add test case for fetch existing systems method

test/loaders/ApiBasedRepositoryLoaderTests.test.ts

@@ -4,6 +4,7 @@ import { UnoptimizedInMemoryRepository } from "../../src/repositories/Unoptimize
 import { fetchSystemDataSuccessfulResponse } from "../jsonSnapshots/fetchSystemData/fetchSystemDataSuccessfulResponse";
 import { fetchSystemDataFailedResponse } from "../jsonSnapshots/fetchSystemData/fetchSystemDataFailedResponse";
 import { fetchRouteDataSuccessfulResponse } from "../jsonSnapshots/fetchRouteData/fetchRouteDataSuccessfulResponse";
+import { ISystem } from "../../src/entities/entities";
 
 /**
  * Function to update behavior of the global `fetch` function.
@@ -79,9 +80,28 @@ describe("ApiBasedRepositoryLoader", () => {
 
 
   describe("fetchAndUpdateRouteDataForExistingSystemsInRepository", () => {
-    // TODO
+    test("calls fetchAndUpdateRouteDataForSystemId for all systems in repository", async () => {
-    // Add a test case to ensure that fetchAndUpdateRouteDataForSystemId
+      const spy = jest.spyOn(loader, "fetchAndUpdateRouteDataForSystemId");
-    // was called for every available system ID
+
+      const systems: ISystem[] = [
+        {
+          name: "Chapman University",
+          id: "1",
+        },
+        {
+          name: "City of Monterey Park",
+          id: "2",
+        }
+      ];
+
+      await Promise.all(systems.map(async (system) => {
+        await loader.repository.addOrUpdateSystem(system);
+      }));
+
+      await loader.fetchAndUpdateRouteDataForExistingSystemsInRepository();
+
+      expect(spy.mock.calls.length).toBe(2);
+    });
   });
 
   describe("fetchAndUpdateRouteDataForSystemId", () => {