A Reducer for the Cart

So far the provider changed the cart with setCart wherever it needed to. With useReducer, every change gets a name. The provider calls dispatch with an action, an object with a type, like { type: "removed", productId: 6 }. One function, the reducer, takes the old state and the action and returns the new state.

A reducer for a counter looks like this:

function countReducer(count, action) {
  switch (action.type) {
    case "increased":
      return count + 1;
    case "reset":
      return 0;
    default:
      return count;
  }
}

A reducer must never change the old state. For an array, return a new one: [...items, line] adds a line, map swaps one line for a copy, filter drops lines. push would change the old array, and React, which compares the old items with the new ones, would see no change at all.

The check adds a Mug twice and a Teapot, removes the Teapot and places the order. Then it calls your reducer on a frozen cart that can't be changed:

> click "Cart (3)"
    item: Mug × 2 $24
    item: Teapot × 1 $22
> click "Remove" in "Teapot"
  request: DELETE /api/cart/6 -> 200
    text: Total: $24
> click "Place order"
    link: Cart (0) -> /cart
> cartReducer, called on a frozen cart: Mug × 2
  added Pen -> Mug × 2, Pen × 1
  quantity_set Mug to 5 -> Mug × 5
  removed Mug -> (empty)
  cleared -> (empty)
  the old cart: Mug × 2

Your task: in src/cartReducer.js, add a case for "added", "quantity_set", "removed" and "cleared" above default, each returning new items.

Assignment
Follow the Coding Tutorial to teach cartReducer the four actions that CartProvider and CheckoutPage dispatch.

Hint
Every case returns a new array: a spread for added, map for quantity_set, filter for removed, and [] for cleared. A case with no return below it falls through to default.