AJAX：在循环中发送xmlhttp请求

Question

I have this function below and I call that function in a loop I get the alert n times but only n-1 or sometimes n-2 responses

if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
        alert(xmlhttp.responseText);
        //document.getElementById("warnings_panel").innerHTML+=xmlhttp.responseText;
    }
}

alert("in ajax) 

xmlhttp.open("GET","getresponse.php?start="+ start + "&end=" + end,true);
xmlhttp.send();

Answer 1

onreadystatechange gets terminated when your function ends because the xmlhttp object gets removed as it is not a global variable. Am I right?

Better would be to put only the AJAX request in a function, so that the xmlhttp object is only created once (I used a shorter version to create the xmlhttp object in this example).

var xmlhttp = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');

function request() {
    xmlhttp.open('GET', 'getresponse.php?start=' + start + '&end=' + end, true);
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            alert(xmlhttp.responseText);
        }
    }
    xmlhttp.send(null);
}