Keep only ids in the cartカートにはIDと数量だけ

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Little Roastery</title>
  <link rel="stylesheet" href="https://wcstack.github.io/tutorial/shop.css">
  <script type="module" src="https://esm.run/@wcstack/state@4/auto"></script>
  <script type="module" src="https://esm.run/@wcstack/fetch@4/auto"></script>
</head>
<body>

<wcs-state>
  <script type="module">
    /** @typedef {{ id: number, name: string, category: string, description: string, price: number, stock: number, icon: string }} Product */

    export default {
      /** @type {Product[] | null} */
      catalog: null,
      loading: false,
      loadError: null,
      reload: false,
      cart: [],
      query: "",
      category: "all",

      get products() {
        return this.catalog ?? [];
      },
      get catalogReady() {
        return this.catalog !== null;
      },
      get "products.*.matches"() {
        const query = this.query.trim().toLowerCase();
        return (this.category === "all" || this["products.*.category"] === this.category) &&
          this["products.*.name"].toLowerCase().includes(query);
      },
      get matchCount() {
        return this.$getAll("products.*.matches", []).filter(Boolean).length;
      },

      get "cart.*.product"() {
        const id = this["cart.*.id"];
        return this.products.find(product => product.id === id);
      },
      get "cart.*.name"() {
        return this["cart.*.product"]?.name ?? "…";
      },
      get "cart.*.price"() {
        return this["cart.*.product"]?.price ?? 0;
      },
      get "cart.*.lineTotal"() {
        return this["cart.*.price"] * this["cart.*.quantity"];
      },
      get itemCount() {
        return this.$getAll("cart.*.quantity", []).reduce((sum, n) => sum + n, 0);
      },
      get subtotal() {
        return this.$getAll("cart.*.lineTotal", []).reduce((sum, n) => sum + n, 0);
      },
      get shipping() {
        return this.subtotal >= 50 ? 0 : 5;
      },
      get total() {
        return this.subtotal + this.shipping;
      },

      retry() {
        this.reload = true;
      },
      addToCart(event, index) {
        const product = this.products[index];
        const line = this.cart.findIndex(item => item.id === product.id);
        if (line >= 0) {
          this[`cart.${line}.quantity`] += 1;
        } else {
          this.cart = this.cart.concat({ id: product.id, quantity: 1 });
        }
      },
      increase(event, index) {
        this[`cart.${index}.quantity`] += 1;
      },
      decrease(event, index) {
        this[`cart.${index}.quantity`] -= 1;
      },
      remove(event, index) {
        this.cart = this.cart.toSpliced(index, 1);
      }
    };
  </script>
</wcs-state>

<wcs-fetch url="https://wcstack.github.io/tutorial/data/products.json"
  data-wcs="value: catalog; loading: loading; error: loadError; trigger: reload"></wcs-fetch>

<header class="shop-header">
  <h1>Little Roastery</h1>
  <p class="cart-badge">Cart: <span data-wcs="textContent: itemCount"></span></p>
</header>

<p class="status" data-wcs="hidden: loading|not">Loading products…</p>
<p class="status error" data-wcs="hidden: loadError|not">
  Couldn't load the products.
  <button type="button" data-wcs="onclick: retry">Try again</button>
</p>

<main class="shop" data-wcs="hidden: catalogReady|not">
  <section class="catalog">
    <h2>Products</h2>
    <div class="toolbar">
      <input type="search" placeholder="Search products" aria-label="Search products" data-wcs="value: query">
      <fieldset class="categories">
        <legend class="visually-hidden">Category</legend>
        <label><input type="radio" name="category" value="all" data-wcs="radio: category"> All</label>
        <label><input type="radio" name="category" value="coffee" data-wcs="radio: category"> Coffee</label>
        <label><input type="radio" name="category" value="tea" data-wcs="radio: category"> Tea</label>
        <label><input type="radio" name="category" value="tools" data-wcs="radio: category"> Tools</label>
      </fieldset>
    </div>
    <div class="grid">
      <template data-wcs="for: products">
        <article class="card" data-wcs="hidden: .matches|not; class.sold-out: .stock|eq(0)">
          <div class="card-icon">{{ .icon }}</div>
          <h3>{{ .name }}</h3>
          <p>{{ .description }}</p>
          <p class="price">${{ .price|toFixed(2) }}</p>
          <p class="badge" data-wcs="hidden: .stock|ne(0)">Sold out</p>
          <button type="button" data-wcs="onclick: addToCart; disabled: .stock|eq(0)">Add to cart</button>
        </article>
      </template>
    </div>
    <p class="no-results" data-wcs="hidden: matchCount|ne(0)">No products match.</p>
  </section>

  <aside class="cart">
    <h2>Cart</h2>
    <template data-wcs="if: cart.length|eq(0)">
      <p class="empty">Your cart is empty.</p>
    </template>
    <template data-wcs="else:">
      <ul class="cart-lines">
        <template data-wcs="for: cart">
          <li>
            <span class="line-name">{{ .name }}</span>
            <span class="line-total">${{ .lineTotal|toFixed(2) }}</span>
            <span class="stepper">
              <button type="button" aria-label="Decrease" data-wcs="onclick: decrease; disabled: .quantity|le(1)">−</button>
              <span>{{ .quantity }}</span>
              <button type="button" aria-label="Increase" data-wcs="onclick: increase">+</button>
            </span>
            <button type="button" class="remove" data-wcs="onclick: remove">Remove</button>
          </li>
        </template>
      </ul>
      <dl class="totals">
        <dt>Subtotal</dt><dd>${{ subtotal|toFixed(2) }}</dd>
        <dt>Shipping</dt><dd>${{ shipping|toFixed(2) }}</dd>
        <dt class="grand">Total</dt><dd class="grand">${{ total|toFixed(2) }}</dd>
      </dl>
      <p class="hint" data-wcs="hidden: shipping|eq(0)">Free shipping on orders of $50 or more.</p>
    </template>
  </aside>
</main>

</body>
</html>