code: function find(predicate) {
          var key, elem;
          for (key in _idMap) {
              if (_idMap.hasOwnProperty(key)) {
                  elem = _idMap[key];
                  if (predicate(elem)) {
                      return elem;
                  }
              }
          }
          return null;
      }
docs: Find the first matched element satisfying the predicate.
  @param {function(Element):boolean} predicate A function to filter elements.
  @return {Element} A matched element.