What is condition-based-waiting?

This Claude skill enables developers to eliminate flaky tests by replacing unreliable, arbitrary timeouts (like setTimeout or sleep) with robust condition polling. By implementing a 'wait-for' pattern, it ensures asynchronous tests are stable, faster, and reliable across different environments, significantly improving CI/CD pass rates and developer productivity.

When should I use condition-based-waiting?

condition-based-waiting is useful in the following scenarios: • Fixing flaky tests that pass locally but fail intermittently in CI/CD environments due to resource contention or timing dependencies. • Waiting for specific UI state changes, DOM elements, or application events in end-to-end (E2E) and integration testing. • Synchronizing tests with asynchronous backend operations such as file system writes, database updates, or message queue processing. • Optimizing test suite execution time by polling for completion instead of waiting for fixed, worst-case duration delays.

name	Condition-Based Waiting
description	Replace arbitrary timeouts with condition polling for reliable async tests
when_to_use	when tests have race conditions, timing dependencies, or inconsistent pass/fail behavior
version	1.1.0
languages	all

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

digraph when_to_use {
    "Test uses setTimeout/sleep?" [shape=diamond];
    "Testing timing behavior?" [shape=diamond];
    "Document WHY timeout needed" [shape=box];
    "Use condition-based waiting" [shape=box];

    "Test uses setTimeout/sleep?" -> "Testing timing behavior?" [label="yes"];
    "Testing timing behavior?" -> "Document WHY timeout needed" [label="yes"];
    "Testing timing behavior?" -> "Use condition-based waiting" [label="no"];
}

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

// ❌ 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 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
  }
}

See @example.ts for complete implementation with domain-specific helpers (waitForEvent, waitForEventCount, waitForEventMatch) from actual debugging session.

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

❌ Stale data: Cache state before loop ✅ Fix: Call getter inside loop for fresh data

When Arbitrary Timeout IS Correct

// Tool ticks every 100ms - need 2 ticks to verify partial output
await waitForEvent(manager, 'TOOL_STARTED'); // First: wait for condition
await new Promise(r => setTimeout(r, 200));   // Then: wait for timed behavior
// 200ms = 2 ticks at 100ms intervals - documented and justified

Requirements:

First wait for triggering condition
Based on known timing (not guessing)
Comment explaining WHY

Real-World Impact

From debugging session (2025-10-03):

Fixed 15 flaky tests across 3 files
Pass rate: 60% → 100%
Execution time: 40% faster
No more race conditions

condition-based-waiting

When & Why to Use This Skill

Use Cases