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.