code: function forEach(callbackfn /* , that = undefined */) {
          var state = getInternalState(this);
          var boundFunction = bind(callbackfn, arguments.length > 1 ? arguments[1] : undefined, 3);
          var entry;
          while (entry = entry ? entry.next : state.first) {
            boundFunction(entry.value, entry.key, this);
            // revert to the last existing entry
            while (entry && entry.removed) entry = entry.previous;
          }
        }
docs: 23.2.3.6 Set.prototype.forEach(callbackfn, thisArg = undefined) 23.1.3.5 Map.prototype.forEach(callbackfn, thisArg = undefined)