| name | condition-based-waiting |
| description | Flaky test fix pattern - replaces arbitrary timeouts with condition polling that waits for actual state changes. |
| trigger | - Tests use setTimeout/sleep with arbitrary values - Tests are flaky (pass sometimes, fail under load) - Tests timeout when run in parallel - Waiting for async operations in tests |
| skip_when | - Testing actual timing behavior (debounce, throttle) → timeout is correct - Synchronous tests → no waiting needed |
Condition-Based Waiting
Overview
Flaky tests often guess at timing with arbitrary delays. This creates race conditions where tests pass on fast machines but fail under load or in CI.
Core principle: Wait for the actual condition you care about, not a guess about how long it takes.
When to Use
Decision flow: Test uses setTimeout/sleep? → Testing actual timing behavior? → (yes: document WHY timeout needed) | (no: use condition-based waiting)
Use when: Arbitrary delays (setTimeout, sleep) | Flaky tests (pass sometimes, fail under load) | Timeouts in parallel runs | Async operation waits
Don't use when: Testing actual timing behavior (debounce, throttle) - document WHY if using arbitrary timeout
Core Pattern
// ❌ BEFORE: Guessing at timing
await new Promise(r => setTimeout(r, 50));
const result = getResult();
expect(result).toBeDefined();
// ✅ AFTER: Waiting for condition
await waitFor(() => getResult() !== undefined);
const result = getResult();
expect(result).toBeDefined();
Quick Patterns
| Scenario | Pattern |
|---|---|
| Wait for event | waitFor(() => events.find(e => e.type === 'DONE')) |
| Wait for state | waitFor(() => machine.state === 'ready') |
| Wait for count | waitFor(() => items.length >= 5) |
| Wait for file | waitFor(() => fs.existsSync(path)) |
| Complex condition | waitFor(() => obj.ready && obj.value > 10) |
Implementation
Generic polling: waitFor(condition, description, timeoutMs=5000) - poll every 10ms, throw on timeout with clear message. See @example.ts for domain-specific helpers (waitForEvent, waitForEventCount, waitForEventMatch).
Common Mistakes
| ❌ Bad | ✅ Fix |
|---|---|
Polling too fast (setTimeout(check, 1)) |
Poll every 10ms |
| No timeout (loop forever) | Always include timeout with clear error |
| Stale data (cache before loop) | Call getter inside loop for fresh data |
When Arbitrary Timeout IS Correct
await waitForEvent(...); await setTimeout(200) - OK when: (1) First wait for triggering condition (2) Based on known timing, not guessing (3) Comment explaining WHY (e.g., "200ms = 2 ticks at 100ms intervals")
Real-World Impact
Fixed 15 flaky tests across 3 files: 60% → 100% pass rate, 40% faster execution, zero race conditions.