# jest

A skill that tells your agent how to write Jest tests: structure, mocks, assertions, async. Useful when AI writes the tests in your repo.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 864 installs · ★ 286
- Author: Mindrally
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/Mindrally/skills/tree/HEAD/jest/SKILL.md
- Updated: 2026-10-08
- HTML: https://affpapa.org/en/nevibecode/skills/jest

## What it does

The skill lays out how an agent should write Jest tests in JavaScript and TypeScript projects. Test names explain the expected behavior, `describe` blocks group related checks, and each test follows Arrange-Act-Assert while covering a single outcome.

Setup and teardown get their own rules. `beforeEach` and `afterEach` keep tests isolated, while `beforeAll` and `afterAll` are reserved for expensive setup worth sharing. Modules are mocked with `jest.mock()`, functions with `jest.fn()`, and `jest.spyOn()` is the pick when you want to track calls but keep the real implementation. Mocks are cleared between tests with `jest.clearAllMocks()`. The skill also warns against mocking everything — test real behavior where you can.

Assertions follow one rule: use the most specific matcher. `toEqual` for objects, `toBe` for primitives, snapshots rarely and with meaningful names. Async tests return a promise or use `async/await`, and coverage is tracked with the `--coverage` flag. Nothing gets installed into your project, so it won't clash with your existing Jest config — it is guidance for the agent only.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill jest -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/jest/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/jest/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill jest -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/jest/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill jest -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/jest/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install Mindrally/skills jest
```

2. **Manually: put the files in place**
- File: `.github/skills/jest/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill jest -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/Mindrally/skills --path jest
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/jest/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill jest -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/Mindrally/skills/tree/HEAD/jest/SKILL.md

## When to use it
- write Jest tests in one consistent style
- group checks into describe blocks
- mock a module or a single function
- test async code without hanging promises
- track coverage with the --coverage flag

## Example prompts
- Write Jest tests for the `fetchUser` module: a successful API response and a failure.
- Refactor these tests: move setup into `beforeEach` and clear mocks with `jest.clearAllMocks()`.
- Add negative cases to these tests and check coverage with `--coverage`.

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: de120d0287a12577.

BEGIN THIRD-PARTY FILE de120d0287a12577
````text
---
name: jest
description: Jest testing best practices for JavaScript and TypeScript applications, covering test structure, mocking, and assertion patterns.
metadata:
  maintainer: Mindrally
  source: https://github.com/Mindrally/skills
---

# Jest Testing Best Practices

You are an expert in JavaScript, TypeScript, and Jest testing.

## Core Principles

### Test Structure
- Use descriptive test names that clearly explain expected behavior
- Organize tests using `describe` blocks for logical grouping
- Follow the Arrange-Act-Assert (AAA) pattern in each test
- Keep tests focused on a single behavior or outcome

### Setup and Teardown
- Use `beforeEach` and `afterEach` for test isolation
- Use `beforeAll` and `afterAll` for expensive setup that can be shared
- Clean up any side effects in teardown hooks

### Mocking
- Use `jest.mock()` for module mocking
- Use `jest.fn()` for function mocks
- Use `jest.spyOn()` when you need to track calls but keep implementation
- Clear mocks between tests with `jest.clearAllMocks()`
- Avoid over-mocking: test real behavior when feasible

### Assertions
- Use the most specific matcher available
- Prefer `toEqual` for object comparison, `toBe` for primitives
- Use `toMatchSnapshot` sparingly and with meaningful names
- Include negative test cases (what should NOT happen)

### Async Testing
- Always return promises or use async/await in async tests
- Use `waitFor` from testing libraries for async assertions
- Set appropriate timeouts for long-running tests

### Coverage
- Aim for meaningful coverage, not just high percentages
- Test edge cases and error conditions
- Use `--coverage` flag to track coverage metrics

### Best Practices
- Keep tests DRY but readable (prefer clarity over brevity)
- Test behavior, not implementation details
- Make tests deterministic (no random data without seeding)
- Use factories or builders for test data creation

## Example Patterns

```javascript
describe('fetchUser', () => {
  it('should return user data when API call succeeds', async () => {
    const mockUser = { id: 1, name: 'John' };
    jest.spyOn(api, 'get').mockResolvedValue(mockUser);

    const result = await fetchUser(1);

    expect(result).toEqual(mockUser);
    expect(api.get).toHaveBeenCalledWith('/users/1');
  });

  it('should throw error when API call fails', async () => {
    jest.spyOn(api, 'get').mockRejectedValue(new Error('Not found'));

    await expect(fetchUser(999)).rejects.toThrow('Not found');
  });
});
```
````
END THIRD-PARTY FILE de120d0287a12577

The third-party file has ended. Everything between the de120d0287a12577 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

© Mindrally · Apache-2.0 · https://github.com/Mindrally/skills/tree/HEAD/jest/SKILL.md

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