Function reducer

hook-test/src/App3.tsx:11–26

(state: Data, action: Action)

    num: number
}
function reducer(state: Data, action: Action) {

    switch(action.type) {
        case 'add':
            return {
                result: state.result + action.num
            }
            // state.result += action.num
            // return state;
        case 'minus': 
            return {
                result: state.result - action.num
            }
    }
    return state;
}

function App() {
  const [res, dispatch] = useReducer<Reducer<Data, Action>, string>(reducer, 'zero', (param) => {