# JavaScript 模块化的发展进程

(function (global) {
  global.myMath = global.myMath || {}
  global.myMath.sum = sum

  function sum (...values) {
    return values.reduce((a, b) => a + b, 0)
  }
})(window)

// checkType 模块需要在 myMath 模块之前运行
(function (global) {
  global.checkType = global.checkType || {}
  global.checkType.type = type

  function type (value) {
    return Object.prototype.toString.call(value).slice(8, -1)
  }
})(window);  // 分号是必须的

// myMath module
(function (global, $) {
  global.myMath = global.myMath || {}
  global.myMath.sum = sum

  function sum (...values) {
    const areAllNumbers = values.every(a => $.type(a) === 'Number')
    return areAllNumbers ? values.reduce((a, b) => a + b, 0) : new Error('numbers are required!')
  }
})(window, window.checkType)

// sum.js
module.exports = exports = (...values) => values.reduce((a, b) => a + b, 0)

// index.js
const sum = require('./sum')
console.log(sum(1,2,3))

// Invoke with makeRequireFunction(module) where |module| is the Module object
// to use as the context for the require() function.
function makeRequireFunction(mod) {
  const Module = mod.constructor;

  function require(path) {
    try {
      exports.requireDepth += 1;
      return mod.require(path); // 通过 index.js 模块的 require 加载 './sum.js'
    } finally {
      exports.requireDepth -= 1;
    }
  }
...

// Loads a module at the given file path. Returns that module's
// `exports` property.
Module.prototype.require = function(id) {
  if (typeof id !== 'string') {
    throw new ERR_INVALID_ARG_TYPE('id', 'string', id);
  }
  if (id === '') {
    throw new ERR_INVALID_ARG_VALUE('id', id,
                                    'must be a non-empty string');
  }
  return Module._load(id, this, /* isMain */ false);
};

// 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.require()` with the
//    filename and return the result.
// 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) {
  if (parent) {
    debug('Module._load REQUEST %s parent: %s', request, parent.id);
  }

  // 获取模块文件名
  var filename = Module._resolveFilename(request, parent, isMain);

  // 处理缓存模块
  var cachedModule = Module._cache[filename];
  if (cachedModule) {
    updateChildren(parent, cachedModule, true);
    return cachedModule.exports;
  }

  // 处理 native 模块
  if (NativeModule.nonInternalExists(filename)) {
    debug('load native module %s', request);
    return NativeModule.require(filename);
  }

  // Don't call updateChildren(), Module constructor already does.
  // 处理第三方模块
  var module = new Module(filename, parent);

  if (isMain) {
    process.mainModule = module;
    module.id = '.';
  }

  Module._cache[filename] = module;

  // 加载第三方模块
  tryModuleLoad(module, filename);

  return module.exports;
};

// loader.js
function tryModuleLoad(module, filename) {
  var threw = true;
  try {
    module.load(filename);
    threw = false;
  } finally {
    if (threw) {
      delete Module._cache[filename];
    }
  }
}
...

// loader.js
// Given a file name, pass it to the proper extension handler.
Module.prototype.load = function(filename) {
  debug('load %j for module %j', filename, this.id);

  assert(!this.loaded);
  this.filename = filename;
  this.paths = Module._nodeModulePaths(path.dirname(filename));

  var extension = path.extname(filename) || '.js';
  if (!Module._extensions[extension]) extension = '.js';

  // 传递给 extension 处理器处理 .js/.json/.node 文件
  Module._extensions[extension](this, filename);
  this.loaded = true;

  // 实验性质的模块额外处理
  if (experimentalModules) {
    if (asyncESM === undefined) lazyLoadESM();
    const ESMLoader = asyncESM.ESMLoader;
    const url = getURLFromFilePath(filename);
    const urlString = `${url}`;
    const exports = this.exports;
    if (ESMLoader.moduleMap.has(urlString) !== true) {
      ESMLoader.moduleMap.set(
        urlString,
        new ModuleJob(ESMLoader, url, async () => {
          const ctx = createDynamicModule(
            ['default'], url);
          ctx.reflect.exports.default.set(exports);
          return ctx;
        })
      );
    } else {
      const job = ESMLoader.moduleMap.get(urlString);
      if (job.reflect)
        job.reflect.exports.default.set(exports);
    }
  }
};

// Native extension for .js
Module._extensions['.js'] = function(module, filename) {
  var content = fs.readFileSync(filename, 'utf8');
  module._compile(stripBOM(content), filename);
};

// Run the file contents in the correct scope or sandbox. Expose
// the correct helper variables (require, module, exports) to
// the file.
// Returns exception, if any.
Module.prototype._compile = function(content, filename) {

  content = stripShebang(content);

  // create wrapper function
  var wrapper = Module.wrap(content);

  var compiledWrapper = vm.runInThisContext(wrapper, {
    filename: filename,
    lineOffset: 0,
    displayErrors: true
  });

  var inspectorWrapper = null;
  if (process._breakFirstLine && process._eval == null) {
    if (!resolvedArgv) {
      // we enter the repl if we're not given a filename argument.
      if (process.argv[1]) {
        resolvedArgv = Module._resolveFilename(process.argv[1], null, false);
      } else {
        resolvedArgv = 'repl';
      }
    }

    // Set breakpoint on module start
    if (filename === resolvedArgv) {
      delete process._breakFirstLine;
      inspectorWrapper = process.binding('inspector').callAndPauseOnStart;
    }
  }
  var dirname = path.dirname(filename);
  var require = makeRequireFunction(this);
  var depth = requireDepth;
  if (depth === 0) stat.cache = new Map();
  var result;
  if (inspectorWrapper) {
    result = inspectorWrapper(compiledWrapper, this.exports, this.exports,
                              require, this, filename, dirname);
  } else {
    result = compiledWrapper.call(this.exports, this.exports, require, this,
                                  filename, dirname);
  }
  if (depth === 0) stat.cache = null;
  return result;
};

Module.wrap = function(script) {
  return Module.wrapper[0] + script + Module.wrapper[1];
};

Module.wrapper = [
  '(function (exports, require, module, __filename, __dirname) { ',
  '\n});'
];

(function (exports, require, module, __filename, __dirname) { module.exports = exports = (...values) => values.reduce((a, b) => a + b, 0)
});

...
result = compiledWrapper.call(this.exports, this.exports, require, this,
                              filename, dirname);
...

// ./index.js
const R = require('./sum')

module.exports = exports = R

// ./sum.js
module.exports = exports = (...values) => values.reduce((a, b) => a + b, 0)

(function(){function r(e,n,t){function o(i,f){if(!n[i]){if(!e[i]){var c="function"==typeof require&&require;if(!f&&c)return c(i,!0);if(u)return u(i,!0);var a=new Error("Cannot find module '"+i+"'");throw a.code="MODULE_NOT_FOUND",a}var p=n[i]={exports:{}};e[i][0].call(p.exports,function(r){var n=e[i][1][r];return o(n||r)},p,p.exports,r,e,n,t)}return n[i].exports}for(var u="function"==typeof require&&require,i=0;i<t.length;i++)o(t[i]);return o}return r})()({1:[function(require,module,exports){
const R = require('./sum')

module.exports = exports = R
},{"./sum":2}],2:[function(require,module,exports){
module.exports = exports = (...values) => values.reduce((a, b) => a + b, 0)
},{}]},{},[1]);