Installation

Install with CLI Recommended 
gh skills-hub install javascript-typescript-jest

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Download and extract to your repository:

.github/skills/javascript-typescript-jest/

Extract the ZIP to .github/skills/ in your repo. The folder name must match javascript-typescript-jest for Copilot to auto-discover it.

Skill Files (1)

SKILL.md 2.2 KB 
---
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)`

License (MIT)

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MIT License

Copyright GitHub, Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.