Detailed explanation of simulated require function and simple implementation mechanism examples in node.js

Require loading mechanism in nodejs

# #1. The order of loading files in require

require('./module');

Require ('./ module.json');

                                                                                                    The file

starts with ./ or ../: then search for the module starting from the folder where the current file is located based on the relative path; /file.js');
=> Find the file.js file in the upper-level directory

Starting with /: Start searching for the module from the system root directory;

        
require('/Users/iceStone/Documents/file.js');

        
require('fs' );

require ('my_module');

If the package.json file does not have a main field, or there is no package.json file at all, the index.js file in the directory will be found by default. As a module:

require('./calcuator');

2. require cache

When a module is loaded for the first time, Node will cache the module. When you load the module in the future, you can directly retrieve the module.exports attribute of the module from the cache (the module will not be executed again)

If you need to execute the code in the module multiple times, you can generally expose the module Behavior (function), module cache can be obtained through require.cache, and can also be deleted

3. All code runs in the module scope and will not pollute the global scope.

Use the incoming module id to find the corresponding module file through the loading rules

Read the code in this file

Execute the code
Get module.exports and return

 nodejs_require.js
 // [require函数模拟]

 "use strict"

 function $require(id) {
 //1、先查看模块是否存在，如果不存在则抛出 Can't found file
 //2、如果存在，就读取代码
 const fs = require('fs');
 const path = require('path');

 // 文件名
 let filename = id;
 //查看是否是绝对路径
 if (!path.isAbsolute(filename)) {
  filename = path.join(__dirname, id);
 }

 // 文件目录
 let dirname = path.dirname(filename);

 //模拟require的缓存机制
 $require.cache = $require.cache || {};
 if($require.cache[filename]){
  return $require.cache[filename].exports;
 }

 // 读取模块代码
 let code="";
 try {
  code = fs.readFileSync(filename,'utf-8'); // 阻塞读取文件，不会放入事件队列 
 } catch (error) {
  console.log(" [*]can't found file! ");
  throw error;
 }

 // console.log(code);

 // 3、执行代码，所要执行的代码，需要营造一个独立的空间

 // 定义一个数据容器，用容器去装模块导出的成员
 let _module = { // 每个js模块都会有一个全局的module变量
  id:'.',
  exports:{},
  parent:module,
  filename:filename
 };
 let _exports = _module.exports;

 // 营造一个独立空间
 code = `(function($require,module,_exports,__dirname,__filename){
  $[code]
 })($require,_module,_exports,dirname,filename)`;

 // 执行代码
 eval(code);

 // 缓存
 $require.cache[filename] = _module;
 // 返回结果
 return _module.exports;
 }


 setInterval(()=>{
  const rr = $require("./test.js");
  console.log(rr);
 },1000);

 //test.js
 const date = $require('./date.js');

 console.log(module);

 module.exports = date;

 //date.js
 module.exports = new Date();

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