Question

In the change_text() function I am trying to pass it the xmhttp.responsetext variable.. but how will I do this? As I see no way yet on how to pass it?

<script type="text/javascript">
function ajax(url, func) {
    this.url = url;
    var xmlhttp =  new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
        if(xmlhttp.readyState == 2 && xmlhttp.status == 200) {
            this.func = func;
        }
    }
    xmlhttp.open();
    xmlhttp.send()
}

function change_text() {
    target = document.getElementById("x");
       target.innerHTML = xmlhttp.responseText;
}

ajax("url.php", change_text);
</script>

Answer 1

Actually, to do this you don’t want to use this at all, since you are not creating any instances of an object and it doesn’t act like a constructor.

You can just:

function ajax(url, func) {
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
        if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            // note: loaded is 4 ^
            func(xmlhttp.responseText);
        }
    }
    xmlhttp.open('GET', url, true);
    //            ^~~ you also forgot to pass parameters here
    xmlhttp.send('');
}

That would make the ajax function work.

For your change_text function, it’s not in the same function as ajax, so it does not have access to the xmlhttp variable. However, the ajax function passed the responseText to it, so just make your function receive it:

function change_text(responseText) {
    var target = document.getElementById("x");
//  ^~~ don't forget to declare local variables with `var`.
    target.innerHTML = responseText;
}

For a working example, see the jsFiddle.