code: function clear() {
          var that = this;
          var state = getInternalState(that);
          var data = state.index;
          var entry = state.first;
          while (entry) {
            entry.removed = true;
            if (entry.previous) entry.previous = entry.previous.next = undefined;
            delete data[entry.index];
            entry = entry.next;
          }
          state.first = state.last = undefined;
          if (DESCRIPTORS) state.size = 0;
          else that.size = 0;
        }
docs: 23.1.3.1 Map.prototype.clear() 23.2.3.2 Set.prototype.clear()