// 非 Node NativeModule
function Module(id, parent) {
  // 模块标识符
  this.id = id;
  // 调用该模块的模块
  this.parent = parent;
  // 模块对外输出的值
  this.exports = {};
  this.path = path.dirname(id);
  this.filename = null;
  this.loaded = false;
  updateChildren(parent, this, false);
  this.children = [];
}

// Native Module
function NativeModule(id) {
  this.filename = `${id}.js`;
  this.id = id;
  this.exports = {};
  this.module = undefined;
  this.exportKeys = undefined;
  this.loaded = false;
  this.loading = false;
  this.canBeRequiredByUsers = !id.startsWith('internal/');
}

// index.js
const jquery = require('jquery');

exports.$ = jquery;
console.log(module);

{
  id: '.',
  exports: { '$': [Function] },
  parent: null,
  filename: '/path/to/example.js',
  loaded: false,
  children: [
      {
        id: '/path/to/node_modules/jquery/dist/jquery.js',
        exports: [Function],
        parent: [Circular],
        filename: '/path/to/node_modules/jquery/dist/jquery.js',
        loaded: true,
        children: [],
        paths: [Object]
      }
    ],
  paths: [
     '/home/user/deleted/node_modules',
     '/home/user/node_modules',
     '/home/node_modules',
     '/node_modules'
  ]
}

if (!module.parent) {
  // ran with `node index.js`
  app.listen(8080, function() {
    console.log('app listening on port 8080');
  });
} else {
  // used with `require('./index.js')`
  module.exports = app;
}

const EventEmitter = require('events').EventEmitter;
module.exports = new EventEmitter();

setTimeout(function() {
  module.exports.emit('ready');
}, 1000);

const eventEmitter = require('./eventEmitter');

eventEmitter.on('ready', function() {
  console.log('module eventEmitter is ready');
});

const exports = module.exports;

exports.area = function(r) {
  return Math.PI * r * r;
};

exports.circumference = function(r) {
  return 2 * Math.PI * r;
};

exports.name = 'mrsingsing';

exports.age = 25;

module.exports = 100;

// 相当于断开了与 module.exports 的联系
// 亦即时 exports 不再指向 module.exports 的内存地址
// 需要 exports.value = 3 才能导出
exports = 3;

exports = function(x) {
  console.log(x);
};

exports.hello = function() {
  return 'hello';
};

module.exports = 'Hello world!';

const dirname = path.dirname(filename);
const require = makeRequireFunction(this, redirects);
let result;
// 相当于 exports 引用 module.exports
const exports = this.exports;
const thisValue = exports;
const module = this;

if (requireDepth === 0) statCache = new Map();

if (inspectorWrapper) {
  result = inspectorWrapper(
    compiledWrapper,
    thisValue,
    exports,
    require,
    module,
    filename,
    dirname
  );
} else {
  result = compiledWrapper.call(thisValue, exports, require, module, filename, dirname);
}

// __filename 当前文件的绝对路径
// __dirname  当前文件所在目录的绝对路径
(function(exports, require, module, __filename, __dirname) {
  exports.a = 3;
});

// Check the cache for the requested file.
// 1. If a module already exists in the cache: return its exports object.
// 2. If the module is native: call
//    `NativeModule.prototype.compileForPublicLoader()` and return the exports.
// 3. Otherwise, create a new module for the file and save it to the cache.
//    Then have it load  the file contents before returning its exports
//    object.
Module._load = function(request, parent, isMain) {
    let relResolveCacheIdentifier;
    if (parent) {
      debug('Module._load REQUEST %s parent: %s', request, parent.id);
      ...
    }
    // 查找文件具体位置
    const filename = Module._resolveFilename(request, parent, isMain);
    // 存在缓存，则不需要再次执行 返回缓存
    const cachedModule = Module._cache[filename];
    if (cachedModule !== undefined) {
      updateChildren(parent, cachedModule, true);
      if (!cachedModule.loaded)
        return getExportsForCircularRequire(cachedModule);
      return cachedModule.exports;
    }
    // 加载node原生模块，原生模块loadNativeModule
    // 如果有 且能被用户引用 返回 mod.exports（这包括node模块的编译创建module对象，将模块运行结果保存在module对象上）
    const mod = loadNativeModule(filename, request);
    if (mod && mod.canBeRequiredByUsers) return mod.exports;

    // 创建一个模块
    // Don't call updateChildren(), Module constructor already does.
    const module = new Module(filename, parent);

    if (isMain) {
      process.mainModule = module;
      module.id = '.';
    }
    // 缓存模块
    Module._cache[filename] = module;
    if (parent !== undefined) {
      relativeResolveCache[relResolveCacheIdentifier] = filename;
    }
    // 加载执行新的模块
    module.load(filename);

    return module.exports;
  };

// index.js
var invisible = function() {
  console.log('invisible');
};

exports.message = 'hi';

exports.say = function() {
  console.log(message);
};

module.exports = function() {
  console.log('hello world!');
};

require('./index.js');

var foo = require('foo');
//  等同于
var foo = require('foo.js');

# Node 安装全局依赖的目录
/usr/local/lib/node/bar.js
# 当前文件夹依赖目录
/home/user/projects/node_modules/bar.js
# 父目录依赖目录
/home/user/node_modules/bar.js
# 父目录的父目录依赖目录
/home/node_modules/bar.js
# 沿路径向上逐级递归，直到根目录下的依赖目录
/node_modules/bar.js

{
  "name": "some-library",
  "main": "./lib/some-library.js"
}

require('./example.js');
require('./example.js').message = 'hello';
require('./example.js').message;
// "hello"

// 删除指定模块的缓存
delete require.cache[moduleName];

// 删除所有模块的缓存
Object.keys(require.cache).forEach(function(key) {
  delete require.cache[key];
});

// foo.js
console.log('foo start');
exports.done = false;
const bar = require('./bar');
console.log('在 foo 中，bar.done = %j', bar.done);
exports.done = true;
console.log('foo end');

// bar.js
console.log('bar start');
exports.done = false;
const foo = require('./foo');
console.log('在 bar 中，foo.done = %j', foo.done);
exports.done = true;
console.log('bar end');

// main.js
console.log('main start');
const foo = require('./foo');
const bar = require('./bar');
console.log('在 main 中，foo.done=%j，bar.done=%j', foo.done, bar.done);

# 执行主文件
$ node main.js

main start
foo start
bar start
在 bar 中，a.done = false
bar 结束
在 foo 中，b.done = true
foo 结束
在 main 中，a.done=true，b.done=true

require.main === module;
// true

// 已安装 React 至 node_modules
const react = require.resolve('react');

console.log(react);
// /Users/mrsingsing/Desktop/demo/node_modules/react/index.js