Agent skill · personamanagmentlayer
jest-expert
Expert in Jest unit testing framework, mocks, snapshots, coverage reports, watch mode, and custom matchers. Use when the user mentions testing, unit testing, JavaScript, QA, mocking, or snapshots, or when the task involves Jest Framework, Test Structure, Advanced Features, or Basic Unit Tests.
What it needs
About 2k tokens when loaded.
What this skill does
Jest Expert Core Concepts Jest Framework Zero Config - Works out of the box Snapshot Testing - UI component testing Mocking - Functions, modules, and timers Code Coverage - Built-in coverage reports Watch Mode - Interactive test runner Parallel Testing - Fast test execution Test Structure describe/it - Test organization Matchers - Assertion library Setup/Teardown - beforeEach, afterEach, beforeAll, afterAll Mock Functions - jest.fn(), jest.spyOn() Async Testing - Promises, async/await Test Lifecycle - Hooks and execution order Advanced Features Custom Matchers - Extend expect Module Mocking - Mock entire modules Timer Mocks - Control time in tests Manual Mocks - mocks directory Configuration - jest.config.js Transformers - Babel, TypeScript support Best Practices Test Organization One test file per source file Group related tests with describe Use descriptive test names Follow AAA pattern (Arrange, Act, Assert) Keep tests simple and focused Test behavior, not implementation Mocking Mock external dependencies Use jest.fn() for function mocks Clear mocks between tests Mock at the right level Avoid over-mocking Document mock behavior Assertions Use specific matchers One assertion per test when possible Test edge cases Verify error conditions Use snapshots judiciously Provide clear failure messages Coverage Aim for high coverage (80%+) Don't chase 100% coverage Focus on critical paths Test edge cases and errors Exclude generated code Review coverage reports Anti-Patterns Common Mistakes Testing implementation details Too many assertions in one test Sharing state between tests Not cleaning up after tests Ignoring async/await Hard-coding test data Mocking Issues Over-mocking everything Not resetting mocks Mocking too deep Inconsistent mock data Forgetting to restore mocks Mocking what you don't need Test Design Problems Tests depending on execution order Flaky tests due to timing No arrange-act-assert structure Testing multiple things at once Duplicate test code Poor test …
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills personamanagmentlayer/jest-expert