What problem does it solve? Jest test suites often break due to configuration errors, module hoisting issues, ESM/CommonJS conflicts, timer mocking problems, and slow CI execution. This Skill provides expert-level diagnosis and fixes for over 50 common Jest problems, plus production-ready configuration patterns. ## Core Features & Use Cases - Configuration Mastery: Production-ready jest.config.js patterns covering module resolution, transforms, coverage thresholds, and TypeScript/ESM integration with ts-jest. - Advanced Mocking & Async Patterns: Solutions for jest.mock hoisting issues, timer control with fake timers, promise testing with resolves/rejects, and race condition handling. - Performance & CI Optimization: Worker parallelization tuning, memory leak detection, cache management, and flaky test debugging for CI pipelines. - Use Case: Your tests fail only in CI with 'Cannot use import statement outside a module' errors. This Skill diagnoses the ESM/CommonJS mismatch and provides the exact ts-jest ESM preset configuration to fix it. ## Quick Start Ask the assistant to diagnose why your Jest tests fail with module resolution or timeout errors and provide a corrected jest.config.js.
