javascript-testing-patterns

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:

// 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:

// 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

// 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

// 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

// 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

// 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

// 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

// 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

Integration tests verify real database operations and HTTP endpoints using supertest and a test database instance. Always truncate tables in beforeEach and tear down in afterAll .

For full API integration test examples (supertest + PostgreSQL) and database repository integration tests, see references/advanced-testing-patterns.md.

Frontend Testing with Testing Library

Test React components by rendering them and querying by role, placeholder, or test ID. Test hooks with renderHook

act . Prefer semantic queries (getByRole , getByPlaceholderText ) over data-testid .

For complete React component test examples (UserForm, hooks with renderHook /act ), see references/advanced-testing-patterns.md.

Test Fixtures and Factories

Use @faker-js/faker to generate realistic test data factories. Factories accept optional overrides so tests can set only the fields they care about:

// tests/fixtures/user.fixture.ts import { faker } from "@faker-js/faker";

export function createUserFixture(overrides?: Partial<User>): User { return { id: faker.string.uuid(), name: faker.person.fullName(), email: faker.internet.email(), createdAt: faker.date.past(), ...overrides, }; }

For snapshot testing, coverage configuration, test organization patterns, promise testing, and timer mocking, see references/advanced-testing-patterns.md.

Best Practices

Follow AAA Pattern: Arrange, Act, Assert
One assertion per test: Or logically related assertions
Descriptive test names: Should describe what is being tested
Use beforeEach/afterEach: For setup and teardown
Mock external dependencies: Keep tests isolated
Test edge cases: Not just happy paths
Avoid implementation details: Test behavior, not implementation
Use test factories: For consistent test data
Keep tests fast: Mock slow operations
Write tests first (TDD): When possible
Maintain test coverage: Aim for 80%+ coverage
Use TypeScript: For type-safe tests
Test error handling: Not just success cases
Use data-testid sparingly: Prefer semantic queries
Clean up after tests: Prevent test pollution

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