Inject or mock time, network, and random sources so tests produce the same result every run.
Tests touching real time, network, or RNG are flaky and fail unpredictably in CI.
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A test that calls Date.now(), hits a real API, or uses Math.random() will eventually fail for reasons unrelated to your code. Inject these dependencies or use fakes so the outcome is fully controlled.
// Bad — real clock makes the assertion brittle
expect(token.expiresAt).toBe(Date.now() + 3600_000);
// Good — inject a fixed clock
const clock = () => 1_000_000;
expect(makeToken(clock).expiresAt).toBe(1_000_000 + 3600_000);