采用异步方式进行的验证码检验

这是网上搜到的源代码，这个东东解决我在B/S模式下做验证码的问题，让我真切的感受到了B/S模式下开发的真实情况。

/* namespacing object */
var net=new Object();

net.READY_STATE_UNINITIALIZED=0;
net.READY_STATE_LOADING=1;
net.READY_STATE_LOADED=2;
net.READY_STATE_INTERACTIVE=3;
net.READY_STATE_COMPLETE=4;
/*--- content loader object for cross-browser requests ---*/
net.ContentLoader=function(url,on_load,on_error,method,params,contentType){
  this.req=null;
  this.on_load=on_load;
  this.on_error=(on_error) ? on_error : this.defaultError;
  this.loadXMLDoc(url,method,params,contentType);
}
net.ContentLoader.prototype.loadXMLDoc=function(url,method,params,contentType){
  if (!method)
  {
	method="GET";
  }
  if (!contentType && method=="POST")
  {
	contentType='application/x-www-form-urlencoded';
  }
  if (window.XMLHttpRequest)
  {
	this.req=new XMLHttpRequest();
  }
  else if (window.ActiveXObject)
  {
  	//add try catch;
	 	try {
		    this.req = new ActiveXObject("Msxml2.XMLHTTP");
			}catch (e1){
					try {
						this.req = new ActiveXObject("Microsoft.XMLHTTP");					
						} catch (e2){
				}
		 }
			 	//
	//this.req=new ActiveXObject("Microsoft.XMLHTTP");
  }
  if (this.req)
  {
	try
	{
	  var loader=this;
	  this.req.onreadystatechange=function()
	  {
		net.ContentLoader.onReadyState.call(loader);
	  }
	  this.req.open(method,url,true);
	  if (contentType)
	  {
		  this.req.setRequestHeader('Content-Type', contentType);
	  }
	  this.req.send(params);
	}
	catch (err)
	{
	  this.on_error.call(this);
	}
  }
}
net.ContentLoader.onReadyState=function(){
  var req=this.req;
  var ready=req.readyState;
  if (ready==net.READY_STATE_COMPLETE){
	var httpStatus=req.status;
	if (httpStatus==200 || httpStatus==0){
	  this.on_load.call(this);
	}else{
	  this.on_error.call(this);
	}
  }
}
net.ContentLoader.prototype.defaultError=function(){
  alert("error fetching data!"
	+"\n\nreadyState:"+this.req.readyState
	+"\nstatus: "+this.req.status
	+"\nheaders: "+this.req.getAllResponseHeaders());
}

调用方式

 new net.ContentLoader(url,subform,null,"GET","num="+obj.idCode.value,null);