Onreadystatechange Function Never Gets Called

Ok...so my code is very simple. The only problem is that the function to be called onreadystatechange is never getting executed. I put in an alert to display the readyState and the

Solution 1:

You need switch the calling order of xmlhttp.onreadystatechange and xmlhttp.open to make sure the onreadystatechange callback is registered before opening.

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