Creating an XHR request with JavaScript

XHR request

function ajax() {
    var xmlhttp;

    xmlhttp = new XMLHttpRequest();

    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 ) {
           if(xmlhttp.status == 200){
               document.getElementById("response").innerHTML = xmlhttp.responseText;
           }
           else if(xmlhttp.status == 400) {
              alert('There was an error 400');
           }
           else {
               alert('something else other than 200 was returned');
           }
        }
    }

    xmlhttp.open("GET", "/ajax", true);
    xmlhttp.send();
}