useReducer.ts

interface State {
  value: number;
}

type Action =
  | { type: 'increment' }
  | { type: 'decrement' }
  | { type: 'incrementAmount'; amount: number };

const counterReducer = (state: State, action: Action) => {
  switch (action.type) {
    case 'increment':
      return { value: state.value + 1 };
    case 'decrement':
      return { value: state.value - 1 };
    case 'incrementAmount':
      return { value: state.value + action.amount };
    default:
      throw new Error();
  }
};

const [state, dispatch] = useReducer(counterReducer, { value: 0 });

dispatch({ type: 'increment' });
dispatch({ type: 'decrement' });
dispatch({ type: 'incrementAmount', amount: 10 });

// TypeScript compilation error
dispatch({ type: 'invalidActionType' });