AJAX fires readyState 4 multiple times in IE

I am experiencing a strange problem where IE8 will fire readyState 4 three or four times on an ajax request. This happens often but not always, and only for a couple of specific links - the rest of my ajax links are fine. My object creation and onreadystatechange handler are basic, ie.

function ajaxPost(params) {

    try {
        // Firefox, Opera 8.0+, Safari
        xmlHttp = new XMLHttpRequest();
    } catch (e) {
        // Internet Explorer
        try {
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                alert('Unsupported Browser');
            }
        }
    }

    xmlHttp.onreadystatechange = function() {
      开发者_运维知识库  if (xmlHttp.readyState == 4) {
            // Do something
        }
    }

    xmlHttp.open("POST", path_to_script);
    xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
    xmlHttp.setRequestHeader("Cache-Control", "must-revalidate");
    xmlHttp.send(params);
}

Not sure if this is relevant - but at the points where the problem happens the user has already been navigating an ajax UI. They have clicked several other ajax links without reloading the page - each link uses a new xmlHttpRequest object with the same global variable name (xmlHttp), but each ajax request finishes before another can be issued. Do I need to do any "cleanup" or something for IE8? This has never happened using Safari, Firefox, Chrome, or any other browser.

Any insight would be much appreciated.

Thanks, Brian

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Since your xmlHttp object is global, may be IE8 assigns a new onreadystatechange each time you call ajaxPost instead of replacing it.

That may explain an erratic behavior, as it depends on the number of calls to ajaxPost.

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I'm wondering if you're having the same problem I had.

Is it possible that the onreadystatechange functions aren't getting executed immediately?

The ready state changes 3 times (1 to 2, 2 to 3, and 3 to 4), but if execution gets delayed, when the functions DO execute, they'll see the current ready state of 4. Because there are 3 readychange events, and they all see readystate = 4, they all execute.