re-add decrease quantity function

Author: Steph Dietz

backend/functions/assembly/crud.ts

@@ -314,6 +314,37 @@ export function addToCart(cartId: string, productId: string): string {
   return "success";
 }
 
+export function decreaseQuantity(cartId: string, productId: string): string {
+  const cartItemId = cartId + "_" + productId;
+  const cartItemQuantity = collections.getText(
+    consts.cartItemsCollection,
+    cartItemId,
+  );
+
+  if (cartItemQuantity === null || cartItemQuantity === "") {
+    console.log("Failed to retrieve cart item quantity for:");
+    return "error";
+  }
+
+  const newQuantity = parseFloat(cartItemQuantity) - 1;
+  if (newQuantity === 0) {
+    return removeFromCart(cartId, productId);
+  }
+
+  const upsertQuantityResult = collections.upsert(
+    consts.cartItemsCollection,
+    cartItemId,
+    newQuantity.toString(),
+  );
+
+  if (!upsertQuantityResult.isSuccessful) {
+    console.log("Failed to update quantity:");
+    return upsertQuantityResult.error;
+  }
+
+  return "success";
+}
+
 export function removeFromCart(cartId: string, productId: string): string {
   const cartItemId = cartId + "_" + productId;
   const result = collections.remove(consts.cartItemsCollection, cartItemId);