Integration vs unit test decision in Express - Hands-On Comparison

Integration vs Unit Test Decision in Express

📖 Scenario: You are building a simple Express server that manages a list of books. You want to learn how to decide when to write unit tests and when to write integration tests.

🎯 Goal: Build a small Express app with a books list, then write a unit test for a helper function and an integration test for the API endpoint.

📋 What You'll Learn

Create an Express app with a GET /books endpoint

Create a helper function to filter books by minimum rating

Write a unit test for the helper function

Write an integration test for the GET /books endpoint

💡 Why This Matters

🌍 Real World

In real projects, you often write unit tests for small functions and integration tests for API endpoints to ensure your app works correctly from small parts to full features.

💼 Career

Understanding when to write unit vs integration tests is essential for backend developers to maintain reliable and maintainable codebases.

1

DATA SETUP: Create the books data array

Create a constant array called books with these exact objects: { id: 1, title: '1984', rating: 4.5 }, { id: 2, title: 'Brave New World', rating: 4.0 }, and { id: 3, title: 'Fahrenheit 451', rating: 4.2 }.

Express

# Your code here

Hint

Use const books = [ ... ] with the exact objects inside.

2

CONFIGURATION: Create the filterBooksByRating helper function

Create a function called filterBooksByRating that takes books and minRating as parameters and returns books with rating greater than or equal to minRating.

Express

const books = [
  { id: 1, title: '1984', rating: 4.5 },
  { id: 2, title: 'Brave New World', rating: 4.0 },
  { id: 3, title: 'Fahrenheit 451', rating: 4.2 }
];

// Your code here

Hint

Use books.filter inside the function to select books with rating >= minRating.

3

CORE LOGIC: Create the Express app with GET /books endpoint

Create an Express app by importing express, calling express() to get app, and add a GET endpoint /books that returns the full books array as JSON.

Express

const books = [
  { id: 1, title: '1984', rating: 4.5 },
  { id: 2, title: 'Brave New World', rating: 4.0 },
  { id: 3, title: 'Fahrenheit 451', rating: 4.2 }
];

function filterBooksByRating(books, minRating) {
  return books.filter(book => book.rating >= minRating);
}

// Your code here

Hint

Import express, create app, and add app.get('/books', ...) that sends books as JSON.

4

COMPLETION: Write unit and integration tests

Write a unit test for filterBooksByRating that checks it returns books with rating >= 4.2. Then write an integration test for GET /books endpoint that checks the response status is 200 and the body contains the full books array. Use jest and supertest.

Express

import express from 'express';

const books = [
  { id: 1, title: '1984', rating: 4.5 },
  { id: 2, title: 'Brave New World', rating: 4.0 },
  { id: 3, title: 'Fahrenheit 451', rating: 4.2 }
];

function filterBooksByRating(books, minRating) {
  return books.filter(book => book.rating >= minRating);
}

const app = express();

app.get('/books', (req, res) => {
  res.json(books);
});

// Your code here

Hint

Use test() from jest and request(app).get('/books') from supertest for integration test.

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