Unit vs integration vs E2E tests?

Definition:

Unit tests check a small piece of logic, integration tests verify collaborating components, and E2E tests exercise a complete user or API flow. They catch different classes of failure and have different setup and execution costs. A balanced strategy puts detailed business-rule checks near the unit level and preserves real integration coverage where mocks would hide bugs.

Key points:

• Unit: one function or small module in isolation.

• Integration: real components together, such as a service and database.

• E2E: a full user flow; keep a focused set for critical behavior.

• Use real database tests for constraints and rollback behavior; a mocked repository cannot prove these work.

Example — a focused unit test:

function canReserve(stock, quantity) {
  return Number.isInteger(quantity) && quantity > 0 && stock >= quantity;
}

test('rejects a negative reservation quantity', () => {
  expect(canReserve(5, -2)).toBe(false);
});

• This checks validation only. A separate real-database integration test is needed to prove concurrent reservations cannot oversell.

Interview point:

• Explain what a test deliberately includes and excludes, and why that boundary is useful.