Testinghard

How do you test asynchronous or concurrent code?

Tags

#async#concurrency#determinism

Back to category Practice quiz

Answer

Use deterministic clocks, await completion, and assert on outcomes. For concurrency, test invariants, use controlled schedulers, and avoid timing-based sleeps.

Advanced answer

Deep dive

Stability comes from determinism:

Use fake timers or virtual clocks.
Await all async work and flush queues.
For concurrency, test invariants (no lost updates, no duplicates).
Use stress tests to increase interleavings.

Examples

Async test pattern:

await doWork();
await flushJobs();
expect(state).toEqual(...)

Common pitfalls

Using sleep instead of waiting for a condition.
Not awaiting tasks (tests pass incorrectly).
Relying on real time or network.

Interview follow-ups

How do you test race conditions deterministically?
When do you use stress tests vs unit tests?
How do you test idempotency under concurrency?

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