Constructing a Type-Safe Micro State Store with Subscriptions

type Listener<S> = (state: Readonly<S>) => void;
 
class Store<S extends object> {
  private state: S;
  private listeners = new Set<Listener<S>>();
 
  constructor(initialState: S) {
    this.state = Object.freeze({ ...initialState });
  }
 
  getState(): Readonly<S> {
    return this.state;
  }
 
  setState(updater: Partial<S> | ((prevState: Readonly<S>) => Partial<S>)): void {
    const changes = typeof updater === "function" ? updater(this.state) : updater;
    this.state = Object.freeze({ ...this.state, ...changes });
    this.listeners.forEach((listener) => listener(this.state));
  }
 
  subscribe(listener: Listener<S>): () => void {
    this.listeners.add(listener);
    return () => this.listeners.delete(listener);
  }
}
 
interface CounterState {
  count: number;
  lastUpdated: string;
}
 
const store = new Store<CounterState>({ count: 0, lastUpdated: "never" });
const unsubscribe = store.subscribe((state) => console.log(`Count: ${state.count}`));
store.setState({ count: 1, lastUpdated: "now" });
unsubscribe();