Integration testing patterns in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this integration test snippet?

Consider this Node.js integration test using Jest and Supertest. What will be the test result output?

Node.js

import request from 'supertest';
import app from './app';

test('GET /users returns 2 users', async () => {
  const response = await request(app).get('/users');
  expect(response.status).toBe(200);
  expect(response.body.length).toBe(2);
});

A Test passes because the /users endpoint returns exactly 2 users with status 200.

B Test fails because response.status is not 200.

C Test fails because response.body.length is not 2.

D Test throws an error because app is not imported correctly.

💡 Hint

Check what the test expects and what the endpoint returns.

The test expects a 200 status and exactly 2 users in the response body. If the app's /users endpoint returns that, the test passes.

❓ lifecycle

intermediate

Which lifecycle hook is best for setting up a database connection before integration tests?

In a Node.js integration test suite using Jest, which lifecycle method should you use to open a database connection before all tests run?

A beforeEach()

B afterEach()

C afterAll()

D beforeAll()

💡 Hint

Think about a setup that runs once before all tests.

beforeAll() runs once before all tests, perfect for opening a database connection once.

🔧 Debug

advanced

Why does this integration test fail with a timeout?

Look at this test code snippet. Why does it fail with a timeout error?

Node.js

test('POST /login returns 200', async () => {
  const response = await request(app).post('/login').send({ username: 'user', password: 'pass' });
  expect(response.status).toBe(200);
});

A The app server is not closed after previous tests, causing port conflicts and hanging requests.

B The test is missing a done callback to signal completion.

C The request is missing headers, so the server never responds.

D The test uses async/await incorrectly, causing the promise to never resolve.

💡 Hint

Consider what happens if the server is not properly closed between tests.

If the server from previous tests is not closed, it can cause port conflicts or hanging requests, leading to timeouts.

📝 Syntax

advanced

Which option correctly mocks a database call in an integration test?

You want to mock the database call getUserById in your integration test. Which code snippet correctly mocks it using Jest?

Node.js

import * as db from './db';

jest.mock('./db');

// Your test here

A db.getUserById = jest.fn(() => ({ id: 1, name: 'Alice' }));

B jest.mockResolvedValue(db.getUserById, { id: 1, name: 'Alice' });

C db.getUserById.mockResolvedValue({ id: 1, name: 'Alice' });

D jest.fn(db.getUserById).mockReturnValue({ id: 1, name: 'Alice' });

💡 Hint

Check how to mock a function's resolved value with Jest.

After jest.mock('./db'), you can set the mock implementation with mockResolvedValue on the mocked function.

❓ state_output

expert

What is the final value of 'count' after running this integration test sequence?

Given this test suite that modifies a shared state, what is the final value of the variable 'count' after all tests run?

Node.js

let count = 0;

describe('Counter tests', () => {
  beforeAll(() => { count = 5; });

  test('Increment count', () => {
    count += 3;
    expect(count).toBe(8);
  });

  test('Decrement count', () => {
    count -= 2;
    expect(count).toBe(6);
  });

  afterAll(() => {
    count += 1;
  });
});

A 9

B 7

C 6

D 8

💡 Hint

Follow the changes to 'count' step by step through the lifecycle hooks and tests.

Initial count is set to 5 before all tests. First test adds 3 → 8. Second test subtracts 2 → 6. After all tests, add 1 → 7.