JavaScript and TypeScript Jest test guidance

Author's original introduction

Best practices for writing JavaScript/TypeScript tests using Jest, including mocking strategies, test structure, and common patterns.

SkillPi interpretation

Tasks

Guide test naming, dependency mocks, asynchronous assertions, snapshots and React component testing.

Audience

JavaScript and TypeScript developers writing or reviewing Jest tests.

Inputs and outputs

Input: code under test and desired observable behavior. Output: test structure and assertion/mocking guidance.

Software, accounts and paid services

A JavaScript/TypeScript project using Jest; the React section refers to React Testing Library. No paid service requirement is stated. Confirm actual file access and external-service permissions in your host. Domains referenced in the documents (not a runtime allowlist): No HTTP(S) domain stated.

Limits and differences

A concise pattern guide, not proof of test coverage or correctness. Snapshot changes and mocks still require review.

Testing

Not tested

Security rating and access scope

caution · Static evidence review; not a runtime safety guarantee.

A JavaScript/TypeScript project using Jest; the React section refers to React Testing Library. No paid service requirement is stated. Confirm actual file access and external-service permissions in your host. Domains referenced in the documents (not a runtime allowlist): No HTTP(S) domain stated.

Before use, confirm and authorize actual file access, external services and network destinations in your host. This review grants no runtime permissions.

Original SKILL.md

---
name: javascript-typescript-jest
description: 'Best practices for writing JavaScript/TypeScript tests using Jest, including mocking strategies, test structure, and common patterns.'
---

### Test Structure
- Name test files with `.test.ts` or `.test.js` suffix
- Place test files next to the code they test or in a dedicated `__tests__` directory
- Use descriptive test names that explain the expected behavior
- Use nested describe blocks to organize related tests
- Follow the pattern: `describe('Component/Function/Class', () => { it('should do something', () => {}) })`

### Effective Mocking
- Mock external dependencies (APIs, databases, etc.) to isolate your tests
- Use `jest.mock()` for module-level mocks
- Use `jest.spyOn()` for specific function mocks
- Use `mockImplementation()` or `mockReturnValue()` to define mock behavior
- Reset mocks between tests with `jest.resetAllMocks()` in `afterEach`

### Testing Async Code
- Always return promises or use async/await syntax in tests
- Use `resolves`/`rejects` matchers for promises
- Set appropriate timeouts for slow tests with `jest.setTimeout()`

### Snapshot Testing
- Use snapshot tests for UI components or complex objects that change infrequently
- Keep snapshots small and focused
- Review snapshot changes carefully before committing

### Testing React Components
- Use React Testing Library over Enzyme for testing components
- Test user behavior and component accessibility
- Query elements by accessibility roles, labels, or text content
- Use `userEvent` over `fireEvent` for more realistic user interactions

## Common Jest Matchers
- Basic: `expect(value).toBe(expected)`, `expect(value).toEqual(expected)`
- Truthiness: `expect(value).toBeTruthy()`, `expect(value).toBeFalsy()`
- Numbers: `expect(value).toBeGreaterThan(3)`, `expect(value).toBeLessThanOrEqual(3)`
- Strings: `expect(value).toMatch(/pattern/)`, `expect(value).toContain('substring')`
- Arrays: `expect(array).toContain(item)`, `expect(array).toHaveLength(3)`
- Objects: `expect(object).toHaveProperty('key', value)`
- Exceptions: `expect(fn).toThrow()`, `expect(fn).toThrow(Error)`
- Mock functions: `expect(mockFn).toHaveBeenCalled()`, `expect(mockFn).toHaveBeenCalledWith(arg1, arg2)`

Copyright and license notice

MIT License

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