code: function _mapCopy(sourceRef, copyRef, rc) {
      return _instanceCopy(sourceRef, copyRef, rc, (map, val, key) => {
          map.set(key, _clone(val, rc));
      });
  }
docs: Copies data from one map to another.
  
  @param sourceRef A reference to the map source.
  @param copyRef A reference to the map copy.
  @param rc The current recurse and reference counter.
  @returns {Map} The reference to the map copy.