调试易

检查状态，必要时把前一个xmlhttp  abort()掉

用串联回调函数取代for
<script>
function createXMLHttpRequest(){ 
    http_request = false; 
    if(window.XMLHttpRequest) {
        http_request = new XMLHttpRequest(); 
    } 
    else if (window.ActiveXObject) { 
        try { 
            http_request = new ActiveXObject("Msxml2.XMLHTTP"); 
        } catch (e) { 
            try { 
                http_request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) { http_request = false;  } 
        } 
    } 
    return http_request; 
}var xmlhttp = createXMLHttpRequest();
var n = 0;function doSend(){
    xmlhttp.onreadystatechange = callback;
    xmlhttp.open("GET", "test1.asp", true); 
    xmlhttp.send(null); 
}
function callback(){
    if (xmlhttp.readyState == 4){
        if (xmlhttp.status == 200){
            alert(xmlhttp.responseText);
            n--;
            if(n>0) doSend();
        }
    } 
}
function startReq(){
n = 4 // 循环次数;
doSend()
}

</script>
<input type="button" value="START" onclick ="startReq();">
 

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,onload,onerror,method,params,contentType){
    this.req=null;
    this.onload=onload;
    this.onerror=(onerror) ? onerror : 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){
            this.req=new ActiveXObject("Microsoft.XMLHTTP");
        }
        if (this.req){
            try{
                var loader=this;
                this.req.onreadystatechange=function(){
                    loader.onReadyState.call(loader);
                }
                this.req.open(method,url,true);
                if (contentType){
                    this.req.setRequestHeader('Content-Type', contentType);
                }
                this.req.send(params);
            }catch (err){
                this.onerror.call(this);
            }
        }
    },    onReadyState:function(){
        var req=this.req;
        var ready=req.readyState;
        var httpStatus=req.status;
        if (ready==net.READY_STATE_COMPLETE){
            if (httpStatus==200 || httpStatus==0){
                this.onload.call(this);
            }else{
                this.onerror.call(this);
            }
        }
    },    defaultError:function(){
        alert("error fetching data!"
            +"\n\nreadyState:"+this.req.readyState
            +"\nstatus: "+this.req.status
            +"\nheaders: "+this.req.getAllResponseHeaders());
    }
}这样用,自己传callback函数就OK