Node.js Testing Interview Questions Jest, Supertest, and Testing Best Practices

Node.js Testing Interview Questions

Testing is essential for production applications. Here are the most common interview questions.

Q1: What Are the Types of Tests?

Answer:

1. Unit tests Test individual functions in isolation
2. Integration tests Test how components work together
3. End-to-end (E2E) tests Test the full application flow
4. Performance tests Test speed and scalability
5. Security tests Test for vulnerabilities

Q2: How Do You Test Express APIs?

Answer: Use Jest + Supertest:

const request = require("supertest");
const app = require("../src/app");

describe("POST /api/signup", () => {
  it("should create a new user", async () => {
    const res = await request(app)
      .post("/api/signup")
      .send({
        firstName: "John",
        email: "[email protected]",
        password: "Test@1234"
      });
    
    expect(res.status).toBe(201);
    expect(res.body.data).toHaveProperty("_id");
    expect(res.headers["set-cookie"]).toBeDefined();
  });
  
  it("should return 400 for missing fields", async () => {
    const res = await request(app)
      .post("/api/signup")
      .send({ email: "[email protected]" });
    
    expect(res.status).toBe(400);
  });
});

Q3: How Do You Test with a Database?

Answer: Use mongodb-memory-server:

const { MongoMemoryServer } = require("mongodb-memory-server");

let mongoServer;

beforeAll(async () => {
  mongoServer = await MongoMemoryServer.create();
  await mongoose.connect(mongoServer.getUri());
});

afterEach(async () => {
  // Clean up collections between tests
  await User.deleteMany({});
});

afterAll(async () => {
  await mongoose.disconnect();
  await mongoServer.stop();
});

Q4: How Do You Mock Functions?

Answer:

// Mock a module
jest.mock("../src/utils/emailService");
const { sendEmail } = require("../src/utils/emailService");

sendEmail.mockResolvedValue({ success: true });

// Mock a Mongoose model method
jest.mock("../src/models/User");
User.findById.mockResolvedValue({ _id: "123", name: "John" });

// Spy on a function
const spy = jest.spyOn(console, "log");
// ... run code ...
expect(spy).toHaveBeenCalledWith("Hello");
spy.mockRestore();

Q5: What Is Test Coverage?

Answer: Coverage measures what percentage of your code is tested:

# Run with coverage
jest --coverage

Metrics:

• Line coverage % of lines executed
• Branch coverage % of if/else branches tested
• Function coverage % of functions called
• Statement coverage % of statements executed

Aim for 80%+ coverage, but focus on critical paths.

Q6: How Do You Test Async Code?

Answer:

// async/await
test("should fetch user", async () => {
  const user = await User.findById("123");
  expect(user.name).toBe("John");
});

// Resolves/rejects
test("should resolve", () => {
  return expect(Promise.resolve("ok")).resolves.toBe("ok");
});

test("should reject", () => {
  return expect(Promise.reject(new Error("fail"))).rejects.toThrow("fail");
});

Q7: What Should You Test?

Answer:

• Happy path Normal successful operation
• Edge cases Empty input, boundary values
• Error cases Invalid input, not found, unauthorized
• Side effects Database changes, emails sent
• Security Unauthenticated access, authorization

The Takeaway

Testing interview questions cover: test types (unit, integration, E2E, performance, security), testing Express APIs (Jest + Supertest), database testing (mongodb-memory-server for isolated tests), mocking (jest.mock for modules, jest.spyOn for spies), test coverage (line, branch, function, statement aim 80%+), async testing (async/await, resolves/rejects), and what to test (happy path, edge cases, errors, side effects, security).