function createRequestObject() {
   var req;
   if(window.XMLHttpRequest){
      // Firefox, Safari, Opera...
      req = new XMLHttpRequest();
   }else if(window.ActiveXObject) {
      // Internet Explorer 5+
      req = new ActiveXObject("Microsoft.XMLHTTP");
   }else {
      // There is an error creating the object,
      // just as an old browser is being used.
      alert('Problem creating the XMLHttpRequest object');
   }
   return req;
}

// Make the XMLHttpRequest object
var http = createRequestObject();

function sendPostcode(postcode) {
   // Open PHP script for requests
   http.open('get', 'get.php?postcode=' + postcode);
   http.onreadystatechange = handleResponse;
   http.send(null);
}

function handleResponse() {
   if(http.readyState == 4 && http.status == 200){
      // Text returned FROM the PHP script	  
      var response = http.responseText;
      if(response) {
         // UPDATE ajaxTest content
         document.getElementById("town").value = response;
      }
   }
}