I'm trying to fetch some content from another source using XHR as shown below:

function fetchPage(str)

{

    if(str==)

    {

        document.getElementById(table).innerHTML=;

        resetFilters();

        $('#progress').hide();  //fetching progress bar <div>

        return;

    }

    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=postCallback;

    xmlhttp.open(GET, fetch.php?url=http://www.sis.itu.edu.tr/tr/ders_programlari/LSprogramlar/prg.php?fb=+str, true);

    xmlhttp.send();



// any stuff that goes here will happen before callback

//  (this is a good place to update a UI element showing a call is resolving.)

//  (for example a spinner or text saying fetching)

$('#progress').show();

progressFetching();

switch(xmlhttp.readyState){ //loading bar adjustments

    case 0:

        $('.bar').css(width,0%);

        $('.bar').text(0%);

        break;

    case 1:

        $('.bar').css(width,25%);

        $('.bar').text(25%);

        break;

    case 2:

        $('.bar').css(width,50%);

        $('.bar').text(50%);

        break;

    case 3:

        $('.bar').css(width,75%);

        $('.bar').text(75%);

        break;

}

}







function postCallback()

{

if (xmlhttp.readyState==4 && xmlhttp.status==200){

    progressDone();   //loading is finished

    $('#error').hide();

    document.getElementById(table).innerHTML=xmlhttp.responseText;                     



    // continue to process post callback.

    resetFilters();

}

else {

    // report error with fetch



    /*if(xmlhttp.status==404 || xmlhttp.responseText == )

        $('#error').show();*/

    //$('#error').show();

}

}

I want my page to display error when connection timeout occurs, or when the computer doesn't have an internet connection (maybe a disconnection occurred while hanging around) or any other situation where the webpage fails to fetch the contents of the other source.

Using the code above, in the else block, if I go for if(xmlhttp.status==404 || xmlhttp.responseText == ) in the /* */ comment section, I won't get an error unless its not a 404 error. If i go for // comment section, error will be displayed after the fetching process is started until it is completed, i.e. between xmlhttp.readyState = 0 through xmlhttp.readyState = 4. How can I display connection error messages using these attributes or something else?

Thank your for your attention:)