Can not receive server response by using ajax

[tadasyan]

I want to use ajax to fill input fields depending on the select element

<select name="service" onchange="fillFields(this.options[this.selectedIndex].value)">

Js code looks like:

function fillFields(inc){
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        var data = JSON.parse(xmlhttp.responseText);
        document.getElementById("datepicker").innerHTML = data.datebeg;
        document.getElementById("ph").innerHTML = data.phone;
        var form = document.forms.mainForm;
        var towns = form.elements["towns[]"];
        var hours = form.elements["hours[]"];
        var minutes = form.elements["minutes[]"]
        var i;
        for (i = 0; i < data.towns.length; i++){
            towns[i] = data.towns[i];
            hours[i] = data.hours[i];
            minutes[i] = data.minutes[i];
        }
    }
};
xmlhttp.open("GET", "fillForm.php?inc=" + inc, true);
xmlhttp.send();
}

fillForm.php retrieves data from db and sends response in JSON format.

It seems to me, I do everything exactly like in examples, but (xmlhttp.readyS tate == 4) condition fails. ReadyState equals to one. What is my problem?

[Dormilich]

if you only ever get a readyState of 1 (and not cancelling the process) then either line 21 produces an error or your server doesn’t respond at all.