User Request: "Add cart" / "Checkout" / "Payments"
    │
    ├─ Checkout approach?
    │   │
    │   ├─ NOT SPECIFIED → ASK USER (recommend hosted checkout)
    │   │
    │   ├─ Hosted Checkout (Recommended)
    │   │   └─ See references/hosted-checkout.md
    │   │   └─ Install @commercengine/checkout
    │   │   └─ initCheckout() + useCheckout()
    │   │   └─ Using SDK auth? → authMode: "provided" + sync tokens (see hosted-checkout.md & Auth Mode)
    │   │
    │   └─ Custom Checkout (Advanced)
    │       └─ See references/checkout-flow.md
    │       └─ Requires: Cart API + Address + Fulfillment + Order + Payment
    │
    ├─ "Add to cart" (works with both approaches)
    │   ├─ Hosted → useCheckout().addToCart(productId, variantId, quantity)
    │   └─ Custom → sdk.cart.addDeleteCartItem()
    │
    ├─ "View cart"
    │   ├─ Hosted → useCheckout().openCart()
    │   └─ Custom → sdk.cart.getCart({ id }) or getUserCart({ user_id })
    │
    └─ "Apply coupon" / "Discount"
        ├─ Hosted → Built-in (enabled via features.coupons)
        └─ Custom → sdk.cart.applyCoupon({ id }, { coupon_code })

npm install @commercengine/checkout @commercengine/storefront-sdk

// Recommended default for new storefront apps:
// Storefront SDK owns auth; Hosted Checkout runs in provided mode.
import StorefrontSDK, { BrowserTokenStorage } from "@commercengine/storefront-sdk";
import { initCheckout, getCheckout } from "@commercengine/checkout";

const tokenStorage = new BrowserTokenStorage("ce_");

const storefront = new StorefrontSDK({
  storeId: process.env.NEXT_PUBLIC_STORE_ID!,
  apiKey: process.env.NEXT_PUBLIC_API_KEY!,
  tokenStorage,
  onTokensUpdated: (accessToken, refreshToken) => {
    // SDK -> checkout
    getCheckout().updateTokens(accessToken, refreshToken);
  },
});

const accessToken = await tokenStorage.getAccessToken();
const refreshToken = await tokenStorage.getRefreshToken();

initCheckout({
  storeId: process.env.NEXT_PUBLIC_STORE_ID!,
  apiKey: process.env.NEXT_PUBLIC_API_KEY!,
  environment: "production",
  authMode: "provided",
  accessToken: accessToken ?? undefined,
  refreshToken: refreshToken ?? undefined,
  onTokensUpdated: ({ accessToken, refreshToken }) => {
    // checkout -> SDK
    storefront.setTokens(accessToken, refreshToken);
  },
});

// Use in any component
import { useCheckout } from "@commercengine/checkout/react";

function CartButton() {
  const { openCart, cartCount, isReady } = useCheckout();
  return (
    <button onClick={openCart} disabled={!isReady}>
      Cart ({cartCount})
    </button>
  );
}

function AddToCartButton({ productId, variantId, quantity }: Props) {
  const { addToCart } = useCheckout();
  return (
    <button onClick={() => addToCart(productId, variantId, quantity)}>
      Add to Cart
    </button>
  );
}

// Create a cart (at least one item required — cannot create empty cart)
const { data, error } = await sdk.cart.createCart({
  items: [
    { product_id: "prod_123", variant_id: "var_456", quantity: 2 },
  ],
});

const cartId = data.cart.id;

// Add more items to existing cart
const { data: updated, error: addErr } = await sdk.cart.addDeleteCartItem(
  { id: cartId },
  { product_id: "prod_789", variant_id: "var_012", quantity: 1 }
);

// Update quantity (same method — addDeleteCartItem handles add, update, and remove)
const { data, error } = await sdk.cart.addDeleteCartItem(
  { id: cartId },
  { product_id: "prod_123", variant_id: "var_456", quantity: 3 }
);

// Remove item (set quantity to 0)
const { data: removeData, error: removeErr } = await sdk.cart.addDeleteCartItem(
  { id: cartId },
  { product_id: "prod_123", variant_id: "var_456", quantity: 0 }
);

// List available coupons
const { data: coupons } = await sdk.cart.getAvailableCoupons();

// Apply a coupon
const { data, error } = await sdk.cart.applyCoupon(
  { id: cartId },
  { coupon_code: "SAVE20" }
);

// Remove coupon
const { data: removeData, error: removeErr } = await sdk.cart.removeCoupon({ id: cartId });