Learn more about Node's module mechanism and talk about the module implementation process

This article will take you to understand the CommonJs specification and the module mechanism of Node, and introduce the basic process of Node implementing the CommonJs specification. I hope it will be helpful to everyone!

Before the CommonJs specification was proposed, Javascript did not have a module system, which meant that it was difficult for us to develop large-scale applications because the organization of the code would be more difficult.

What is CommonJs specification

---

First of all, CommonJS is not unique to Node. CommonJs is a module specification that defines how to reference and export modules,Nodejs Just implements this specification. The CommonJS module specification is mainly divided into three parts: module reference, module definition and module identification.

Module reference

Module reference means that we can introduce other modules through require.

const { add } = require('./add');
const result = add(1 ,2);

Module definition

A file is a module, and the module will provide two variables, namely module and exports. module is the current module itself, exports is the content to be exported, and exports is an attribute of the module, that is, exports is module.exports. The content imported by other modules through require is the content of module.exports.

// add.js
exports.add = (a, b) => {
    return a + b;
}

Module identification

The module identification is the content in require, such as require('./add'), the module identification is ./add.

The module import and export mechanism built through CommonJS allows users to easily build large-scale applications without having to consider variable pollution.

Node’s module implementation

---

Node implements the CommonJs specification and adds some features it needs. Node mainly does the following three things to implement the CommonJs specification:

• Path analysis

• File positioning

• Compile and execute

Path analysis

When require() is executed, require receives The parameter is the module identifier, and node performs path analysis through the module identifier. The purpose of path analysis is to find the path where this module is located through the module identifier. First of all, node modules are divided into two categories, namely core modules and file modules. The core module is the module that comes with node, and the file module is the module written by the user. At the same time, file modules are divided into file modules in the form of relative paths, file modules in the form of absolute paths, and file modules in the form of non-paths (such as express).

When node finds a file module, it will compile, execute and cache the module. The general principle is to use the full path of the module as the key, and the compiled content as value, when this module is introduced for the second time, there is no need to perform these steps of path analysis, file location, compilation and execution. The compiled content can be read directly from the cache.

// 缓存的模块示意：
const cachedModule = {
    '/Usr/file/src/add.js': 'add.js编译后的内容',
    'http': 'Node自带的http模块编译后的内容',
    'express': '非路径形式自定义文件模块express编译后的内容'
    // ...
}

When you want to find the module imported by require, the order of searching for the module is to first check whether the module is already in the cache. If it is not in the cache, then check the core module, and then look for the file module. Among them, file modules in the form of paths are easier to find. The complete file path can be obtained based on the relative or absolute path. It is relatively troublesome to find custom file modules in non-path form. Node will search for the file from the node_modules folder.

Where is the node_modules directory? For example, the file we are currently executing is /Usr/file/index.js;

/** 
* /Usr/file/index.js;
*/

const { add } = require('add');
const result = add(1, 2);

In this module we have introduced an add module, this add It is not a core module nor a file module in the form of a path, so how to find the add module at this time.

Module has a paths attribute. The path to find the add module is in the paths attribute. We can type this attribute to take a look:

/** 
* /Usr/file/index.js;
*/

console.log(module.paths);

We execute node index in the file directory. js can print out the value of paths. The value in paths is an array, as follows:

[
'/Usr/file/node_modules',
'/Usr/node_modules',
'/node_modules',
]

That is, Node will sequentially search from the above directory to see if it contains the add module. The principle is similar to the prototype chain. First, start searching in the node_modules folder in the directory of the same level as the currently executed file. If the node_modules directory is not found or does not exist, continue searching to the upper level.

File positioning

Path analysis and file positioning are used together. The file identifier can be without a suffix, or it can be passed Path analysis finds a directory or a package. At this time, locating specific files requires some additional processing.

File extension analysis

const { add } = require('./add');

比如上面这段代码，文件标识符是不带扩展名的，这个时候node会依次查找是否存在.js、.json、.node文件。

目录和包分析

同样是上面这段代码，通过./add查找到的可能不是一个文件，可能是一个目录或者包（通过判断add文件夹下是否有package.json文件来判断是目录还是包）。这个时候文件定位的步骤是这样的：

• 查看是否有package.json文件
  • 有
    • 读取package.json里的main字段的值作为文件
  • 没有
    • 寻找目录下的index作为文件（依次查找index.js、index.json、index.node）

如果package.json里没有main字段，那么也会将index作为文件，然后进行扩展名分析找到对应后缀的文件。

模块编译

我们开发中主要遇到的模块为json模块和js模块。

json模块编译

当我们require一个json模块的时候，实际上Node会帮我们使用fs.readFilcSync去读取对应的json文件，得到json字符串，然后调用JSON.parse解析得到json对象，再赋值给module.exports，然后给到require。

js模块编译

当我们require一个js模块的时候，比如

// index.js
const { add } = require('./add');

// add.js
exports.add = (a, b) => {
    return a + b;
}

这个时候发生了什么呢，为什么我们可以直接在模块里使用module、exports、require这些变量。这是因为Node在编译js模块的时候对模块的内容进行了首尾的包装。

比如add.js这个模块，实际编译的时候是会被包装成类似这样的结构：

(function(require, exports, module) {
  exports.add = (a, b) => {
    return a + b;
  }
  return module.exports;
})(require, module.exports, module)

即我们编写的js文件是会被包装成一个函数，我们编写的只是这个函数里的内容，Node后续的包装的过程对我们隐藏了。这个函数支持传入一些参数，其中就包括require、exports和module。

当编译完js文件后，就会执行这个文件，node会将对应的参数传给这个函数然后执行，并且返回module.exports值给到require函数。

以上就是Node实现CommonJs规范的基本流程。

更多node相关知识，请访问：nodejs 教程！

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