Problem med Undefined i objekt

Hej

Jeg har et stykke Javascript der driller. Problemet er at "this.request" pludselig er "undefined" i min doCallBack() metode, selvom "this" stadig er ok.

Her er koden:

function doHttpRequest(formRef) {
    if(formRef.elements.length == this.refNames.length) {
        var url = "stat-search.php?";
        for(var i = 0; i < formRef.elements.length; i++) {
            url = url + refNames[i] + "=\"" + escape(formRef.elements[i].value) + "\";";
        }
        this.request.open("GET", url, true);
        this.request.onreadystatechange = this.callBack;
        this.request.send(null);
    } else {
        // throw exception
    }
}

function doCallBack() {
    if (this.request.readyState == 4) {
        if(this.request.status == 200) {
            document.getElementById("listView").innerHTML = this.request.responseText;
        }
    }
}

/*
        Constructor
*/

function ajax(listViewRef, refNames) {
    this.listView = listViewRef;
    this.refNames = refNames;
    this.request = null;
 
    try {
        this.request = new XMLHttpRequest();
    } catch (trymicrosoft) {
        try {
            this.request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (othermicrosoft) {
            try {
                this.request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (failed) {
                this.request = null;
            }
        }
    }
    if (this.request == null) {
        // throw exception
    }

    this.httpRequest = doHttpRequest;
    this.callBack = doCallBack;
}