function createStore(reducer, initialState) {
  let currentState = initialState
  let currentReducer = reducer
  const listeners = []

  return {
    getState() {
      return currentState
    },
    dispatch(action) {
      currentState = currentReducer(currentState, action)
      listeners.forEach(fn => fn())
      return action
    },
    subscribe(newListener) {
      listeners.push(newListener)
      return () => listeners.splice(listeners.indexOf(newListener), 1)
    }
  }
}

function counter(state = 0, action) {
  switch (action.type) {
    case 'INCREMENT': return state + 1
    case 'DECREMENT': return state - 1
    default: return state
  }
}

let store = createStore(counter)

const unsubscribe = store.subscribe(() => {
  console.log(store.getState())
})

store.dispatch({ type: 'INCREMENT' })
store.dispatch({ type: 'DECREMENT' })
unsubscribe()
store.dispatch({ type: 'DECREMENT' })