react useReducer quick

app.js

const initialState = {
  counter: 0
}

const counterReducer= (state, event) => {
  if (event.type === 'INC') {
    return {
      ...state,
      counter: state.counter +1
    }
  }
  if (event.type === 'DEC') {
    return {
      ...state,
      counter: state.counter -1
    }
  }
}

const App = () => {
  const [state, dispatch] = React.useReducer(counterReducer, initialState)
  return (
    <div>
      <h1>{state.counter}</h1>
      <p onClick={() => dispatch({type: 'INC'})}> + </p>
      <p onClick={() => dispatch({type: 'DEC'})}> - </p>
    </div>
  )
}

const el = document.getElementById('app')
ReactDOM.render(<App />, el)