Integration vs unit test decision in Express - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a unit test that mocks the database call.

Express

test('should return user data', () => {
  const mockDb = jest.fn().mockReturnValue({ id: 1, name: 'Alice' });
  const result = getUser([1]);
  expect(result.name).toBe('Alice');
});

Drag options to blanks, or click blank then click option'

AmockDb

BmockDb()

CdbCall()

DdbCall

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the mock function immediately instead of passing it.

Passing the real database call instead of the mock.

In unit tests, you pass the mock function itself, not its call result, to replace the real database call.

2fill in blank

medium

Complete the code to write an integration test that sends a request to the Express app.

Express

const request = require('supertest');
const app = require('./app');

test('GET /users returns 200', async () => {
  const response = await request(app).[1]('/users');
  expect(response.statusCode).toBe(200);
});

Drag options to blanks, or click blank then click option'

Adelete

Bpost

Cput

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using post instead of get for a GET request.

Forgetting to await the request.

To test a GET request, use the get method from supertest.

3fill in blank

hard

Fix the error in the unit test by completing the mock setup correctly.

Express

jest.mock('./db', () => ({
  fetchUser: jest.fn().[1]({ id: 2, name: 'Bob' })
}));

Drag options to blanks, or click blank then click option'

AmockReturnValue

BmockImplementation

CmockResolvedValue

DmockReturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using mockReturnValue for async functions.

Using a non-existent mockReturn method.

Since fetchUser returns a promise, use mockResolvedValue to mock an async resolved value.

4fill in blank

hard

Fill both blanks to create a test that checks the response body and status code in an integration test.

Express

test('POST /login returns token', async () => {
  const response = await request(app).post('/login').send({ username: 'user', password: 'pass' });
  expect(response.statusCode).toBe([1]);
  expect(response.body).toHaveProperty([2]);
});

Drag options to blanks, or click blank then click option'

A200

B'token'

C'error'

D201

Attempts:

3 left

💡 Hint

Common Mistakes

Using status code 201 which means created, not typical for login.

Checking for 'error' property instead of 'token'.

The expected status code for a successful login is 200, and the response body should have a 'token' property.

5fill in blank

hard

Fill all three blanks to write a unit test that mocks a service call and verifies the result.

Express

const service = require('./service');

jest.mock('./service');

test('should return processed data', async () => {
  service.processData.mock[1]({ success: true });
  const result = await processHandler([2]);
  expect(result).toEqual([3]);
});

Drag options to blanks, or click blank then click option'

AResolvedValue

B'inputData'

C{ success: true }

DReturnValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using mockReturnValue for async functions.

Passing wrong input data to the handler.

Expecting wrong output object.

Use mockResolvedValue to mock async service calls. Pass the input data string, and expect the mocked success object.

Practice

1. Which type of test in Express focuses on checking a single function or module in isolation?

easy

A. Unit test

B. Integration test

C. End-to-end test

D. Performance test

Solution

1. Step 1: Understand test types in Express

   Unit tests focus on small pieces like functions or modules alone, without dependencies.

2. Step 2: Identify test purpose

   Integration tests check how parts work together, so they are not isolated.

3. Final Answer:

   Unit test -> Option A

4. Quick Check:

   Single module check = Unit test [OK]

Hint: Unit tests isolate one part; integration tests combine parts [OK]

Common Mistakes:

• Confusing integration tests with unit tests
• Thinking end-to-end tests are unit tests
• Assuming performance tests check single functions

2. Which of the following is the correct way to write a simple unit test for an Express middleware function?

easy

A. Use a browser to manually check middleware behavior

B. Test the middleware by calling it with mock request and response objects

C. Start the Express server and send HTTP requests to test the middleware

D. Run the entire app and check logs for middleware output

Solution

1. Step 1: Understand unit test scope

   Unit tests isolate the middleware by calling it directly with mock objects, no server needed.

2. Step 2: Identify correct testing method

   Starting the server or manual checks are integration or manual tests, not unit tests.

3. Final Answer:

   Test the middleware by calling it with mock request and response objects -> Option B

4. Quick Check:

   Mock objects for unit test [OK]

Hint: Unit test = call function with mocks, no server start [OK]

Common Mistakes:

• Starting server for unit tests
• Using manual browser checks for unit tests
• Confusing integration test methods with unit tests

3. Consider this Express test snippet:

request(app)
  .get('/users')
  .expect(200)
  .end((err, res) => {
    if (err) throw err;
    console.log(res.body.length);
  });

What type of test is this and what does it check?

medium

A. Unit test; mocks the database call

B. Unit test; checks a single function's return value

C. Integration test; checks the route and middleware working together

D. Performance test; measures response time

Solution

1. Step 1: Analyze the test code

   The test sends an HTTP GET request to '/users' on the app, expecting status 200.

2. Step 2: Identify test type

   This checks the route handler and middleware together, so it's an integration test.

3. Final Answer:

   Integration test; checks the route and middleware working together -> Option C

4. Quick Check:

   HTTP request test = Integration test [OK]

Hint: HTTP request tests check multiple parts together [OK]

Common Mistakes:

• Calling this a unit test
• Assuming it mocks database calls
• Thinking it measures performance

4. You wrote a test that starts the Express server and sends HTTP requests, but it runs very slowly and sometimes fails due to timing issues. What is the best way to fix this problem?

medium

A. Rewrite the test as a unit test using mocks instead of starting the server

B. Add more delays in the test to wait for responses

C. Increase server timeout settings in Express

D. Run the test on a faster machine

Solution

1. Step 1: Identify the problem cause

   Starting the server and sending real HTTP requests makes tests slow and flaky.

2. Step 2: Choose a better testing approach

   Using unit tests with mocks avoids server start and speeds up tests, reducing timing issues.

3. Final Answer:

   Rewrite the test as a unit test using mocks instead of starting the server -> Option A

4. Quick Check:

   Mocks speed tests, avoid server start [OK]

Hint: Mocks speed tests; avoid real server start [OK]

Common Mistakes:

• Adding delays instead of fixing test design
• Changing server settings unnecessarily
• Blaming hardware instead of test approach

5. You want to test an Express app's user login flow. Which approach best balances speed and coverage by combining unit and integration tests?

hard

A. Only write integration tests for the login route to cover everything at once

B. Test manually in the browser without automated tests

C. Write unit tests for the entire Express app without any integration tests

D. Write unit tests for the login function logic and integration tests for the full login route with database

Solution

1. Step 1: Understand testing goals

   Unit tests check small parts fast; integration tests check how parts work together.

2. Step 2: Combine tests for best results

   Writing unit tests for login logic and integration tests for the route with database balances speed and coverage.

3. Final Answer:

   Write unit tests for the login function logic and integration tests for the full login route with database -> Option D

4. Quick Check:

   Combine unit + integration tests for best coverage [OK]

Hint: Use unit tests for logic, integration for routes [OK]

Common Mistakes:

• Relying only on integration tests
• Skipping integration tests entirely
• Ignoring automated tests in favor of manual