var check = {} require = function(name, callback){    //name切割成多个模块名，然后转换为URL    //for循环    //这里会运行于iframe中    var s = document.createElement("script")    document.body.appendChild(s)    s.src = name    //for循环结束    check[name] = callback //以后check[name]会对应一个对象}//这是执行回调require.exec = function(factory, name, list){  var ret = factory.apply(null, list || [])  check[name](ret)}//这是模块用到define方法define = function(name, callback, list){    //这大大被简化了，还要从URL还原name，处理依赖列表    var str = "require.exec(" + callback.toString() +",'"+name +"')"    var s = document.createElement("script")    document.body.appendChild(s)    s.text = str;}