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Thread: successful AJAX query results in "undefined"-alert...

  1. #1
    Join Date
    Aug 2011

    successful AJAX query results in "undefined"-alert...


    I seem to have a sneaky little problem in my ajax-based function. The function is supposed to check at the server if the given username/password-combination is valid. On server-side everything is working correctly (Output is either "OK" or "Error") but within the js-application I always receive an alert-box stating "undefined" in the very end.

    Could you please take a look at my code and give me a hint where the problem is?

    ajaxRequest = function(u,f,m,b,h,s)
        alert ('ajaxRequest');
        this.url = u;
        this.wState = f || function() { };
        this.method = m || "GET";
        this.body = b || null;
        this.headers = h || false;
        this.sync = s || true;
        this.abortReq = false;
        this.req = (window.XMLHttpRequest) //Gecko-Browser?
            new XMLHttpRequest() //Gecko-Browser!
            ((window.ActiveXObject) //Internet Explorer?
            new ActiveXObject("Microsoft.XMLHTTP") //Internet Explorer!
            false //some Error occured
        this.doRequest = function() //function to perform the request
  ,this.url,this.sync); //Open connection
            if (this.headers) //additional headers given?
                for (var i=0; i<this.headers.length; i+=2)
                    this.headers[i],this.headers[i+1] //construct keyvalue pairs for the header
            this.req.onreadystatechange = this.wState; //function-call
            (!this.abortReq) ? this.req.send(this.body) : this.req.abort();
            //send request if not aborted somehow
    *   function to check whether the connection can
    *   be established for given username and password
    *   @param: string username Username
    *   @param: string password Password
    var checkConnection = function (username, password) {
        xmlhttp = new ajaxRequest(
                var r = xmlhttp.req;
                if (r.readyState==4) //transfer complete
                    document.getElementById('test-label').value =
                    (r.status == 200) //insert Data in DOM
                        ? r.responseText : r.status+"ERROR"; //Error occured
                    alert ('responsetext inserted');
            'POST',                                     //type
            '&username='+username+'&password='+password, //query
            ["Content-Type","application/x-www-form-urlencoded"]    //header
    When I perform the query the debug messages appear in following order:
    1.) ajaxRequest
    2.) doRequest
    3.) callback-function
    4.) callback-function
    5.) callback-function
    6.) readystate!
    7.) responsetext inserted (*it actually is updated*)
    8.) callback-function
    9.) readystate!
    10.) responsetext inserted
    11.) undefined (appears always)

    My question is: Why is the callback-function called again (debug no. 8) and why do I always receive the alert-box "undefined", whithout calling any alert?!

    Thanks for your help in advance!


  2. #2
    Join Date
    Aug 2008

    Re: successful AJAX query results in "undefined"-alert...

    I can't say for sure what the problem is, as I never use XMLHttpRequest directly. Have you considered using a library like jQuery or prototype? They make DOM manipulation and AJAX queries much simpler.

  3. #3
    Join Date
    Aug 2011

    Re: successful AJAX query results in "undefined"-alert...

    Hallo Chris_F,

    indeed, Frameworks make DOM-Modifications easier but I am developing an extension for firefox and I do not want to embed any overloaded Framework into it, as my aim is to keep the extension as small as possible. Though, thanks for the hint

    Anyone else any idea?

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