jest-testing

Q: How do I install jest-testing?

Run npx skills add https://github.com/pluginagentmarketplace/custom-plugin-nodejs --skill jest-testing 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-testing/ (personal) or .claude/skills/jest-testing/ (project).

by pluginagentmarketplaceTesting & QA

Test Node.js applications with Jest including unit tests, integration tests, mocking, code coverage, and CI/CD integration

Installs: 120
Last 24h: —
Trending rank: —
Security: Not audited

Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/pluginagentmarketplace/custom-plugin-nodejs --skill jest-testing

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-testing 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

Master testing Node.js applications with Jest - the delightful JavaScript testing framework.

Quick Start

Test in 3 steps:

Install - npm install --save-dev jest supertest
Write Test - Create *.test.js files
Run - npm test

Core Concepts

Basic Test Structure

// sum.test.js
const sum = require('./sum');

describe('sum function', () => {
  test('adds 1 + 2 to equal 3', () => {
    expect(sum(1, 2)).toBe(3);
  });

  test('adds negative numbers', () => {
    expect(sum(-1, -2)).toBe(-3);
  });
});

Jest Configuration

// package.json
{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage"
  },
  "jest": {
    "testEnvironment": "node",
    "coverageThreshold": {
      "global": {
        "branches": 80,
        "functions": 80,
        "lines": 80
      }
    }
  }
}

Unit Testing

// userService.test.js
const UserService = require('./userService');
const User = require('./models/User');

jest.mock('./models/User');

describe('UserService', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe('createUser', () => {
    it('should create user successfully', async () => {
      const userData = {
        name: 'John',
        email: 'john@example.com'
      };

      User.create.mockResolvedValue({ id: 1, ...userData });

      const result = await UserService.createUser(userData);

      expect(User.create).toHaveBeenCalledWith(userData);
      expect(result.id).toBe(1);
    });

    it('should throw error for duplicate email', async () => {
      User.create.mockRejectedValue(new Error('Email exists'));

      await expect(UserService.createUser({}))
        .rejects
        .toThrow('Email exists');
    });
  });
});

Learning Path

Beginner (1-2 weeks)

✅ Setup Jest and write basic tests
✅ Understand test structure (describe/it/expect)
✅ Learn matchers (toBe, toEqual, etc.)
✅ Test synchronous functions

Intermediate (3-4 weeks)

✅ Test async functions
✅ Mock modules and functions
✅ API testing with Supertest
✅ Code coverage reports

Advanced (5-6 weeks)

✅ Integration testing
✅ Test database operations
✅ CI/CD integration
✅ Performance testing

API Testing with Supertest

const request = require('supertest');
const app = require('./app');

describe('User API', () => {
  describe('POST /api/users', () => {
    it('should create new user', async () => {
      const userData = {
        name: 'John',
        email: 'john@example.com',
        password: 'password123'
      };

      const response = await request(app)
        .post('/api/users')
        .send(userData)
        .expect('Content-Type', /json/)
        .expect(201);

      expect(response.body).toHaveProperty('id');
      expect(response.body.email).toBe(userData.email);
    });

    it('should return 400 for invalid email', async () => {
      const response = await request(app)
        .post('/api/users')
        .send({ email: 'invalid' })
        .expect(400);

      expect(response.body).toHaveProperty('error');
    });
  });

  describe('GET /api/users/:id', () => {
    it('should return user by id', async () => {
      const response = await request(app)
        .get('/api/users/123')
        .expect(200);

      expect(response.body.id).toBe('123');
    });

    it('should return 404 for non-existent user', async () => {
      await request(app)
        .get('/api/users/999')
        .expect(404);
    });
  });
});

Mocking Patterns

// Mock entire module
jest.mock('axios');
const axios = require('axios');

test('fetches data from API', async () => {
  axios.get.mockResolvedValue({ data: { id: 1 } });

  const result = await fetchUser(1);

  expect(axios.get).toHaveBeenCalledWith('/api/users/1');
  expect(result.id).toBe(1);
});

// Spy on function
test('calls callback', () => {
  const callback = jest.fn();

  processData('test', callback);

  expect(callback).toHaveBeenCalledWith('test');
  expect(callback).toHaveBeenCalledTimes(1);
});

// Mock timers
jest.useFakeTimers();

test('delays execution', () => {
  const callback = jest.fn();

  setTimeout(callback, 1000);
  jest.advanceTimersByTime(1000);

  expect(callback).toHaveBeenCalled();
});

Test Lifecycle Hooks

describe('User Tests', () => {
  beforeAll(async () => {
    // Setup test database
    await connectTestDB();
  });

  afterAll(async () => {
    // Cleanup
    await disconnectTestDB();
  });

  beforeEach(async () => {
    // Clear data before each test
    await User.deleteMany({});
  });

  afterEach(() => {
    // Cleanup after each test
    jest.clearAllMocks();
  });

  test('...', () => {});
});

Jest Matchers

// Equality
expect(value).toBe(expected)        // Strict equality (===)
expect(value).toEqual(expected)     // Deep equality
expect(value).not.toBe(expected)    // Negation

// Truthiness
expect(value).toBeDefined()
expect(value).toBeNull()
expect(value).toBeTruthy()
expect(value).toBeFalsy()

// Numbers
expect(value).toBeGreaterThan(3)
expect(value).toBeGreaterThanOrEqual(3)
expect(value).toBeLessThan(5)
expect(value).toBeCloseTo(0.3)      // Floating point

// Strings
expect(string).toMatch(/pattern/)
expect(string).toContain('substring')

// Arrays
expect(array).toContain(item)
expect(array).toHaveLength(3)

// Objects
expect(obj).toHaveProperty('key')
expect(obj).toMatchObject({ key: 'value' })

// Exceptions
expect(() => fn()).toThrow()
expect(() => fn()).toThrow('error message')

// Async
await expect(promise).resolves.toBe(value)
await expect(promise).rejects.toThrow()

Code Coverage

# Run with coverage
npm test -- --coverage

# Coverage report shows:
# - Statements: % of code executed
# - Branches: % of if/else paths
# - Functions: % of functions called
# - Lines: % of lines executed

Testing Best Practices

✅ AAA pattern: Arrange, Act, Assert
✅ One assertion per test (ideally)
✅ Descriptive test names
✅ Test edge cases
✅ Mock external dependencies
✅ Clean up after tests
✅ Avoid test interdependence
✅ Aim for 80%+ coverage

CI/CD Integration

# .github/workflows/test.yml
name: Tests

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-node@v3
        with:
          node-version: 18
      - run: npm ci
      - run: npm test -- --coverage
      - uses: codecov/codecov-action@v3

When to Use

Use Jest testing when:

Building Node.js applications
Need comprehensive test coverage
Want fast, parallel test execution
Require mocking and snapshot testing
Implementing CI/CD pipelines

Related Skills

Express REST API (test API endpoints)
Async Programming (test async code)
Database Integration (test DB operations)
JWT Authentication (test auth flows)

Resources

FAQ

What does the jest-testing skill do?

Test Node.js applications with Jest including unit tests, integration tests, mocking, code coverage, and CI/CD integration

How do I install jest-testing?

Run npx skills add https://github.com/pluginagentmarketplace/custom-plugin-nodejs --skill jest-testing 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-testing/ (personal) or .claude/skills/jest-testing/ (project).

Which AI agents can use jest-testing?

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-testing 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-testing?

It is published by pluginagentmarketplace in pluginagentmarketplace/custom-plugin-nodejs. It has 120 recorded installs.

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jest-testing

Install

Install

Describe the task

Review the result

SKILL.md

Jest Testing Skill

Quick Start

Core Concepts

Basic Test Structure

Jest Configuration

Unit Testing

Learning Path

Beginner (1-2 weeks)

Intermediate (3-4 weeks)

Advanced (5-6 weeks)

API Testing with Supertest

Mocking Patterns

Test Lifecycle Hooks

Jest Matchers

Code Coverage

Testing Best Practices

CI/CD Integration

When to Use

Related Skills

Resources

FAQ

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