feat(tests): add unit tests for accounts and transactions APIs

- Updated package.json to include Vitest for testing and added necessary devDependencies.
- Created accounts.test.ts to test the accounts API endpoints for listing and retrieving accounts.
- Implemented setup.ts to reset test data before each test run.
- Developed transactions.test.ts to cover creating, updating, and deleting transactions through the API.
- Added vitest.config.ts for configuring Vitest with appropriate settings and coverage options.

src/test/transactions.test.ts

@@ -0,0 +1,128 @@
+import { describe, it, expect } from "vitest";
+import { POST as createTransaction } from "../pages/api/transactions/index";
+import {
+  PUT as updateTransaction,
+  DELETE as deleteTransaction,
+} from "../pages/api/transactions/[id]/index";
+import { accounts, transactions } from "../data/store";
+
+describe("Transactions API", () => {
+  describe("POST /api/transactions", () => {
+    it("should create a new transaction", async () => {
+      const initialBalance = accounts[0].balance;
+      const newTransaction = {
+        accountId: "1",
+        date: "2025-04-24",
+        description: "Test New Transaction",
+        amount: -25.0,
+      };
+
+      const response = await createTransaction({
+        request: new Request("http://localhost:4321/api/transactions", {
+          method: "POST",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify(newTransaction),
+        }),
+      });
+
+      const result = await response.json();
+
+      expect(response.status).toBe(201);
+      expect(result).toHaveProperty("id");
+      expect(result.description).toBe(newTransaction.description);
+      expect(accounts[0].balance).toBe(initialBalance + newTransaction.amount);
+    });
+
+    it("should reject transaction with missing fields", async () => {
+      const invalidTransaction = {
+        accountId: "1",
+        // Missing required fields
+      };
+
+      const response = await createTransaction({
+        request: new Request("http://localhost:4321/api/transactions", {
+          method: "POST",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify(invalidTransaction),
+        }),
+      });
+
+      const error = await response.json();
+
+      expect(response.status).toBe(400);
+      expect(error).toHaveProperty("error", "Missing required fields");
+    });
+  });
+
+  describe("PUT /api/transactions/:id", () => {
+    it("should update an existing transaction", async () => {
+      const initialBalance = accounts[0].balance;
+      const originalAmount = transactions[0].amount;
+      const updates = {
+        description: "Updated Description",
+        amount: -75.0,
+      };
+
+      const response = await updateTransaction({
+        params: { id: "1" },
+        request: new Request("http://localhost:4321/api/transactions/1", {
+          method: "PUT",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify(updates),
+        }),
+      });
+
+      const result = await response.json();
+
+      expect(response.status).toBe(200);
+      expect(result.description).toBe(updates.description);
+      expect(result.amount).toBe(updates.amount);
+      expect(accounts[0].balance).toBe(
+        initialBalance - originalAmount + updates.amount
+      );
+    });
+
+    it("should reject update for non-existent transaction", async () => {
+      const response = await updateTransaction({
+        params: { id: "999" },
+        request: new Request("http://localhost:4321/api/transactions/999", {
+          method: "PUT",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify({ description: "Test" }),
+        }),
+      });
+
+      const error = await response.json();
+
+      expect(response.status).toBe(404);
+      expect(error).toHaveProperty("error", "Transaction not found");
+    });
+  });
+
+  describe("DELETE /api/transactions/:id", () => {
+    it("should delete a transaction", async () => {
+      const initialBalance = accounts[0].balance;
+      const transactionAmount = transactions[0].amount;
+      const initialCount = transactions.length;
+
+      const response = await deleteTransaction({
+        params: { id: "1" },
+      });
+
+      expect(response.status).toBe(204);
+      expect(transactions).toHaveLength(initialCount - 1);
+      expect(accounts[0].balance).toBe(initialBalance - transactionAmount);
+    });
+
+    it("should return 404 for non-existent transaction", async () => {
+      const response = await deleteTransaction({
+        params: { id: "999" },
+      });
+
+      const error = await response.json();
+
+      expect(response.status).toBe(404);
+      expect(error).toHaveProperty("error", "Transaction not found");
+    });
+  });
+});