Detailed explanation of the usage of require.js_javascript skills

1. Why use require.js?

In the earliest days, all Javascript codes were written in one file, and it was enough to load this one file. Later, there were more and more codes, and one file was no longer enough. It had to be divided into multiple files and loaded in sequence. I believe many people have seen the web page code below.

　<script src="1.js"></script>
　　<script src="2.js"></script>
　　<script src="3.js"></script>
　　<script src="4.js"></script>
　　<script src="5.js"></script>
　　<script src="6.js"></script>

This code loads multiple js files in sequence.

This way of writing has big disadvantages. First of all, when loading, the browser will stop rendering the web page. The more files are loaded, the longer the web page will lose response. Secondly, due to the dependencies between js files, the loading order must be strictly guaranteed (such as the above example) 1.js should be in front of 2.js), and the module with the greatest dependency must be loaded last. When the dependencies are complex, code writing and maintenance will become difficult.

require.js was born to solve these two problems:

(1) Implement asynchronous loading of js files to avoid web pages losing response;

(2) Manage dependencies between modules to facilitate code writing and maintenance.

2. Loading of require.js

The first step to use require.js is to download the latest version from the official website.

After downloading, it is assumed that it is placed under the js subdirectory and it can be loaded.

<script src="js/require.js"></script>

Some people may think that loading this file may also cause the web page to become unresponsive. There are two solutions. One is to load it at the bottom of the web page, and the other is to write it like this:

<script src="js/require.js" defer async="true" ></script>

The async attribute indicates that this file needs to be loaded asynchronously to avoid the webpage becoming unresponsive. IE does not support this attribute and only supports defer, so defer is also written.

After loading require.js, the next step is to load our own code. Assume that our own code file is main.js and is also placed under the js directory. Then, just write it like this:

　<script src="js/require.js" data-main="js/main"></script>

The data-main attribute is used to specify the main module of the web program. In the above example, it is main.js under the js directory. This file will be loaded by require.js first. Since the default file extension of require.js is js, main.js can be abbreviated to main.

3. How to write the main module

The main.js in the previous section, I call it the "main module", which means the entry code of the entire web page. It's a bit like the main() function in C language, all code starts running from here.

Let’s see how to write main.js.

If our code does not depend on any other modules, we can write javascript code directly.

// main.js
alert("Loading successful!");

But in this case, there is no need to use require.js. A really common situation is that the main module depends on other modules, in which case the require() function defined by the AMD specification must be used.

// main.js
　　require(['moduleA', 'moduleB', 'moduleC'], function (moduleA, moduleB, moduleC){
　　　　// some code here
　　});

The require() function accepts two parameters. The first parameter is an array, indicating the modules it depends on. The above example is ['moduleA', 'moduleB', 'moduleC'], that is, the main module depends on these three modules; the second parameter is a callback function. Currently It will be called after all the modules specified above are loaded successfully. Loaded modules will be passed into this function as parameters, so these modules can be used inside the callback function.

require() loads moduleA, moduleB and moduleC asynchronously, and the browser will not lose response; the callback function it specifies will only run after the previous modules are loaded successfully, solving the dependency problem.

Below, let’s look at a practical example.

Assuming that the main module depends on the three modules jquery, underscore and backbone, main.js can be written like this:

　　require(['jquery', 'underscore', 'backbone'], function ($, _, Backbone){
　　　　// some code here
　　});

require.js will first load jQuery, underscore and backbone, and then run the callback function. The code of the main module is written in the callback function.

4. Module loading

In the last example of the previous section, the dependent modules of the main module are ['jquery', 'underscore', 'backbone']. By default, require.js assumes that these three modules are in the same directory as main.js, and the file names are jquery.js, underscore.js and backbone.js respectively, and then load them automatically.

Using the require.config() method, we can customize the loading behavior of the module. require.config() is written at the head of the main module (main.js). The parameter is an object, and the paths attribute of this object specifies the loading path of each module.

require.config({
　　　　paths: {
　　　　　　"jquery": "jquery.min",
　　　　　　"underscore": "underscore.min",
　　　　　　"backbone": "backbone.min"
　　　　}
　　});

上面的代码给出了三个模块的文件名，路径默认与main.js在同一个目录（js子目录）。如果这些模块在其他目录，比如js/lib目录，则有两种写法。一种是逐一指定路径。

require.config({
　　　　paths: {
　　　　　　"jquery": "lib/jquery.min",
　　　　　　"underscore": "lib/underscore.min",
　　　　　　"backbone": "lib/backbone.min"
　　　　}
　　});

另一种则是直接改变基目录（baseUrl）。

require.config({
　　　　baseUrl: "js/lib",
　　　　paths: {
　　　　　　"jquery": "jquery.min",
　　　　　　"underscore": "underscore.min",
　　　　　　"backbone": "backbone.min"
　　　　}
　　});

如果某个模块在另一台主机上，也可以直接指定它的网址，比如：

require.config({
　　　　paths: {
　　　　　　"jquery": "https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min"
　　　　}
　　});

require.js要求，每个模块是一个单独的js文件。这样的话，如果加载多个模块，就会发出多次HTTP请求，会影响网页的加载速度。因此，require.js提供了一个优化工具，当模块部署完毕以后，可以用这个工具将多个模块合并在一个文件中，减少HTTP请求数。

五、AMD模块的写法

require.js加载的模块，采用AMD规范。也就是说，模块必须按照AMD的规定来写。

具体来说，就是模块必须采用特定的define()函数来定义。如果一个模块不依赖其他模块，那么可以直接定义在define()函数之中。

假定现在有一个math.js文件，它定义了一个math模块。那么，math.js就要这样写：

// math.js
　　define(function (){
　　　　var add = function (x,y){
　　　　　　return x+y;
　　　　};
　　　　return {
　　　　　　add: add
　　　　};
　　});

加载方法如下：

　　// main.js
　　require(['math'], function (math){
　　　　alert(math.add(1,1));
　　});

如果这个模块还依赖其他模块，那么define()函数的第一个参数，必须是一个数组，指明该模块的依赖性。

define(['myLib'], function(myLib){
　　　　function foo(){
　　　　　　myLib.doSomething();
　　　　}
　　　　return {
　　　　　　foo : foo
　　　　};
　　});

当require()函数加载上面这个模块的时候，就会先加载myLib.js文件。

六、加载非规范的模块

理论上，require.js加载的模块，必须是按照AMD规范、用define()函数定义的模块。但是实际上，虽然已经有一部分流行的函数库（比如jQuery）符合AMD规范，更多的库并不符合。那么，require.js是否能够加载非规范的模块呢？回答是可以的。这样的模块在用require()加载之前，要先用require.config()方法，定义它们的一些特征。举例来说，underscore和backbone这两个库，都没有采用AMD规范编写。如果要加载它们的话，必须先定义它们的特征。

require.config({
　　　　shim: {
　　　　　　'underscore':{
　　　　　　　　exports: '_'
　　　　　　},
　　　　　　'backbone': {
　　　　　　　　deps: ['underscore', 'jquery'],
　　　　　　　　exports: 'Backbone'
　　　　　　}
　　　　}
　　});

require.config()接受一个配置对象，这个对象除了有前面说过的paths属性之外，还有一个shim属性，专门用来配置不兼容的模块。具体来说，每个模块要定义（1）exports值（输出的变量名），表明这个模块外部调用时的名称；（2）deps数组，表明该模块的依赖性。

比如，jQuery的插件可以这样定义：

　shim: {
　　　　'jquery.scroll': {
　　　　　　deps: ['jquery'],
　　　　　　exports: 'jQuery.fn.scroll'
　　　　}
　　}

七、require.js插件

require.js还提供一系列插件，实现一些特定的功能。

domready插件，可以让回调函数在页面DOM结构加载完成后再运行。

　shim: {
　　　　'jquery.scroll': {
　　　　　　deps: ['jquery'],
　　　　　　exports: 'jQuery.fn.scroll'
　　　　}
　　}

text和image插件，则是允许require.js加载文本和图片文件。

define([
　　　　'text!review.txt',
　　　　'image!cat.jpg'
　　　　],
　　　　function(review,cat){
　　　　　　console.log(review);
　　　　　　document.body.appendChild(cat);
　　　　}
　　);

类似的插件还有json和mdown，用于加载json文件和markdown文件。

以上就是本文的全部所述，希望本文分享对大家有所帮助。