The onreadystatechange event handler remains untriggered

Hi there! I'm struggling with a simple code issue. The problem is that the function assigned to onreadystatechange never seems to be executed. I added an alert to display the readyState and status of xmlhttp, which both showed as 1 and 0 respectively. I'm baffled as to why the state isn't changing, especially since everything else in the code seems to be working fine. I even used alert boxes to confirm that the username being fetched from the form displays correctly. Can someone please help me out here? I'm really stuck...

function checkAvailability() {
    var xmlhttp;
    if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();
    } else {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    if (xmlhttp) {
        var regform = document.getElementById("regform");
        var username = regform.username.value;
        xmlhttp.open("POST", "http://localhost:8080/UsernameAvailability", true);
        xmlhttp.onreadystatechange = function() {
            alert(xyz);
        }
        xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        xmlhttp.send("username=" + username.value);
    }
}

Answer №1

To ensure the proper registration of the onreadystatechange callback, you must switch the calling order of xmlhttp.onreadystatechange and xmlhttp.open. This will guarantee that the callback is set up before opening the connection.

xmlhttp.onreadystatechange = function() {
  alert(xyz);
};
xmlhttp.open("POST", "http://localhost:8080/UsernameAvailability", true);

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