testing

TDD with Effect

Red-Green-Refactor with typed effects:

1. Red: Write failing test with expected behavior
2. Green: Implement minimal code to pass
3. Refactor: Improve while tests stay green

describe("createOrder", () => {
  it("fails if product not found", async () => {
    const result = await Effect.runPromiseExit(
      createOrder({ productId: "missing" }).pipe(
        Effect.provide(TestLayers)
      )
    );
    expect(Exit.isFailure(result)).toBe(true);
  });
});

Test Layers

// Create test implementations
const ProductRepoTest = Layer.succeed(ProductRepository, {
  findById: () => Effect.succeed(testProduct),
  save: () => Effect.succeed(undefined),
});

const OrderRepoTest = Layer.succeed(OrderRepository, {
  create: (order) => Effect.succeed({ ...order, id: "test-id" }),
});

// Combine for tests
const TestLayers = Layer.mergeAll(
  ProductRepoTest,
  OrderRepoTest,
  ClockTest,
);

Testing Error Paths

it("handles payment failure", async () => {
  const FailingPayment = Layer.succeed(PaymentService, {
    charge: () => Effect.fail(new PaymentDeclinedError({ reason: "test" })),
  });

  const result = await Effect.runPromiseExit(
    processOrder(testOrder).pipe(
      Effect.provide(FailingPayment),
      Effect.provide(OtherTestLayers)
    )
  );

  expect(Exit.isFailure(result)).toBe(true);
});

Anti-Patterns

• jest.mock() → Use Layer substitution
• vi.fn() → Use real test implementations
• Partial mocks → Full test adapters
• Testing implementation → Test behavior via public API