code: function forEach(callbackfn /*, that = undefined */) {
                                  anInstance(this, C, 'forEach');
                                  var f = ctx(callbackfn, arguments.length > 1 ? arguments[1] : undefined, 3)
                                      , entry;
                                  while (entry = entry ? entry.n : this._f) {
                                      f(entry.v, entry.k, this);
                                      // revert to the last existing entry
                                      while (entry && entry.r) entry = entry.p;
                                  }
                              }
docs: 23.2.3.6 Set.prototype.forEach(callbackfn, thisArg = undefined) 23.1.3.5 Map.prototype.forEach(callbackfn, thisArg = undefined)