AJAX请求返回'undefined'

Question

For some reason I always receive undefined if I return the value but if I'm trying to display it in alert I receive the php values.

function getXMLHttp() {
  var xmlHttp

  try {
    //Firefox, Opera 8.0+, Safari
    xmlHttp = new XMLHttpRequest();
  } catch(e) {
    //Internet Explorer
    try {
      xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch(e) {
      try {
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
      } catch(e) {
        //Browser does not support AJAX
        return false;
      }
    }
  }

  return xmlHttp;
}

function isUsernameExists() {
  var xmlHttp = getXMLHttp();

  xmlHttp.onreadystatechange = function() {
    if (xmlHttp.readyState == 4) {
    handleResponse(xmlHttp.responseText);
    }
  }

  var str = document.getElementById('username').value.toString();
  xmlHttp.open("GET", "ajax.php?username="+str, true);
  xmlHttp.send(null);

}

Edit:

function handleResponse(response) {
    return response.toString();
}

Thanks,

Guy Dor

Answer 1

It looks like you are trying to read the return value from this function:

function isUsernameExists() {

It doesn't have a return statement, so it will always be undefined.

I'm guessing you expect this return statement to pass the value you want:

return xmlHttp.responseText.toString();

But that is part of this function:

xmlHttp.onreadystatechange = function() {

Which is called automatically when the readystatechange event fires, and not by any function call you make.

Asynchronous JavaScript and XML rarely uses XML but is asynchronous. Anything you want to do with the data fetched needs to be done by the callback function you assign to onreadystatechange. It can call other functions, but it cannot return anything (at least not that will be received anywhere useful).