javascript - Post Ajax function to return true or false

I'm trying to get an Ajax function to return true or false (it's a sort of validation function which is called from another function). So the other function needs to get a true or false value from this one so it would know what to do next

This is the function:

function validateLogin( uid, pass, i ){

        var uvalue = uid.value;
        var pvalue = pass.value;

        var xmlhttp;
        if (window.XMLHttpRequest)

          {// code for IE7+, Firefox, Chrome, Opera, Safari
          xmlhttp=new XMLHttpRequest();
          }
        else
          {// code for IE6, IE5
          xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
          }

        xmlhttp.open("POST","validateCheck.php",true);
        xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");


        xmlhttp.onreadystatechange=function()
          {
          if (xmlhttp.readyState==4 && xmlhttp.status==200)
            {

                if( xmlhttp.responseText == "Error" )
                    return false;
                else if( xmlhttp.responseText == "Yes" )
                    return a;

                else if( xmlhttp.responseText == 1 )
                        return false;               
                else if( xmlhttp.responseText == 2 )
                        return false;
                else if( xmlhttp.responseText == 3 )
                        return false;
                else if( xmlhttp.responseText == 4 )
                         return false;
                else if( xmlhttp.responseText == 5 )
                        return false;

                else
                    return true;
            }
          }

          xmlhttp.send("uid="+uvalue+"&pass="+pvalue);  
    }

Calling function is:

function validateForm(i){
   var uid = document.myform1.username;
   var pass = document.myform1.password;

   if( validateLogin(uid,pass,i) )
        alert(validateLogin(uid,pass,i));

}

But the function validateLogin is not returning anything to the calling function(validateForm).