javascript ajax sometimes failes to execute

I'm using the following ajax function:

function callAjax(request,callback)
{
    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)
        {
            if(callback != null)callback(xmlhtt开发者_高级运维p.responseText);
        }
    }
    xmlhttp.open("GET",request,true);
    xmlhttp.send();
}

And I've found that when I call this function 3 or more times in sequence, some of the callbacks are sometimes not executed. The ajax request occurs, and a result is returned, but somehow the callback function does not get called. Worse, this doesn't even happen consistently - sometimes it works, sometimes not. What I want to know is, why? And more importantly, how can I fix it? As far as I can tell, the callbacks will be run if there is at least a 4 millisecond gap between them. If two of the pages return within that timeframe, only one of them (the second I believe, but I'm not sure it always is) will actually be handled.

I know I can work around this by making sure that I'm only waiting for one ajax request to return at a time (through liberal use of sleep()). However, this slows down the page significantly; I want to be able to fire off all three (or more) requests, and have them execute their callbacks whenever they return.

What am I doing wrong? How do I make it work?

NB: I'm not looking for someone to say 'oh, you should use jquery'. I do not want to use any external libraries for this - please just tell me how to fix the code I have.

---

xmlhttp is a global variable so it will be overwritten if you have more than one call happening at the same time.

You are also using a GET request. GET requests get cached if the improper heading are not set. See "heartbeat" logger qustion for details on how to avoid caching.

---

Most likely the status is not 200. Your callback will only happen for status 200 in you current code.

xmlhttp.onreadystatechange=function()
{
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
        if(typeof callback == 'function')
            callback.call(xmlhttp, xmlhttp.responseText);
    }
    else{
        alert("status is " + xmlhttp.status);
    }

}

---

Could be a page caching issue, which you can simply fix by adding a random number variable to your request.

Also, If I'm not mistaken, browsers do have a maximum number of simultaneous ajax requests at once.