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javascript-testing-patterns
Implement comprehensive testing strategies using Jest, Vitest, and
# JavaScript Testing Patterns
Comprehensive guide for implementing robust testing strategies in JavaScript/TypeScript applications using modern testing frameworks and best practices.
## When to Use This Skill
- Setting up test infrastructure for new projects
- Writing unit tests for functions and classes
- Creating integration tests for APIs and services
- Implementing end-to-end tests for user flows
- Mocking external dependencies and APIs
- Testing React, Vue, or other frontend components
- Implementing test-driven development (TDD)
- Setting up continuous testing in CI/CD pipelines
## Testing Frameworks
### Jest - Full-Featured Testing Framework
**Setup:**
```typescript
// jest.config.ts
import type { Config } from "jest";
const config: Config = {
preset: "ts-jest",
testEnvironment: "node",
roots: ["<rootDir>/src"],
testMatch: ["**/__tests__/**/*.ts", "**/?(*.)+(spec|test).ts"],
collectCoverageFrom: [
"src/**/*.ts",
"!src/**/*.d.ts",
"!src/**/*.interface.ts",
],
coverageThreshold: {
global: {
branches: 80,
functions: 80,
lines: 80,
statements: 80,
},
},
setupFilesAfterEnv: ["<rootDir>/src/test/setup.ts"],
};
export default config;
```
### Vitest - Fast, Vite-Native Testing
**Setup:**
```typescript
// vitest.config.ts
import { defineConfig } from "vitest/config";
export default defineConfig({
test: {
globals: true,
environment: "node",
coverage: {
provider: "v8",
reporter: ["text", "json", "html"],
exclude: ["**/*.d.ts", "**/*.config.ts", "**/dist/**"],
},
setupFiles: ["./src/test/setup.ts"],
},
});
```
## Unit Testing Patterns
### Pattern 1: Testing Pure Functions
```typescript
// utils/calculator.ts
export function add(a: number, b: number): number {
return a + b;
}
export function divide(a: number, b: number): number {
if (b === 0) {
throw new Error("Division by zero");
}
return a / b;
}
// utils/calculator.test.ts
import { describe, it, expect } from "vitest";
import { add, divide } from "./calculator";
describe("Calculator", () => {
describe("add", () => {
it("should add two positive numbers", () => {
expect(add(2, 3)).toBe(5);
});
it("should add negative numbers", () => {
expect(add(-2, -3)).toBe(-5);
});
it("should handle zero", () => {
expect(add(0, 5)).toBe(5);
expect(add(5, 0)).toBe(5);
});
});
describe("divide", () => {
it("should divide two numbers", () => {
expect(divide(10, 2)).toBe(5);
});
it("should handle decimal results", () => {
expect(divide(5, 2)).toBe(2.5);
});
it("should throw error when dividing by zero", () => {
expect(() => divide(10, 0)).toThrow("Division by zero");
});
});
});
```
### Pattern 2: Testing Classes
```typescript
// services/user.service.ts
export class UserService {
private users: Map<string, User> = new Map();
create(user: User): User {
if (this.users.has(user.id)) {
throw new Error("User already exists");
}
this.users.set(user.id, user);
return user;
}
findById(id: string): User | undefined {
return this.users.get(id);
}
update(id: string, updates: Partial<User>): User {
const user = this.users.get(id);
if (!user) {
throw new Error("User not found");
}
const updated = { ...user, ...updates };
this.users.set(id, updated);
return updated;
}
delete(id: string): boolean {
return this.users.delete(id);
}
}
// services/user.service.test.ts
import { describe, it, expect, beforeEach } from "vitest";
import { UserService } from "./user.service";
describe("UserService", () => {
let service: UserService;
beforeEach(() => {
service = new UserService();
});
describe("create", () => {
it("should create a new user", () => {
const user = { id: "1", name: "John", email: "john@example.com" };
const created = service.create(user);
expect(created).toEqual(user);
expect(service.findById("1")).toEqual(user);
});
it("should throw error if user already exists", () => {
const user = { id: "1", name: "John", email: "john@example.com" };
service.create(user);
expect(() => service.create(user)).toThrow("User already exists");
});
});
describe("update", () => {
it("should update existing user", () => {
const user = { id: "1", name: "John", email: "john@example.com" };
service.create(user);
const updated = service.update("1", { name: "Jane" });
expect(updated.name).toBe("Jane");
expect(updated.email).toBe("john@example.com");
});
it("should throw error if user not found", () => {
expect(() => service.update("999", { name: "Jane" })).toThrow(
"User not found",
);
});
});
});
```
### Pattern 3: Testing Async Functions
```typescript
// services/api.service.ts
export class ApiService {
async fetchUser(id: string): Promise<User> {
const response = await fetch(`https://api.example.com/users/${id}`);
if (!response.ok) {
throw new Error("User not found");
}
return response.json();
}
async createUser(user: CreateUserDTO): Promise<User> {
const response = await fetch("https://api.example.com/users", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify(user),
});
return response.json();
}
}
// services/api.service.test.ts
import { describe, it, expect, vi, beforeEach } from "vitest";
import { ApiService } from "./api.service";
// Mock fetch globally
global.fetch = vi.fn();
describe("ApiService", () => {
let service: ApiService;
beforeEach(() => {
service = new ApiService();
vi.clearAllMocks();
});
describe("fetchUser", () => {
it("should fetch user successfully", async () => {
const mockUser = { id: "1", name: "John", email: "john@example.com" };
(fetch as any).mockResolvedValueOnce({
ok: true,
json: async () => mockUser,
});
const user = await service.fetchUser("1");
expect(user).toEqual(mockUser);
expect(fetch).toHaveBeenCalledWith("https://api.example.com/users/1");
});
it("should throw error if user not found", async () => {
(fetch as any).mockResolvedValueOnce({
ok: false,
});
await expect(service.fetchUser("999")).rejects.toThrow("User not found");
});
});
describe("createUser", () => {
it("should create user successfully", async () => {
const newUser = { name: "John", email: "john@example.com" };
const createdUser = { id: "1", ...newUser };
(fetch as any).mockResolvedValueOnce({
ok: true,
json: async () => createdUser,
});
const user = await service.createUser(newUser);
expect(user).toEqual(createdUser);
expect(fetch).toHaveBeenCalledWith(
"https://api.example.com/users",
expect.objectContaining({
method: "POST",
body: JSON.stringify(newUser),
}),
);
});
});
});
```
## Mocking Patterns
### Pattern 1: Mocking Modules
```typescript
// services/email.service.ts
import nodemailer from "nodemailer";
export class EmailService {
private transporter = nodemailer.createTransport({
host: process.env.SMTP_HOST,
port: 587,
auth: {
user: process.env.SMTP_USER,
pass: process.env.SMTP_PASS,
},
});
async sendEmail(to: string, subject: string, html: string) {
await this.transporter.sendMail({
from: process.env.EMAIL_FROM,
to,
subject,
html,
});
}
}
// services/email.service.test.ts
import { describe, it, expect, vi, beforeEach } from "vitest";
import { EmailService } from "./email.service";
vi.mock("nodemailer", () => ({
default: {
createTransport: vi.fn(() => ({
sendMail: vi.fn().mockResolvedValue({ messageId: "123" }),
})),
},
}));
describe("EmailService", () => {
let service: EmailService;
beforeEach(() => {
service = new EmailService();
});
it("should send email successfully", async () => {
await service.sendEmail(
"test@example.com",
"Test Subject",
"<p>Test Body</p>",
);
expect(service["transporter"].sendMail).toHaveBeenCalledWith(
expect.objectContaining({
to: "test@example.com",
subject: "Test Subject",
}),
);
});
});
```
### Pattern 2: Dependency Injection for Testing
```typescript
// services/user.service.ts
export interface IUserRepository {
findById(id: string): Promise<User | null>;
create(user: User): Promise<User>;
}
export class UserService {
constructor(private userRepository: IUserRepository) {}
async getUser(id: string): Promise<User> {
const user = await this.userRepository.findById(id);
if (!user) {
throw new Error("User not found");
}
return user;
}
async createUser(userData: CreateUserDTO): Promise<User> {
// Business logic here
const user = { id: generateId(), ...userData };
return this.userRepository.create(user);
}
}
// services/user.service.test.ts
import { describe, it, expect, vi, beforeEach } from "vitest";
import { UserService, IUserRepository } from "./user.service";
describe("UserService", () => {
let service: UserService;
let mockRepository: IUserRepository;
beforeEach(() => {
mockRepository = {
findById: vi.fn(),
create: vi.fn(),
};
service = new UserService(mockRepository);
});
describe("getUser", () => {
it("should return user if found", async () => {
const mockUser = { id: "1", name: "John", email: "john@example.com" };
vi.mocked(mockRepository.findById).mockResolvedValue(mockUser);
const user = await service.getUser("1");
expect(user).toEqual(mockUser);
expect(mockRepository.findById).toHaveBeenCalledWith("1");
});
it("should throw error if user not found", async () => {
vi.mocked(mockRepository.findById).mockResolvedValue(null);
await expect(service.getUser("999")).rejects.toThrow("User not found");
});
});
describe("createUser", () => {
it("should create user successfully", async () => {
const userData = { name: "John", email: "john@example.com" };
const createdUser = { id: "1", ...userData };
vi.mocked(mockRepository.create).mockResolvedValue(createdUser);
const user = await service.createUser(userData);
expect(user).toEqual(createdUser);
expect(mockRepository.create).toHaveBeenCalled();
});
});
});
```
### Pattern 3: Spying on Functions
```typescript
// utils/logger.ts
export const logger = {
info: (message: string) => console.log(`INFO: ${message}`),
error: (message: string) => console.error(`ERROR: ${message}`),
};
// services/order.service.ts
import { logger } from "../utils/logger";
export class OrderService {
async processOrder(orderId: string): Promise<void> {
logger.info(`Processing order ${orderId}`);
// Process order logic
logger.info(`Order ${orderId} processed successfully`);
}
}
// services/order.service.test.ts
import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
import { OrderService } from "./order.service";
import { logger } from "../utils/logger";
describe("OrderService", () => {
let service: OrderService;
let loggerSpy: any;
beforeEach(() => {
service = new OrderService();
loggerSpy = vi.spyOn(logger, "info");
});
afterEach(() => {
loggerSpy.mockRestore();
});
it("should log order processing", async () => {
await service.processOrder("123");
expect(loggerSpy).toHaveBeenCalledWith("Processing order 123");
expect(loggerSpy).toHaveBeenCalledWith("Order 123 processed successfully");
expect(loggerSpy).toHaveBeenCalledTimes(2);
});
});
```
## Integration Testing
### Pattern 1: API Integration Tests
```typescript
// tests/integration/user.api.test.ts
import request from "supertest";
import { app } from "../../src/app";
import { pool } from "../../src/config/database";
describe("User API Integration Tests", () => {
beforeAll(async () => {
// Setup test database
await pool.query("CREATE TABLE IF NOT EXISTS users (...)");
});
afterAll(async () => {
// Cleanup
await pool.query("DROP TABLE IF EXISTS users");
await pool.end();
});
beforeEach(async () => {
// Clear data before each test
await pool.query("TRUNCATE TABLE users CASCADE");
});
describe("POST /api/users", () => {
it("should create a new user", async () => {
const userData = {
name: "John Doe",
email: "john@example.com",
password: "password123",
};
const response = await request(app)
.post("/api/users")
.send(userData)
.expect(201);
expect(response.body).toMatchObject({
name: userData.name,
email: userData.email,
});
expect(response.body).toHaveProperty("id");
expect(response.body).not.toHaveProperty("password");
});
it("should return 400 if email is invalid", async () => {
const userData = {
name: "John Doe",
email: "invalid-email",
password: "password123",
};
const response = await request(app)
.post("/api/users")
.send(userData)
.expect(400);
expect(response.body).toHaveProperty("error");
});
it("should return 409 if email already exists", async () => {
const userData = {
name: "John Doe",
email: "john@example.com",
password: "password123",
};
await request(app).post("/api/users").send(userData);
const response = await request(app)
.post("/api/users")
.send(userData)
.expect(409);
expect(response.body.error).toContain("already exists");
});
});
describe("GET /api/users/:id", () => {
it("should get user by id", async () => {
const createResponse = await request(app).post("/api/users").send({
name: "John Doe",
email: "john@example.com",
password: "password123",
});
const userId = createResponse.body.id;
const response = await request(app)
.get(`/api/users/${userId}`)
.expect(200);
expect(response.body).toMatchObject({
id: userId,
name: "John Doe",
email: "john@example.com",
});
});
it("should return 404 if user not found", async () => {
await request(app).get("/api/users/999").expect(404);
});
});
describe("Authentication", () => {
it("should require authentication for protected routes", async () => {
await request(app).get("/api/users/me").expect(401);
});
it("should allow access with valid token", async () => {
// Create user and login
await request(app).post("/api/users").send({
name: "John Doe",
email: "john@example.com",
password: "password123",
});
const loginResponse = await request(app).post("/api/auth/login").send({
email: "john@example.com",
password: "password123",
});
const token = loginResponse.body.token;
const response = await request(app)
.get("/api/users/me")
.set("Authorization", `Bearer ${token}`)
.expect(200);
expect(response.body.email).toBe("john@example.com");
});
});
});
```
### Pattern 2: Database Integration Tests
```typescript
// tests/integration/user.repository.test.ts
import { describe, it, expect, beforeAll, afterAll, beforeEach } from "vitest";
import { Pool } from "pg";
import { UserRepository } from "../../src/repositories/user.repository";
describe("UserRepository Integration Tests", () => {
let pool: Pool;
let repository: UserRepository;
beforeAll(async () => {
pool = new Pool({
host: "localhost",
port: 5432,
database: "test_db",
user: "test_user",
password: "test_password",
});
repository = new UserRepository(pool);
// Create tables
await pool.query(`
CREATE TABLE IF NOT EXISTS users (
id SERIAL PRIMARY KEY,
name VARCHAR(255) NOT NULL,
email VARCHAR(255) UNIQUE NOT NULL,
password VARCHAR(255) NOT NULL,
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
)
`);
});
afterAll(async () => {
await pool.query("DROP TABLE IF EXISTS users");
await pool.end();
});
beforeEach(async () => {
await pool.query("TRUNCATE TABLE users CASCADE");
});
it("should create a user", async () => {
const user = await repository.create({
name: "John Doe",
email: "john@example.com",
password: "hashed_password",
});
expect(user).toHaveProperty("id");
expect(user.name).toBe("John Doe");
expect(user.email).toBe("john@example.com");
});
it("should find user by email", async () => {
await repository.create({
name: "John Doe",
email: "john@example.com",
password: "hashed_password",
});
const user = await repository.findByEmail("john@example.com");
expect(user).toBeTruthy();
expect(user?.name).toBe("John Doe");
});
it("should return null if user not found", async () => {
const user = await repository.findByEmail("nonexistent@example.com");
expect(user).toBeNull();
});
});
```
## Frontend Testing with Testing Library
### Pattern 1: React Component Testing
```typescript
// components/UserForm.tsx
import { useState } from 'react';
interface Props {
onSubmit: (user: { name: string; email: string }) => void;
}
export function UserForm({ onSubmit }: Props) {
const [name, setName] = useState('');
const [email, setEmail] = useState('');
const handleSubmit = (e: React.FormEvent) => {
e.preventDefault();
onSubmit({ name, email });
};
return (
<form onSubmit={handleSubmit}>
<input
type="text"
placeholder="Name"
value={name}
onChange={(e) => setName(e.target.value)}
data-testid="name-input"
/>
<input
type="email"
placeholder="Email"
value={email}
onChange={(e) => setEmail(e.target.value)}
data-testid="email-input"
/>
<button type="submit">Submit</button>
</form>
);
}
// components/UserForm.test.tsx
import { render, screen, fireEvent } from '@testing-library/react';
import { describe, it, expect, vi } from 'vitest';
import { UserForm } from './UserForm';
describe('UserForm', () => {
it('should render form inputs', () => {
render(<UserForm onSubmit={vi.fn()} />);
expect(screen.getByPlaceholderText('Name')).toBeInTheDocument();
expect(screen.getByPlaceholderText('Email')).toBeInTheDocument();
expect(screen.getByRole('button', { name: 'Submit' })).toBeInTheDocument();
});
it('should update input values', () => {
render(<UserForm onSubmit={vi.fn()} />);
const nameInput = screen.getByTestId('name-input') as HTMLInputElement;
const emailInput = screen.getByTestId('email-input') as HTMLInputElement;
fireEvent.change(nameInput, { target: { value: 'John Doe' } });
fireEvent.change(emailInpu
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