家必须以同样的方式编写模块，否则你有你的写法，我有我的写法，岂不是乱了套！考虑到Javascript模块现在还没有官方规范，这一点就更重要了。

目前，通行的Javascript模块规范共有两种：[CommonJS][]和[AMD][]。我主要介绍AMD，但是要先从CommonJS讲起。

八、CommonJS

2009年，美国程序员Ryan Dahl创造了[node.js][]项目，将javascript语言用于服务器端编程。

![bg2012103002.jpg][]

这标志"Javascript模块化编程"正式诞生。因为老实说，在浏览器环境下，没有模块也不是特别大的问题，毕竟网页程序的复杂性有限；但是在服务器端，一定要有模块，与操作系统和其他应用程序互动，否则根本没法编程。

node.js的[模块系统][Link 2]，就是参照[CommonJS][]规范实现的。在CommonJS中，有一个全局性方法require()，用于加载模块。假定有一个数学模块math.js，就可以像下面这样加载。

然后，就可以调用模块提供的方法：

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因为这个系列主要针对浏览器编程，不涉及node.js，所以对CommonJS就不多做介绍了。我们在这里只要知道，require()用于加载模块就行了。

九、浏览器环境

有了服务器端模块以后，很自然地，大家就想要客户端模块。而且最好两者能够兼容，一个模块不用修改，在服务器和浏览器都可以运行。

但是，由于一个重大的局限，使得CommonJS规范不适用于浏览器环境。还是上一节的代码，如果在浏览器中运行，会有一个很大的问题，你能看出来吗？

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第二行math.add(2, 3)，在第一行require('math')之后运行，因此必须等math.js加载完成。也就是说，如果加载时间很长，整个应用就会停在那里等。

这对服务器端不是一个问题，因为所有的模块都存放在本地硬盘，可以同步加载完成，等待时间就是硬盘的读取时间。但是，对于浏览器，这却是一个大问题，因为模块都放在服务器端，等待时间取决于网速的快慢，可能要等很长时间，浏览器处于"假死"状态。

因此，浏览器端的模块，不能采用"同步加载"（synchronous），只能采用"异步加载"（asynchronous）。这就是AMD规范诞生的背景。

十、AMD

[AMD][]是"Asynchronous Module Definition"的缩写，意思就是"异步模块定义"。它采用异步方式加载模块，模块的加载不影响它后面语句的运行。所有依赖这个模块的语句，都定义在一个回调函数中，等到加载完成之后，这个回调函数才会运行。

AMD也采用require()语句加载模块，但是不同于CommonJS，它要求两个参数：

第一个参数\[module\]，是一个数组，里面的成员就是要加载的模块；第二个参数callback，则是加载成功之后的回调函数。如果将前面的代码改写成AMD形式，就是下面这样：

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>      </div> 
>     </div> </td> 
>   </tr> 
>  </tbody> 
> </table>

math.add()与math模块加载不是同步的，浏览器不会发生假死。所以很显然，AMD比较适合浏览器环境。

目前，主要有两个Javascript库实现了AMD规范：[require.js][]和[curl.js][]。本系列的第三部分，将通过介绍require.js，进一步讲解AMD的用法，以及如何将模块化编程投入实战。

（完）

[Link 1]: http://www.ruanyifeng.com/blog/2012/10/javascript_module.html
[bg2012103001.jpg]: http://image.beekka.com/blog/201210/bg2012103001.jpg
[CommonJS]: http://wiki.commonjs.org/wiki/Modules/1.1
[AMD]: https://github.com/amdjs/amdjs-api/wiki/AMD
[node.js]: http://nodejs.org/
[bg2012103002.jpg]: http://image.beekka.com/blog/201210/bg2012103002.jpg
[Link 2]: http://nodejs.org/docs/latest/api/modules.html
[require.js]: http://requirejs.org/
[curl.js]: https://github.com/cujojs/curl