Stub external systems (network, DB, filesystem) but let internal objects run for real.
Over-mocking internals tests the mocks, not the code, and hides real integration bugs.
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Mocking internal collaborators couples tests to implementation and verifies your mocks rather than your logic. Mock only at architectural boundaries — HTTP, database, filesystem, clock — and exercise real code within the unit.
// Bad — mocks an internal pure function
jest.spyOn(priceCalc, "applyTax").mockReturnValue(11);
// Good — mock the external gateway, run logic for real
const gateway = { fetchRate: async () => 0.1 };
expect(await total(items, gateway)).toBe(11);