1class StateManager {
2    #states = [];
3    #subscribers = new Set();
4    #currentIndex = -1;
5
6    dispatch(action) {
7        const newState = this.#reducer(this.currentState, action);
8        this.#states = this.#states.slice(0, this.#currentIndex + 1);
9        this.#states.push(newState);
10        this.#currentIndex++;
11        this.#notifySubscribers();
12    }
13
14    #reducer(state = { count: 0, history: [] }, action) {
15        switch (action.type) {
16            case 'INCREMENT':
17                return {
18                    count: state.count + action.payload,
19                    history: [...state.history, `Added ${action.payload}`]
20                };
21            case 'DECREMENT':
22                return {
23                    count: state.count - action.payload,
24                    history: [...state.history, `Subtracted ${action.payload}`]
25                };
26            case 'RESET':
27                return {
28                    count: 0,
29                    history: [...state.history, 'Reset']
30                };
31            default:
32                return state;
33        }
34    }
35
36    get currentState() {
37        return this.#currentIndex >= 0 
38            ? this.#states[this.#currentIndex] 
39            : this.#reducer(undefined, {});
40    }
41
42    timeTravel(index) {
43        if (index >= 0 &amp;&amp; index < this.#states.length) {
44            this.#currentIndex = index;
45            this.#notifySubscribers();
46        }
47    }
48
49    #notifySubscribers() {
50        this.#subscribers.forEach(subscriber => subscriber(this.currentState));
51    }
52}
53
54const manager = new StateManager();
55manager.dispatch({ type: 'INCREMENT', payload: 5 });
56manager.dispatch({ type: 'DECREMENT', payload: 2 });
57manager.dispatch({ type: 'INCREMENT', payload: 3 });
58manager.timeTravel(1);
59manager.dispatch({ type: 'RESET' });
60console.log(manager.currentState);