fix(rest/nodejs): express discount allocation path as a JSONPath

rest/nodejs/src/api/checkout.ts

@@ -412,7 +412,12 @@ export class CheckoutService {
             code,
             title: "10% Off",
             amount: discountAmount,
-            allocations: [{ path: "subtotal", amount: discountAmount }],
+            allocations: [
+              {
+                path: "$.totals[?(@.type=='subtotal')]",
+                amount: discountAmount,
+              },
+            ],
           });
           checkout.totals.push({ type: "discount", amount: -discountAmount });
         } else if (upperCode === "WELCOME20") {
@@ -422,7 +427,12 @@ export class CheckoutService {
             code,
             title: "Welcome 20% Off",
             amount: discountAmount,
-            allocations: [{ path: "subtotal", amount: discountAmount }],
+            allocations: [
+              {
+                path: "$.totals[?(@.type=='subtotal')]",
+                amount: discountAmount,
+              },
+            ],
           });
           checkout.totals.push({ type: "discount", amount: -discountAmount });
         } else if (upperCode === "FIXED500") {
@@ -432,7 +442,12 @@ export class CheckoutService {
             code,
             title: "$5.00 Off",
             amount: discountAmount,
-            allocations: [{ path: "subtotal", amount: discountAmount }],
+            allocations: [
+              {
+                path: "$.totals[?(@.type=='subtotal')]",
+                amount: discountAmount,
+              },
+            ],
           });
           checkout.totals.push({ type: "discount", amount: -discountAmount });
         }

rest/nodejs/test/discount.test.ts

@@ -132,3 +132,33 @@ test("an applied discount's allocations sum to its amount", () => {
   }
   assert.ok(checked > 0, "expected at least one discount carrying allocations");
 });
+
+// An allocation target must be expressed as a JSONPath (discount.json:
+// allocation.path), so a platform can resolve where the discount applied —
+// not a bare label like "subtotal".
+test("an applied discount's allocation path is a JSONPath", () => {
+  const checkout = checkoutWithCodes(["10OFF"]);
+  new CheckoutService()["recalculateTotals"](checkout);
+
+  const applied = (
+    checkout as unknown as {
+      discounts: {
+        applied: Array<{
+          allocations?: Array<{ path: string; amount: number }>;
+        }>;
+      };
+    }
+  ).discounts.applied;
+
+  let checked = 0;
+  for (const a of applied) {
+    for (const alloc of a.allocations ?? []) {
+      checked += 1;
+      assert.ok(
+        alloc.path.startsWith("$."),
+        `allocation path "${alloc.path}" must be a JSONPath rooted at "$."`
+      );
+    }
+  }
+  assert.ok(checked > 0, "expected at least one allocation to check");
+});