Testing Strategies

Test Structure (Arrange-Act-Assert)

describe("OrderService", () => {
  describe("createOrder", () => {
    it("creates an order with valid items and returns order ID", async () => {
      const repo = new InMemoryOrderRepository();
      const service = new OrderService(repo);
      const input = { customerId: "c1", items: [{ productId: "p1", quantity: 2 }] };

      const result = await service.createOrder(input);

      expect(result.id).toBeDefined();
      expect(result.status).toBe("pending");
      expect(result.items).toHaveLength(1);
      const saved = await repo.findById(result.id);
      expect(saved).toEqual(result);
    });

    it("rejects order with empty items", async () => {
      const service = new OrderService(new InMemoryOrderRepository());

      await expect(
        service.createOrder({ customerId: "c1", items: [] })
      ).rejects.toThrow("Order must have at least one item");
    });
  });
});

Name tests by behavior, not method name. Each test should be independent and self-contained.

Contract Testing (Pact)

import { PactV4 } from "@pact-foundation/pact";

const provider = new PactV4({
  consumer: "OrderService",
  provider: "UserService",
});

describe("UserService contract", () => {
  it("returns user by ID", async () => {
    await provider
      .addInteraction()
      .given("user with id user-1 exists")
      .uponReceiving("a request for user user-1")
      .withRequest("GET", "/api/users/user-1")
      .willRespondWith(200, (builder) => {
        builder.jsonBody({
          id: "user-1",
          name: "Alice",
          email: "alice@example.com",
        });
      })
      .executeTest(async (mockServer) => {
        const client = new UserClient(mockServer.url);
        const user = await client.getUser("user-1");
        expect(user.name).toBe("Alice");
      });
  });
});