XMLhttprequest问题

I'm trying to work on xmlhttprequests which isn't working. When inserting an alert box I receive a status of 0. Cant quite figure out what's wrong.

function submitChat() {
    if (form1.uname.value != '' && form1.msg.value != '') {
      var uname = form1.uname.value;
      var msg = form1.msg.value;
      var xmlhttp = new XMLHttpRequest();
      alert(xmlhttp.status);
      xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readystate == 4 && xmlhttp.status == 200) {
          document.getElementById('chatlogs').innerHTML = xmlhttp.responseText;
          }
      }
      xmlhttp.open("GET", "insert.php?uname=" + uname + "&msg=" + msg, true);
      xmlhttp.send();
    } else {
      alert("All Fields are Mandatory !!!");
      return;
    }
}

<form name="form1" action="#">`` Chatname:
  <input type="text" name="uname" />
  <br/>
  <textarea name="msg"></textarea>
  <a href="javascript: submitChat()"> Send</a> 
  <br/>
  <br/>
</form>

<div id="chatlogs">
  Loading Chat History !!!!!!!!!!
</div>

</div>

You should do .open() before your alert. So your code becomes like this:

function submitChat() {
    if (form1.uname.value != '' && form1.msg.value != '') {

        var uname = form1.uname.value;

        var msg = form1.msg.value;
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("GET","insert.php?uname="+uname+"&msg="+msg,true);
        alert (xmlhttp.status);
        xmlhttp.onreadystatechange = function() {
            if(xmlhttp.readystate==4 && xmlhttp.status==200) {
                document.getElementById('chatlogs').innerHTML = xmlhttp.responseText;


            }

        }

        xmlhttp.send(null); 



    } else {
        alert("All Fields are Mandatory !!!");
        return;
    }
}