간단한 Redux 스타일 Store 구현

type Reducer<S, A> = (state: S, action: A) => S
type Listener<S> = (state: S) => void

interface Store<S, A> {
  getState: () => S
  subscribe: (listener: Listener<S>) => () => void
  dispatch: (action: A) => A
}

function createStore<S, A>(
  reducer: Reducer<S, A>,
  preloadedState: S
): Store<S, A> {
  let currentState = preloadedState
  let listeners: Listener<S>[] = []

  return {
    getState: () => currentState,
    subscribe: (listener) => {
      listener(currentState)
      listeners.push(listener)
      return () => {
        listeners = listeners.filter((l) => l !== listener)
      }
    },
    dispatch: (action) => {
      currentState = reducer(currentState, action)
      listeners.forEach((l) => l(currentState))
      return action
    },
  }
}