Allow calling the cart .get() method right after cart mutation methods

.changeset/itchy-weeks-search.md

@@ -0,0 +1,27 @@
+---
+'@shopify/hydrogen': patch
+---
+
+Allow calling the cart `.get()` method right after creating a new cart with
+one of the mutation methods: `create()`, `addLines()`, `updateDiscountCodes()`, `updateBuyerIdentity()`, `updateNote()`, `updateAttributes()`, `setMetafields()`.
+
+```ts
+import {
+  createCartHandler,
+  cartGetIdDefault,
+  cartSetIdDefault,
+} from '@shopify/hydrogen';
+
+const cartHandler = createCartHandler({
+  storefront,
+  getCartId: cartGetIdDefault(request.headers),
+  setCartId: cartSetIdDefault(),
+  cartQueryFragment: CART_QUERY_FRAGMENT,
+  cartMutateFragment: CART_MUTATE_FRAGMENT,
+});
+
+await cartHandler.addLines([{merchandiseId: '...'}]);
+// This change fixes a bug where `cart` would be null, even though a
+// new cart was created when adding a line item
+const cart = await cartHandler.get();
+```

packages/hydrogen/src/cart/createCartHandler.test.ts

@@ -399,4 +399,17 @@ describe('createCartHandler', () => {
 
     expect(result.cart).toHaveProperty('id', 'gid://shopify/Cart/c1-456');
   });
+
+  it('stores the cartId in memory when a new cart is created and returns that result if available', async () => {
+    const cart = getCartHandler();
+
+    await cart.addLines([
+      {
+        merchandiseId: '1',
+        quantity: 1,
+      },
+    ]);
+
+    expect(await cart.get()).toHaveProperty('id', 'c1-new-cart-id');
+  });
 });

packages/hydrogen/src/cart/createCartHandler.ts

@@ -96,22 +96,31 @@ export function createCartHandler<TCustomMethods extends CustomMethodsBase>(
   options: CartHandlerOptions | CartHandlerOptionsWithCustom<TCustomMethods>,
 ): CartHandlerReturn<TCustomMethods> {
   const {
-    getCartId,
+    getCartId: _getCartId,
     setCartId,
     storefront,
     customerAccount,
     cartQueryFragment,
     cartMutateFragment,
   } = options;
 
+  let cartId = _getCartId();
+
+  const getCartId = () => cartId || _getCartId();
+
   const mutateOptions = {
     storefront,
     getCartId,
     cartFragment: cartMutateFragment,
   };
 
-  const cartId = getCartId();
-  const cartCreate = cartCreateDefault(mutateOptions);
+  const _cartCreate = cartCreateDefault(mutateOptions);
+
+  const cartCreate: CartCreateFunction = async function (...args) {
+    const result = await _cartCreate(...args);
+    cartId = result?.cart?.id;
+    return result;
+  };
 
   const methods: HydrogenCart = {
     get: cartGetDefault({