概述如何实现一个简单的浏览器端js模块加载器

define(factory() {
 var x = {
  a: 1
 };
 return x;
});

// a.js
define([ 'c.js', 'd.js' ], factory (c, d) {
 var x = c + d;
 return x;
});

var zmm = {
 _modules: {},
 _configs: {
  // 用于拼接相对路径
  basePath: (function (path) {
   if (path.charAt(path.length - 1) === '/') {
    path = path.substr(0, path.length - 1);
   }
   return path.substr(path.indexOf(location.host) + location.host.length + 1);
  })(location.href),
  // 用于拼接相对根路径
  host: location.protocol + '//' + location.host + '/'
 }
};
zmm.hasModule = function (_uri) {
 // 判断是否已有该模块，不论加载中或已加载好
 return this._modules.hasOwnProperty(_uri);
};
zmm.isModuleLoaded = function (_uri) {
 // 判断该模块是否已加载好
 return !!this._modules[_uri];
};
zmm.pushModule = function (_uri) {
 // 新模块占坑，但此时还未加载完成，表示加载中；防止重复加载
 if (!this._modules.hasOwnProperty(_uri)) {
  this._modules[_uri] = null;
 }
};
zmm.installModule = function (_uri, mod) {
 this._modules[_uri] = mod;
};
zmm.load = function (uris) {
 var i, nsc;
 for (i = 0; i < uris.length; i++) {
  if (!this.hasModule(uris[i])) {
   this.pushModule(uris[i]);
   // 开始加载
   var nsc = document.createElement('script');
    nsc.src = uri;
   document.body.appendChild(nsc);
  }
 }
};
zmm.resolvePath = function (path) {
 // 返回绝对路径
 var res = '', paths = [], resPaths;
 if (path.match(/.*://.*/)) {
  // 绝对路径
  res = path.match(/.*://.*?//)[0]; // 协议+域名
  path = path.substr(res.length);
 } else if (path.charAt(0) === '/') {
  // 相对根路径 /开头
  res = this._configs.host;
  path = path.substr(1);
 } else {
  // 相对路径 ./或../开头或直接文件名
  res = this._configs.host;
  resPaths = this._configs.basePath.split('/');
 }
 resPaths = resPaths || [];
 paths = path.split('/');
 for (var i = 0; i < paths.length; i++) {
  if (paths[i] === '..') {
   resPaths.pop();
  } else if (paths[i] === '.') {
   // do nothing
  } else {
   resPaths.push(paths[i]);
  }
 }
 res += resPaths.join('/');
 return res;
};
var define = zmm.define = function (dependPaths, fac) {
 var _uri = document.currentScript.src;
 if (zmm.isModuleLoaded(_uri)) {
  return;
 }
 var factory, depPaths, uris = [];
 if (arguments.length === 1) {
  factory = arguments[0];
  // 挂载到模块组中
  zmm.installModule(_uri, factory());
  // 告诉proxy该模块已装载好
  zmm.proxy.emit(_uri);
 } else {
  // 有依赖的情况
  factory = arguments[1];
  // 装载完成的回调函数
  zmm.use(arguments[0], function () {
   zmm.installModule(_uri, factory.apply(null, arguments));
   zmm.proxy.emit(_uri);
  });
 }
};
zmm.use = function (paths, callback) {
 if (!Array.isArray(paths)) {
  paths = [paths];
 }
 var uris = [], i;
 for (i = 0; i < paths.length; i++) {
  uris.push(this.resolvePath(paths[i]));
 }
 // 先注册事件，再加载
 this.proxy.watch(uris, callback);
 this.load(uris);
};
zmm.proxy = function () {
 var proxy = {};
 var taskId = 0;
 var taskList = {};
 var execute = function (task) {
  var uris = task.uris,
   callback = task.callback;
  for (var i = 0, arr = []; i < uris.length; i++) {
   arr.push(zmm._modules[uris[i]]);
  }
  callback.apply(null, arr);
 };
 var deal_loaded = function (_uri) {
  var i, k, task, sum;
  // 当一个模块加载完成时，遍历当前任务栈
  for (k in taskList) {
   if (!taskList.hasOwnProperty(k)) {
    continue;
   }
   task = taskList[k];
   if (task.uris.indexOf(_uri) > -1) {
    // 查看这个任务中的模块是否都已加载好
    for (i = 0, sum = 0; i < task.uris.length; i++) {
     if (zmm.isModuleLoaded(task.uris[i])) {
      sum ++;
     }
    }
    if (sum === task.uris.length) {
     // 都加载完成 删除任务
     delete(taskList[k]);
     execute(task);
    }
   }
  }
 };
 proxy.watch = function (uris, callback) {
  // 先检查一遍是否都加载好了
  for (var i = 0, sum = 0; i < uris.length; i++) {
   if (zmm.isModuleLoaded(uris[i])) {
    sum ++;
   }
  }
  if (sum === uris.length) {
   execute({
    uris: uris,
    callback: callback
   });
  } else {
   // 订阅新加载任务
   var task = {
    uris: uris,
    callback: callback
   };
   taskList['' + taskId] = task;
   taskId ++;
  }
 };
 proxy.emit = function (_uri) {
  console.log(_uri + ' is loaded!');
  deal_loaded(_uri);
 };
 return proxy;
}();

// main.html
zmm.use('/a.js', function(){...});
// a.js
define('/b.js', function(b) {
 var a = 1;
 a = b + 1;
 return a;
});
// b.js
define('/a.js', function(a) {
 var b = a + 1;
 return b;
});

// a.js
define('./js/a', function (require, exports, module) {
 var a = 1;
 require.async('./b', function (b) {
  a = b + 1;
  module.exports = a; //a= 3
 });
 module.exports = a; // a= 1
});
// b.js
define('./js/b', function (require, exports, module) {
 var a = require('./a');
 var b = a + 1;
 module.exports = b;
});
// main.html
seajs.use('./js/a', function (a) {
 console.log(a); // 1
});

// a.js
var a = 1;
module.exports = a;
var b = require('./b');
a = b + 1;
module.exports = a;
// b.js
var a = require('./a');
var b = a + 1;
module.exports = b;
// main.js
var a = require('./a');
console.log(a); //3

// even.js
import { odd } from './odd';
export var counter = 0;
export function even(n) { counter++; return n == 0 || odd(n - 1);}
// odd.js
import { even } from './even';
export function odd(n) { return n != 0 && even(n - 1);}
// main.js
import * as m from './even.js';
m.even(10); // true; m.counter = 6

// even.js
var odd = require('./odd');
var counter = 0;
exports.counter = counter;
exports.even = function(n) {
counter++;
return n == 0 || odd(n - 1);
}
// odd.js
var even = require('./even').even;
module.exports = function(n) {
return n != 0 && even(n - 1);
}
// main.js
var m = require('./even');
m.even(10); // TypeError: even is not a function