const store = createStore((state = { counter: 0 }, action) => {
  switch(action.type) {
    case "INCREMENT":
      return { counter: state.counter + 1 }
    case "DECREMENT":
      return { counter: state.counter - 1 }
    default:
      return state
  }
})

store.dispatch({ type: "INCREMENT" })
store.dispatch({ type: "INCREMENT" })

console.log(store.getState())