JS模拟多线程

<script src="prototype.js"></script>
<script type="text/javascript" defer="defer">
function test(obj,info,callback){ 
 callback = callback || new Function();
 new Ajax.Request('test.xml',{method:'get',
  onSuccess:function(o){

   $(obj).innerHTML += info + '完成<br>';
   callback('完成');
  },
  onFailure : function(o){
   $(obj).innerHTML += info + '失败<br>';
   callback('失败');
  },
  onComplete : function(o){
   document.body.scrollTop = 9999;
  }
 });
}

var Thread = {
 runNum : 0,  //当前正式运行的线程数
 maxNum : 5, //最大同时执行的线程数 -1表示不限
 commandList : new Array(), 
 start : function(){
  //window.status = this.runNum; 
  if(this.maxNum != -1 && this.runNum >= this.maxNum){   
   return; 
  }
  if(this.commandList.length <= 0){  
   this.runNum = 0;
   return false; 
  }  
  this.runNum++;
  var _this =this;  
  var tFun = function(){
   if(!_this.commandList[0]) return;
   var command = _this.commandList[0].shift();   
   command.apply(_this,_this.commandList[0].concat(
    function(){  //alert(2)