block3.ts

// actions.ts
// Every action in the entire application would be part of this union type
type MyActions = 
  AddToCartAction | FilterItemsAction | SomeOtherAction /* ... */;

// reducer.ts
function reducer(state: MyAppState, action: MyActions) {
  // action has type MyAction, which contains every possible action
  if (action.type === 'ADD_TO_CART') {
    // Since only one action has `type` === 'ADD_TO_CART', TypeScript
    // knows the narrowed type of this action has `payload.productIds`
    const addedProductIds = action.payload.productIds;
    // etc...
  }

  if (action.type === 'FILTER_ITEMS') {
    // Also type-safe
    const filter = action.payload.filterType;
    // etc...
  }

  return state;
}