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 names

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

• Implementation Examples — Basic Unit Tests, Testing Async Code, Mocking and Spies, Snapshot Testing, Testing React Components with React Testing Library, Timer Mocks, Custom Matchers, Configuration (jest.config.js)

Resources

Official Documentation

• Jest Documentation - Complete guide
• API Reference - API docs
• Expect Matchers - Assertion reference
• Mock Functions - Mocking guide

Learning Resources

• Testing JavaScript - Kent C. Dodds course
• Jest Crash Course - Video tutorial
• React Testing Library - React testing
• Jest Cheat Sheet - Quick reference

Tools & Extensions

• VS Code Jest Extension - IDE integration
• jest-watch-typeahead - Better watch mode
• jest-extended - Additional matchers
• snapshot-diff - Snapshot comparison

Community Resources

• GitHub Jest - Source code and issues
• Stack Overflow - Community help
• Discord Jest - Community chat
• Jest Blog - Updates and articles