Counter.js

import React, { useReducer } from 'react'

const initialState = {count: 0}

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1}
    case 'decrement':
      return {count: state.count - 1}
    default:
      throw new Error()
  }
}

export default function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState)
  return (
    <>
      Count: {state.count}
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  )
}