function substr_count (haystack, needle, offset, length) {
  // http://kevin.vanzonneveld.net
  // +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   bugfixed by: Onno Marsman
  // +   improved by: Brett Zamir (http://brett-zamir.me)
  // +   improved by: Thomas
  // *     example 1: substr_count('Kevin van Zonneveld', 'e');
  // *     returns 1: 3
  // *     example 2: substr_count('Kevin van Zonneveld', 'K', 1);
  // *     returns 2: 0
  // *     example 3: substr_count('Kevin van Zonneveld', 'Z', 0, 10);
  // *     returns 3: false

  var cnt = 0;

  haystack += '';
  needle += '';
  if (isNaN(offset)) {
    offset = 0;
  }
  if (isNaN(length)) {
    length = 0;
  }
  if (needle.length == 0) {
    return false;
  }
  offset--;

  while ((offset = haystack.indexOf(needle, offset + 1)) != -1) {
    if (length > 0 && (offset + needle.length) > length) {
      return false;
    }
    cnt++;
  }

  return cnt;
}

substr_count('Kevin van Zonneveld', 'e');

3

substr_count('Kevin van Zonneveld', 'K', 1);

0

substr_count('Kevin van Zonneveld', 'Z', 0, 10);

false