javascript-testing-patterns – Coding skill for AI agents

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: "[email protected]" };
      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: "[email protected]" };
      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: "[email protected]" };
      service.create(user);

      const updated = service.update("1", { name: "Jane" });

      expect(updated.name).toBe("Jane");
      expect(updated.email).toBe("[email protected]");
    });

    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: "[email protected]" };

      (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: "[email protected]" };
      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(
      "[email protected]",
      "Test Subject",
      "<p>Test Body</p>",
    );

    expect(service["transporter"].sendMail).toHaveBeenCalledWith(
      expect.objectContaining({
        to: "[email protected]",
        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<Us

… (full content is in the download package)