Why testing APIs matters in Express - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple Express server that listens on port 3000.

Express

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen([1], () => {
  console.log('Server running');
});

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

A5000

B8080

C3000

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 80 without admin rights

Using a string instead of a number

The server listens on port 3000, which is a common default for Express apps.

2fill in blank

medium

Complete the code to add JSON body parsing middleware to the Express app.

Express

const express = require('express');
const app = express();

app.use([1]());

app.post('/data', (req, res) => {
  res.json(req.body);
});

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

Aexpress.json

BbodyParser.json

CjsonParser

Dexpress.body

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated body-parser package

Calling middleware without parentheses

Express has built-in JSON parsing middleware called express.json().

3fill in blank

hard

Fix the error in the route handler to correctly send a JSON response with a message.

Express

app.get('/status', (req, res) => {
  res.[1]({ message: 'API is working' });
});

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

Ajson

BsendJson

CsendText

DsendMessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like sendJson

Using sendText which sends plain text

The res.json() method sends a JSON response.

4fill in blank

hard

Fill both blanks to create a middleware that logs the HTTP method and URL of each request.

Express

app.use((req, res, next) => {
  console.log(req.[1] + ' ' + req.[2]);
  next();
});

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

Amethod

Burl

Cpath

Dbody

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.path instead of req.url

Logging req.body which is the request data

req.method gives the HTTP method and req.url gives the requested URL.

5fill in blank

hard

Fill all three blanks to create a route that responds with a status code 201 and a JSON message.

Express

app.post('/create', (req, res) => {
  res.status([1]).[2]([3]);
});

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

A201

Bjson

C{ message: 'Resource created' }

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using status code 200 instead of 201

Using send instead of json for JSON response

The status code 201 means 'Created'. Use res.status(201).json({ message: 'Resource created' }) to send the response.

Practice

1. Why is testing APIs important in Express applications?

easy

A. It automatically updates the API documentation.

B. It makes the app run faster on the server.

C. It helps find and fix problems early before users see them.

D. It reduces the size of the API code.

Solution

1. Step 1: Understand the purpose of API testing

   Testing APIs helps catch bugs and errors early in development.

2. Step 2: Identify the benefit of early problem detection

   Finding problems early prevents issues from reaching users and causing failures.

3. Final Answer:

   It helps find and fix problems early before users see them. -> Option C

4. Quick Check:

   API testing = early bug detection [OK]

Hint: Testing finds bugs early to avoid user issues [OK]

Common Mistakes:

• Thinking testing speeds up server runtime
• Confusing testing with documentation generation
• Believing testing reduces code size

2. Which of the following is the correct way to write a simple test for an Express API endpoint using a testing library like Jest and Supertest?

easy

A. app.get('/api').expect(200);

B. request(app).get('/api').expect(200).end(done);

C. request('/api').get(app).expect(200);

D. expect(request(app).get('/api')).toBe(200);

Solution

1. Step 1: Recognize the correct testing syntax

   Using Supertest, you call request(app).get('/api') to test the endpoint.

2. Step 2: Confirm chaining with expect and end

   The correct pattern chains .expect(200) and .end(done) to complete the test.

3. Final Answer:

   request(app).get('/api').expect(200).end(done); -> Option B

4. Quick Check:

   Supertest syntax = request(app).get().expect() [OK]

Hint: Supertest uses request(app).get(url).expect(status) [OK]

Common Mistakes:

• Calling expect directly on app.get instead of request
• Swapping app and URL in request call
• Using expect incorrectly without chaining

3. Consider this Express API test snippet using Jest and Supertest:

test('GET /users returns 200 and JSON', async () => {
  const response = await request(app).get('/users');
  expect(response.status).toBe(200);
  expect(response.headers['content-type']).toMatch(/json/);
});

What will happen if the API returns status 404 instead of 200?

medium

A. The test will fail because the status is not 200.

B. The test will pass because content-type is checked.

C. The test will throw a syntax error.

D. The test will ignore the status and pass.

Solution

1. Step 1: Analyze the status check in the test

   The test expects response.status to be exactly 200.

2. Step 2: Understand test behavior on mismatch

   If the API returns 404, the expect(response.status).toBe(200) assertion fails and the test fails.

3. Final Answer:

   The test will fail because the status is not 200. -> Option A

4. Quick Check:

   Status check fails test if not 200 [OK]

Hint: Expect status 200; else test fails [OK]

Common Mistakes:

• Assuming test passes if content-type matches
• Thinking 404 triggers syntax error
• Believing test ignores status mismatch

4. You wrote this Express API test but it always times out:

test('POST /login returns 200', (done) => {
  request(app)
    .post('/login')
    .send({ username: 'user', password: 'pass' })
    .expect(200);
});

What is the main reason for the timeout?

medium

A. Missing call to done() to signal test completion.

B. Incorrect HTTP method used for /login endpoint.

C. The .send() method is not supported in Supertest.

D. The test expects 200 but server returns 500.

Solution

1. Step 1: Check test callback usage

   The test uses done callback but never calls done(), so Jest waits forever.

2. Step 2: Understand how to end async tests

   Calling done() inside .end() or returning a promise is needed to finish the test.

3. Final Answer:

   Missing call to done() to signal test completion. -> Option A

4. Quick Check:

   Call done() to end async test [OK]

Hint: Always call done() or return promise in async tests [OK]

Common Mistakes:

• Forgetting to call done() in callback tests
• Assuming .send() is unsupported
• Guessing wrong HTTP method without checking

5. You want to ensure your Express API handles errors correctly. Which testing approach best helps catch unexpected server errors early?

hard

A. Skip testing and rely on console logs for errors.

B. Manually test the API endpoints once after deployment.

C. Only test the API when users report bugs.

D. Write automated tests that simulate invalid inputs and check for 4xx or 5xx responses.

Solution

1. Step 1: Identify best practice for error handling tests

   Automated tests simulating invalid inputs help verify API returns proper error codes.

2. Step 2: Understand benefits of early automated error detection

   This approach finds bugs before deployment and prevents user impact.

3. Final Answer:

   Write automated tests that simulate invalid inputs and check for 4xx or 5xx responses. -> Option D

4. Quick Check:

   Automated error tests catch bugs early [OK]

Hint: Test invalid inputs automatically to catch errors early [OK]

Common Mistakes:

• Relying only on manual or post-deployment testing
• Ignoring error cases in tests
• Depending on logs instead of tests