code: function (key) {
          var that = this;
          var state = getInternalState(that);
          var entry = getEntry(that, key);
          if (entry) {
            var next = entry.next;
            var prev = entry.previous;
            delete state.index[entry.index];
            entry.removed = true;
            if (prev) prev.next = next;
            if (next) next.previous = prev;
            if (state.first == entry) state.first = next;
            if (state.last == entry) state.last = prev;
            if (DESCRIPTORS) state.size--;
            else that.size--;
          } return !!entry;
        }
docs: 23.1.3.3 Map.prototype.delete(key) 23.2.3.4 Set.prototype.delete(value)