Fake timers vs promises

A production symptom appears first: flaky tests, stale state, slow UI, late cleanup, or growing memory. This is common in Jest/Vitest tests for debounced inputs, retries, loading spinners, and async React components.

Writing flaky tests that assume all async work finished after advancing timers.

Fix the root scheduling, cleanup, or concurrency pattern instead of adding arbitrary waits.

try {
  await operation();
} finally {
  cleanup();
}

The console lane proves the visible order: timer -> promise. The active line and queues show why source order changed.

Reproduce the symptom with a small test, apply the safer pattern, then verify the flaky, slow, or leaking behavior disappears.