1const initialState = {count: 0};
2
3function reducer(state, action) {
4  switch (action.type) {
5    case 'increment':
6      return {count: state.count + 1};
7    case 'decrement':
8      return {count: state.count - 1};
9    default:
10      throw new Error();
11  }
12}
13
14function Counter() {
15  const [state, dispatch] = useReducer(reducer, initialState);
16  return (
17    <>
18      Count: {state.count}
19      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
20      <button onClick={() => dispatch({type: 'increment'})}>+</button>
21    </>
22  );
23}

1function init(initialCount) {  return {count: initialCount};}
2function reducer(state, action) {
3  switch (action.type) {
4    case 'increment':
5      return {count: state.count + 1};
6    case 'decrement':
7      return {count: state.count - 1};
8    case 'reset':      return init(action.payload);    default:
9      throw new Error();
10  }
11}
12
13function Counter({initialCount}) {
14  const [state, dispatch] = useReducer(reducer, initialCount, init);  return (
15    <>
16      Count: {state.count}
17      <button
18        onClick={() => dispatch({type: 'reset', payload: initialCount})}>        Reset
19      </button>
20      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
21      <button onClick={() => dispatch({type: 'increment'})}>+</button>
22    </>
23  );
24}

1const getTotalItems = (items: CartItemType[]) =>
2    items.reduce((ack: number, item) => ack + item.amount, 0);
3