jest-skill
Generates Jest unit and integration tests in JavaScript or TypeScript. Covers mocking, snapshots, async testing, and React component testing. Use when user mentions "Jest", "describe/it/expect", "jest.mock", "toMatchSnapshot". Triggers on: "Jest", "expect().toBe()", "jest.mock", "snapshot test", "JS test", "React test".
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/lambdatest/agent-skills --skill jest-skill- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads jest-skill automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Jest Testing Skill
Core Patterns
Basic Test
describe('Calculator', () => {
let calc;
beforeEach(() => { calc = new Calculator(); });
test('adds two numbers', () => {
expect(calc.add(2, 3)).toBe(5);
});
test('throws on division by zero', () => {
expect(() => calc.divide(10, 0)).toThrow('Division by zero');
});
});
Matchers
expect(value).toBe(exact); // === strict
expect(value).toEqual(object); // deep equality
expect(value).toBeTruthy();
expect(value).toBeNull();
expect(value).toBeGreaterThan(3);
expect(value).toBeCloseTo(0.3, 5);
expect(str).toMatch(/regex/);
expect(arr).toContain(item);
expect(arr).toHaveLength(3);
expect(obj).toHaveProperty('name');
expect(obj).toMatchObject({ name: 'Alice' });
expect(() => fn()).toThrow(CustomError);
Mocking
// Mock function
const mockFn = jest.fn();
mockFn.mockReturnValue(42);
mockFn.mockResolvedValue({ data: 'test' });
expect(mockFn).toHaveBeenCalledWith('arg1');
expect(mockFn).toHaveBeenCalledTimes(1);
// Mock module
jest.mock('./database');
const db = require('./database');
db.getUser.mockResolvedValue({ name: 'Alice' });
// Mock with implementation
jest.mock('./api', () => ({
fetchUsers: jest.fn().mockResolvedValue([{ name: 'Alice' }]),
}));
// Spy
const spy = jest.spyOn(console, 'log').mockImplementation();
expect(spy).toHaveBeenCalledWith('expected');
spy.mockRestore();
// Fake timers
jest.useFakeTimers();
jest.advanceTimersByTime(1000);
jest.useRealTimers();
Async Testing
test('fetches users', async () => {
const users = await fetchUsers();
expect(users).toHaveLength(3);
});
test('resolves with data', () => {
return expect(fetchData()).resolves.toEqual({ data: 'value' });
});
test('rejects with error', () => {
return expect(fetchBadData()).rejects.toThrow('not found');
});
React Component Testing (Testing Library)
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import '@testing-library/jest-dom';
import LoginForm from './LoginForm';
test('submits login form', async () => {
const onSubmit = jest.fn();
render(<LoginForm onSubmit={onSubmit} />);
fireEvent.change(screen.getByLabelText('Email'), {
target: { value: 'user@test.com' },
});
fireEvent.change(screen.getByLabelText('Password'), {
target: { value: 'password123' },
});
fireEvent.click(screen.getByRole('button', { name: /login/i }));
await waitFor(() => {
expect(onSubmit).toHaveBeenCalledWith({
email: 'user@test.com', password: 'password123',
});
});
});
Snapshot Testing
test('renders correctly', () => {
const tree = renderer.create(<Button label="Click" />).toJSON();
expect(tree).toMatchSnapshot();
});
// Update: jest --updateSnapshot
Anti-Patterns
| Bad | Good | Why |
|---|---|---|
expect(x === y).toBe(true) | expect(x).toBe(y) | Better errors |
No await on async | Always await | Swallows failures |
| Snapshot everything | Snapshot UI, assert logic | Snapshot fatigue |
Quick Reference
| Task | Command |
|---|---|
| Run all | npx jest |
| Watch | npx jest --watch |
| Coverage | npx jest --coverage |
| Update snapshots | npx jest --updateSnapshot |
| Run file | npx jest tests/calc.test.js |
| Single test | test.only('name', () => {}) |
Deep Patterns
For production-grade patterns, see reference/playbook.md:
| Section | What's Inside |
|---|---|
| §1 Production Config | Node + React configs, path aliases, coverage thresholds |
| §2 Mocking Deep Dive | Module/partial/manual mocks, spies, timers, env vars |
| §3 Async Patterns | Promises, rejections, event emitters, streams |
| §4 test.each | Array, tagged template, describe.each for table-driven tests |
| §5 Custom Matchers | toBeWithinRange, toBeValidEmail, TypeScript declarations |
| §6 React Testing Library | userEvent, hooks, context providers |
| §7 Snapshot Testing | Component, inline, property matchers |
| §8 API Service Testing | Mocked axios, CRUD patterns, error handling |
| §9 Global Setup | Multi-project config, DB setup/teardown |
| §10 CI/CD | GitHub Actions with coverage gates |
| §11 Debugging Table | 10 common problems with fixes |
| §12 Best Practices | 15-item production checklist |
FAQ
What does the jest-skill skill do?
Generates Jest unit and integration tests in JavaScript or TypeScript. Covers mocking, snapshots, async testing, and React component testing. Use when user mentions "Jest", "describe/it/expect", "jest.mock", "toMatchSnapshot". Triggers on: "Jest", "expect().toBe()", "jest.mock", "snapshot test", "JS test", "React test".
How do I install jest-skill?
Run npx skills add https://github.com/lambdatest/agent-skills --skill jest-skill in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/jest-skill/ (personal) or .claude/skills/jest-skill/ (project).
Which AI agents can use jest-skill?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is jest-skill safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains jest-skill?
It is published by lambdatest in lambdatest/agent-skills. It has 280 recorded installs.
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