ajax - undefined

function ajax_return(httpRequest) {
  if (httpRequest.readystate == 4) {
    if (httpRequest.status == 200) {
     var odpoved = httpRequest.responseText;    
    } else {
     var odpoved = "Error Occured: "+httpRequest.status+" "+httpRequest.status.Text;
    }
  }
  
    document.getElementById('chat').innerHTML = odpoved;
}

function ajax(data) {
  var httpRequest;
  
  if (window.ActiveXObject) {
    httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
  } else {
    httpRequest = new XMLHttpRequest();
  }

  httpRequest.open("POST", "includes/chat_load.php", true);
  httpRequest.onreadystatechange = function() {ajax_return(httpRequest);};
  httpRequest.send(null);  
  
}


/* -- */

    function chat_reload() {
      ajax();
    }
    
    function chat_send() {
      var input = document.chatform.chat_message;
      ajax(input.value);
      input.value = "";
    }