Good and Bad Tests

Good Tests

Integration-style: Test through real interfaces, not mocks of internal parts.

// GOOD: Tests observable behavior
test("user can checkout with valid cart", async () => {
  const cart = createCart();
  cart.add(product);
  const result = await checkout(cart, paymentMethod);
  expect(result.status).toBe("confirmed");
});

Characteristics:

• Tests behavior users/callers care about
• Uses public API only
• Survives internal refactors
• Describes WHAT, not HOW
• One logical assertion per test

Bad Tests

Implementation-detail tests: Coupled to internal structure.

// BAD: Tests implementation details
test("checkout calls paymentService.process", async () => {
  const mockPayment = jest.mock(paymentService);
  await checkout(cart, payment);
  expect(mockPayment.process).toHaveBeenCalledWith(cart.total);
});

Red flags:

• Mocking internal collaborators
• Testing private methods
• Asserting on call counts/order
• Test breaks when refactoring without behavior change
• Test name describes HOW not WHAT
• Verifying through external means instead of interface

// BAD: Bypasses interface to verify
test("createUser saves to database", async () => {
  await createUser({ name: "Alice" });
  const row = await db.query("SELECT * FROM users WHERE name = ?", ["Alice"]);
  expect(row).toBeDefined();
});

// GOOD: Verifies through interface
test("createUser makes user retrievable", async () => {
  const user = await createUser({ name: "Alice" });
  const retrieved = await getUser(user.id);
  expect(retrieved.name).toBe("Alice");
});

Tautological tests: Expected value restates the implementation, so the test passes by construction.

// BAD: Expected value is recomputed the way the code computes it
test("calculateTotal sums line items", () => {
  const items = [{ price: 10 }, { price: 5 }];
  const expected = items.reduce((sum, i) => sum + i.price, 0);
  expect(calculateTotal(items)).toBe(expected);
});

// GOOD: Expected value is an independent, known literal
test("calculateTotal sums line items", () => {
  expect(calculateTotal([{ price: 10 }, { price: 5 }])).toBe(15);
});