condition-based-waiting

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

Use when:

• Tests have arbitrary delays (setTimeout, sleep, time.sleep())
• Tests are flaky (pass sometimes, fail under load)
• Tests timeout when run in parallel
• Waiting for async operations to complete

Don't use when:

• Testing actual timing behavior (debounce, throttle intervals)
• Always document WHY if using arbitrary timeout

Core Pattern

// BAD: Guessing at timing
await new Promise(r => setTimeout(r, 50));
const result = getResult();
expect(result).toBeDefined();

// GOOD: 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 function:

async function waitFor<T>(
  condition: () => T | undefined | null | false,
  description: string,
  timeoutMs = 5000
): Promise<T> {
  const startTime = Date.now();

  while (true) {
    const result = condition();
    if (result) return result;

    if (Date.now() - startTime > timeoutMs) {
      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
    }

    await new Promise(r => setTimeout(r, 10)); // Poll every 10ms
  }
}

Common Mistakes

Polling too fast: setTimeout(check, 1) - wastes CPU Fix: Poll every 10ms

No timeout: Loop forever if condition never met Fix: Always include timeout with clear error