Added handlers for the cart API

lib/bigcommerce/api/cart/handlers/add-item.ts

@@ -0,0 +1,37 @@
+import parseItem from '../../utils/parse-item'
+import getCartCookie from '../../utils/get-cart-cookie'
+import type { CartHandlers } from '..'
+
+// Return current cart info
+const addItem: CartHandlers['addItem'] = async ({
+  res,
+  body: { cartId, item },
+  config,
+}) => {
+  if (!item) {
+    return res.status(400).json({
+      data: null,
+      errors: [{ message: 'Missing item' }],
+    })
+  }
+  if (!item.quantity) item.quantity = 1
+
+  const options = {
+    method: 'POST',
+    body: JSON.stringify({
+      line_items: [parseItem(item)],
+    }),
+  }
+  const { data } = cartId
+    ? await config.storeApiFetch(`/v3/carts/${cartId}/items`, options)
+    : await config.storeApiFetch('/v3/carts', options)
+
+  // Create or update the cart cookie
+  res.setHeader(
+    'Set-Cookie',
+    getCartCookie(config.cartCookie, data.id, config.cartCookieMaxAge)
+  )
+  res.status(200).json({ data })
+}
+
+export default addItem