app/src/main/webapp/theme/scripts/clientSideInclude.js

/*
* Licensed to the Apache Software Foundation (ASF) under one or more
*  contributor license agreements.  The ASF licenses this file to You
* under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.  For additional information regarding
* copyright in this work, please see the NOTICE file in the top level
* directory of this distribution.
*/
// global vars for access from callback function
var req = false;
var element;

function clientSideInclude(id, url) {
  // For Safari, Firefox, and other non-MS browsers
  if (window.XMLHttpRequest) {
    try {
      req = new XMLHttpRequest();
    } catch (e) {
      req = false;
    }
  } else if (window.ActiveXObject) {
    // For Internet Explorer on Windows
    try {
      req = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
      try {
        req = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (er) {
        req = false;
      }
    }
  }
 // var element = document.getElementById(id);
 element = document.getElementById(id);
 if (!element) {
  alert("Bad id " + id +
   "passed to clientSideInclude." +
   "You need a div or span element " +
   "with this id in your page.");
  return;
 }
  if (req) {
    // Asynchronous request using processReqChange callback
    req.onreadystatechange = processReqChange;
    req.open('GET', url, true);
    req.send(null);
  }
}

// handle onreadystatechange event of req object
function processReqChange() {
    // only if req shows "loaded"
    if (req.readyState == 4) {
        // only if "OK"
        if (req.status == 200) {
            element.innerHTML = req.responseText;
         } else {
            alert("There was a problem retrieving the XML data:\n" +
                req.statusText);
         }
    }
}