code: function _toNaturalIndex (idx, len) {
      idx = _toInteger(idx);
  
      return idx >= -len && idx < len ? idx < 0 ? idx + len : idx : NaN;
  }
docs: Checks if the given number, even negative, represents an array-like index
  within the provided length. If so returns its natural number equivalent.<br/>
  Returns <code>NaN<code> otherwise.
  @private
  @param {Number} idx
  @param {Number} len
  @returns {Number}