/** Function count the occurrences of substring in a string;
 * @param {String} string   Required. The string;
 * @param {String} subString    Required. The string to search for;
 * @param {Boolean} allowOverlapping    Optional. Default: false;
 */
function occurrences(string, subString, allowOverlapping) {
  string += "";
  subString += "";
  if (subString.length <= 0) return string.length + 1;

  var n = 0, pos = 0;
  var step = (allowOverlapping) ? (1):(subString.length);

  while (true) {
    pos = string.indexOf(subString, pos);
    if (pos >= 0) {
      n++;
      pos += step;
    } else break;
  }
  return(n);
}