forEach(callback) {
      var thisArg = arguments[1];
      var it = this._map.keys();
      var next;
      while (!(next = it.next()).done) {
        callback.call(thisArg, next.value, next.value, this);
      }
    }
23.2.3.6

Iterate over the collection calling `callback` with (value, value, set).

@param {function} callback