Implement a Hydrogen cart using createCartHandler and wire it to cart actions in a Remix app

In-depth guide

Steps

1. In server.ts call createCartHandler with storefront and getCartId / setCartId callbacks; getCartId reads the cart ID from a cookie (or session), setCartId writes it back on the response headers
2. The handler returns a cart object with methods: cart.get(), cart.addLines(), cart.updateLines(), cart.removeLines(), cart.updateDiscountCodes(), and others — call these in Remix action functions
3. Create a cart.tsx route (or an action in root.tsx) with a Remix action that reads a formData action field to dispatch the correct cart mutation (for example, 'add-line', 'remove-line')
4. In the action call await cart.addLines([{ merchandiseId, quantity }]) and return the response using cart.setCartId inside a redirect or json response so the updated cart cookie is propagated
5. In the root loader call cart.get() to fetch the current cart and pass it to the Outlet context; components subscribe to cart state via useCart() or useMatches()
6. Ensure the CartProvider from @shopify/hydrogen is wrapping the app root for client-side optimistic cart updates if you use the built-in cart components