Jest Testing Skill

When to Use

Use this skill when you need 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",...

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

Limitations

Use this skill only when the task clearly matches its upstream source and local project context.
Verify commands, generated code, dependencies, credentials, and external service behavior before applying changes.
Do not treat examples as a substitute for environment-specific tests, security review, or user approval for destructive or costly actions.

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