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Modern JavaScript testing strategies with Jest, Mocha, and testing best practices covering unit testing, integration testing, mocking, async patterns, and DOM testing.
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| name | JavaScript Testing Patterns |
| description | Modern JavaScript testing strategies with Jest, Mocha, and testing best practices covering unit testing, integration testing, mocking, async patterns, and DOM testing. |
| version | 1.0.0 |
| author | thetestingacademy |
| license | MIT |
| tags | ["javascript","jest","mocha","unit-testing","mocking","async-testing","dom-testing"] |
| testingTypes | ["unit","integration"] |
| frameworks | ["jest"] |
| languages | ["javascript","typescript"] |
| domains | ["web"] |
| agents | ["claude-code","cursor","github-copilot","windsurf","codex","aider","continue","cline","zed","bolt"] |
You are an expert JavaScript developer specializing in testing patterns and methodologies. When the user asks you to write, review, or improve JavaScript tests, follow these detailed instructions.
project/
src/
services/
user.service.js
user.service.test.js
utils/
validators.js
validators.test.js
components/
Button.jsx
Button.test.jsx
__tests__/
integration/
api.test.js
e2e/
checkout.test.js
__mocks__/
axios.js
localStorage.js
jest.config.js
// jest.config.js
module.exports = {
testEnvironment: 'jsdom',
roots: ['<rootDir>/src'],
testMatch: ['**/__tests__/**/*.js', '**/?(*.)+(spec|test).js'],
collectCoverageFrom: [
'src/**/*.{js,jsx}',
'!src/**/*.test.{js,jsx}',
'!src/**/index.js',
],
coverageThresholds: {
global: {
branches: 80,
functions: 80,
lines: 80,
statements: 80,
},
},
setupFilesAfterEnv: ['<rootDir>/jest.setup.js'],
moduleNameMapper: {
'^@/(.*)$': ,
: ,
},
: {
: ,
},
: ,
: ,
};
// validators.js
export function isValidEmail(email) {
const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
return regex.test(email);
}
export function calculateTotal(items) {
return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
}
export function formatCurrency(amount, currency = 'USD') {
return new Intl.NumberFormat('en-US', {
style: 'currency',
currency,
}).format(amount);
}
// validators.test.js
import { isValidEmail, calculateTotal, formatCurrency } from './validators';
describe('Validators', () => {
describe('isValidEmail', () => {
it('should return true for valid emails', () => {
expect(isValidEmail('user@example.com')).toBe(true);
expect(isValidEmail('first.last@domain.co.uk')).toBe(true);
expect(isValidEmail('user+tag@example.com')).toBe(true);
});
it('should return false for invalid emails', () => {
expect(isValidEmail('')).toBe(false);
expect(isValidEmail('not-an-email')).toBe(false);
expect(isValidEmail('@missing-local.com')).toBe(false);
expect(isValidEmail('missing@domain')).toBe(false);
});
});
describe('calculateTotal', () => {
it('should calculate total for single item', () => {
const items = [{ price: 10, quantity: 2 }];
expect(calculateTotal(items)).toBe(20);
});
it('should calculate total for multiple items', () => {
const items = [
{ price: 10, quantity: 2 },
{ price: 5, quantity: 3 },
];
expect(calculateTotal(items)).toBe(35);
});
it('should return 0 for empty array', () => {
expect(calculateTotal([])).toBe(0);
});
});
describe('formatCurrency', () => {
it('should format USD by default', () => {
expect(formatCurrency(1234.56)).toBe('$1,234.56');
});
it('should format different currencies', () => {
expect(formatCurrency(1234.56, 'EUR')).toContain('1,234.56');
expect(formatCurrency(1234.56, 'GBP')).toContain('1,234.56');
});
});
});
// user.service.js
export class UserService {
constructor(apiClient, cache) {
this.apiClient = apiClient;
this.cache = cache;
}
async getUser(id) {
const cached = this.cache.get(`user:${id}`);
if (cached) return JSON.parse(cached);
const user = await this.apiClient.get(`/users/${id}`);
this.cache.set(`user:${id}`, JSON.stringify(user));
return user;
}
async createUser(userData) {
const user = await this.apiClient.post('/users', userData);
return user;
}
async deleteUser(id) {
const user = await this.getUser(id);
if (!user) throw new Error('User not found');
await this.apiClient.delete(`/users/${id}`);
this.cache.delete(`user:${id}`);
}
}
// user.service.test.js
import { UserService } from './user.service';
describe('UserService', () => {
let userService;
let mockApiClient;
let mockCache;
beforeEach(() => {
mockApiClient = {
get: jest.fn(),
post: jest.fn(),
delete: jest.fn(),
};
mockCache = {
get: jest.fn(),
set: jest.fn(),
delete: jest.fn(),
};
userService = new UserService(mockApiClient, mockCache);
});
describe('getUser', () => {
it('should return cached user if available', async () => {
const cachedUser = { id: '1', name: 'Cached User' };
mockCache.get.mockReturnValue(JSON.stringify(cachedUser));
const result = await userService.getUser('1');
expect(result).toEqual(cachedUser);
expect(mockCache.get).toHaveBeenCalledWith('user:1');
expect(mockApiClient.get).not.toHaveBeenCalled();
});
it('should fetch from API if not cached', async () => {
const apiUser = { id: '1', name: 'API User' };
mockCache.get.mockReturnValue(null);
mockApiClient.get.mockResolvedValue(apiUser);
const result = await userService.getUser('1');
expect(result).toEqual(apiUser);
expect(mockApiClient.get).toHaveBeenCalledWith('/users/1');
expect(mockCache.set).toHaveBeenCalledWith('user:1', JSON.stringify(apiUser));
});
});
describe('createUser', () => {
it('should create a new user', async () => {
const newUser = { id: '2', name: 'New User' };
mockApiClient.post.mockResolvedValue(newUser);
const result = await userService.createUser({ name: 'New User' });
expect(result).toEqual(newUser);
expect(mockApiClient.post).toHaveBeenCalledWith('/users', { name: 'New User' });
});
});
describe('deleteUser', () => {
it('should delete user and clear cache', async () => {
const user = { id: '1', name: 'User' };
mockCache.get.mockReturnValue(JSON.stringify(user));
await userService.deleteUser('1');
expect(mockApiClient.delete).toHaveBeenCalledWith('/users/1');
expect(mockCache.delete).toHaveBeenCalledWith('user:1');
});
it('should throw error when user not found', async () => {
mockCache.get.mockReturnValue(null);
mockApiClient.get.mockResolvedValue(null);
await expect(userService.deleteUser('nonexistent')).rejects.toThrow('User not found');
});
});
});
describe('Async operations', () => {
it('should handle resolved promises', async () => {
const result = await fetchData();
expect(result.status).toBe('success');
});
it('should handle rejected promises', async () => {
await expect(fetchData('invalid')).rejects.toThrow('Invalid ID');
});
it('should use resolves matcher', async () => {
await expect(fetchUser('1')).resolves.toEqual({ id: '1', name: 'User' });
});
it('should use rejects matcher', async () => {
await expect(fetchUser('invalid')).rejects.toThrow('Not found');
});
it('should test callback-based async code', (done) => {
fetchDataCallback('1', (err, data) => {
expect(err).toBeNull();
expect(data.id).toBe('1');
done();
});
});
});
describe('Timer functions', () => {
beforeEach(() => {
jest.useFakeTimers();
});
afterEach(() => {
jest.useRealTimers();
});
it('should debounce function calls', () => {
const fn = jest.fn();
const debounced = debounce(fn, 300);
debounced();
debounced();
debounced();
expect(fn).not.toHaveBeenCalled();
jest.advanceTimersByTime(300);
expect(fn).toHaveBeenCalledTimes(1);
});
it('should throttle function calls', () => {
const fn = jest.fn();
const throttled = throttle(fn, 100);
throttled();
expect(fn).toHaveBeenCalledTimes(1);
throttled();
expect(fn).toHaveBeenCalledTimes(1);
jest.advanceTimersByTime(100);
throttled();
expect(fn).toHaveBeenCalledTimes(2);
});
});
describe('Function mocking', () => {
it('should mock a simple function', () => {
const mockFn = jest.fn();
mockFn.mockReturnValue(42);
expect(mockFn()).toBe(42);
expect(mockFn).toHaveBeenCalledTimes(1);
});
it('should mock with implementation', () => {
const mockFn = jest.fn((x) => x * 2);
expect(mockFn(5)).toBe(10);
expect(mockFn).toHaveBeenCalledWith(5);
});
it('should mock resolved values', async () => {
const mockFn = jest.fn();
mockFn.mockResolvedValue({ id: 1, name: 'Test' });
const result = await mockFn();
expect(result).toEqual({ id: 1, name: 'Test' });
});
it('should mock different return values', () => {
const mockFn = jest.fn();
mockFn
.mockReturnValueOnce(1)
.mockReturnValueOnce(2)
.mockReturnValue(3);
expect(mockFn()).toBe(1);
expect(mockFn()).toBe(2);
expect(mockFn()).toBe(3);
expect(mockFn()).toBe(3);
});
});
// __mocks__/axios.js
module.exports = {
get: jest.fn(() => Promise.resolve({ data: {} })),
post: jest.fn(() => Promise.resolve({ data: {} })),
put: jest.fn(() => Promise.resolve({ data: {} })),
delete: jest.fn(() => Promise.resolve({ data: {} })),
};
// api.test.js
jest.mock('axios');
import axios from 'axios';
import { fetchUsers } from './api';
describe('API functions', () => {
it('should fetch users', async () => {
const mockUsers = [{ id: 1, name: 'User 1' }];
axios.get.mockResolvedValue({ data: mockUsers });
const users = await fetchUsers();
expect(users).toEqual(mockUsers);
expect(axios.get).toHaveBeenCalledWith('/api/users');
});
});
jest.mock('./utils', () => ({
...jest.requireActual('./utils'),
fetchData: jest.fn(),
}));
import { fetchData, formatData } from './utils';
test('should use real formatData but mocked fetchData', () => {
fetchData.mockResolvedValue({ raw: 'data' });
// formatData uses the real implementation
});
describe('Spying', () => {
it('should spy on console.error', () => {
const spy = jest.spyOn(console, 'error').mockImplementation(() => {});
console.error('Test error');
expect(spy).toHaveBeenCalledWith('Test error');
spy.mockRestore();
});
it('should spy on object methods', () => {
const obj = {
method: (x) => x * 2,
};
const spy = jest.spyOn(obj, 'method');
const result = obj.method(5);
expect(result).toBe(10);
expect(spy).toHaveBeenCalledWith(5);
});
});
// Button.jsx
export function Button({ onClick, disabled, children, variant = 'primary' }) {
return (
<button
onClick={onClick}
disabled={disabled}
className={`btn btn-${variant}`}
>
{children}
</button>
);
}
// Button.test.jsx
import { render, screen, fireEvent } from '@testing-library/react';
import { Button } from './Button';
describe('Button', () => {
it('should render with text', () => {
render(<Button>Click me</Button>);
expect(screen.getByText('Click me')).toBeInTheDocument();
});
it('should handle click events', () => {
const onClick = jest.fn();
render(<Button onClick={onClick}>Click me</Button>);
fireEvent.click(screen.getByText('Click me'));
expect(onClick).toHaveBeenCalledTimes(1);
});
it('should be disabled when disabled prop is true', () => {
render(<Button disabled>Click me</Button>);
expect(screen.getByRole('button')).toBeDisabled();
});
it('should apply variant class', () => {
render(<Button variant="secondary">Click me</Button>);
expect(screen.getByRole('button')).toHaveClass('btn-secondary');
});
});
describe('Snapshot tests', () => {
it('should match snapshot', () => {
const data = {
id: 1,
name: 'Test User',
roles: ['admin', 'user'],
};
expect(data).toMatchSnapshot();
});
it('should match inline snapshot', () => {
const formatted = formatUserName({ first: 'John', last: 'Doe' });
expect(formatted).toMatchInlineSnapshot(`"John Doe"`);
});
it('should snapshot React component', () => {
const { container } = render(<Button>Click me</Button>);
expect(container).toMatchSnapshot();
});
});
describe('UserManager', () => {
describe('creating users', () => {
it('should create user with valid data', () => {});
it('should reject duplicate emails', () => {});
});
describe('retrieving users', () => {
it('should get user by id', () => {});
it('should return null for non-existent user', () => {});
});
describe('deleting users', () => {
it('should delete existing user', () => {});
it('should throw error for non-existent user', () => {});
});
});
describe('Test suite with setup', () => {
let db;
beforeAll(() => {
// Runs once before all tests
db = createDatabase();
});
afterAll(() => {
// Runs once after all tests
db.close();
});
beforeEach(() => {
// Runs before each test
db.clear();
});
afterEach(() => {
// Runs after each test
jest.clearAllMocks();
});
it('test 1', () => {});
it('test 2', () => {});
});
const { expect } = require('chai');
const sinon = require('sinon');
describe('Mocha tests', () => {
it('should test with Chai assertions', () => {
expect(true).to.be.true;
expect([1, 2, 3]).to.have.lengthOf(3);
expect({ name: 'Test' }).to.have.property('name', 'Test');
});
it('should use Sinon spies', () => {
const callback = sinon.spy();
const obj = { method: callback };
obj.method('arg1', 'arg2');
expect(callback.calledOnce).to.be.true;
expect(callback.calledWith('arg1', 'arg2')).to.be.true;
});
it('should use Sinon stubs', () => {
const stub = sinon.stub().returns(42);
expect(stub()).to.equal(42);
expect(stub.called).to.be.true;
});
});
# Run all tests
npm test
# Run in watch mode
npm test -- --watch
# Run specific file
npm test user.service.test.js
# Run with coverage
npm test -- --coverage
# Run matching pattern
npm test -- --testNamePattern="user"
# Update snapshots
npm test -- --updateSnapshot
# Run in CI mode
CI=true npm test
JavaScript testing is about building confidence in your code. Write tests that give you the freedom to refactor and the safety to ship with confidence.