Testing GET endpoints in Express - Mini Project: Build & Apply

Testing GET endpoints

📖 Scenario: You are building a simple Express server that has a GET endpoint to return a list of books. You want to write tests to make sure this endpoint works correctly.

🎯 Goal: Create a basic Express app with a GET endpoint at /books that returns a list of books. Then write a test using a testing library to check that the endpoint returns the correct data and status code.

📋 What You'll Learn

Create an Express app with a GET endpoint /books

The endpoint should return a JSON array of book objects

Write a test that sends a GET request to /books

The test should check that the response status is 200

The test should check that the response body matches the expected books array

💡 Why This Matters

🌍 Real World

Testing GET endpoints is essential for backend APIs to ensure they return the correct data and status codes before deploying to production.

💼 Career

Backend developers and QA engineers often write tests like these to maintain API quality and prevent bugs.

1

Create the books data array

Create a constant array called books with these exact objects: { id: 1, title: '1984' } and { id: 2, title: 'Brave New World' }.

Express

// Create the books array with two book objects
// Your code here

Hint

Use const books = [ ... ] to create the array with two objects inside.

2

Set up the Express app and import modules

Import express and create an app variable called app by calling express().

Express

const books = [
  { id: 1, title: '1984' },
  { id: 2, title: 'Brave New World' }
];

// Import express and create app
// Your code here

Hint

Use const express = require('express') and const app = express().

3

Create the GET endpoint for /books

Use app.get with path '/books' and a callback with parameters req and res. Inside the callback, send the books array as JSON using res.json(books).

Express

const books = [
  { id: 1, title: '1984' },
  { id: 2, title: 'Brave New World' }
];

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

// Create GET /books endpoint
// Your code here

Hint

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

4

Write a test for the GET /books endpoint

Import supertest and use it to create a request object with supertest(app). Write a test named 'GET /books returns books list' that sends a GET request to /books and expects status 200 and the JSON body to equal the books array.

Express

const books = [
  { id: 1, title: '1984' },
  { id: 2, title: 'Brave New World' }
];

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

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

// Import supertest and write test for GET /books
// Your code here

Hint

Use supertest to send a GET request and check status and body with expect.

Practice

1. What is the main purpose of testing GET endpoints in an Express app?

easy

A. To verify the server returns the correct data and status code

B. To check if the server can handle POST requests

C. To test database connection speed

D. To validate CSS styles on the page

Solution

1. Step 1: Understand GET endpoint testing

   Testing GET endpoints means checking if the server responds correctly when a client asks for data.

2. Step 2: Identify the goal of testing

   The goal is to confirm the server sends the right data and proper HTTP status code (like 200 for success).

3. Final Answer:

   To verify the server returns the correct data and status code -> Option A

4. Quick Check:

   Testing GET endpoints = verify data and status [OK]

Hint: GET endpoint tests check response data and status code [OK]

Common Mistakes:

• Confusing GET with POST request testing
• Testing unrelated features like CSS or database speed
• Ignoring status codes in tests

2. Which of the following is the correct way to start a test for a GET endpoint using supertest with Express?

easy

A. app.get('/path').send(200);

B. request(app).get('/path').expect(200, done);

C. supertest.get('/path').expect(200);

D. request.get(app, '/path').expect(200);

Solution

1. Step 1: Recall supertest syntax

   Supertest uses request(app).get('/path') to simulate a GET request to the Express app.

2. Step 2: Check the correct chaining

   Chaining .expect(200, done) checks the status code and signals test completion.

3. Final Answer:

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

4. Quick Check:

   supertest GET syntax = request(app).get(path).expect(status) [OK]

Hint: Use request(app).get(path).expect(status, done) for GET tests [OK]

Common Mistakes:

• Calling app.get() instead of request(app).get()
• Missing the done callback in async tests
• Incorrect method chaining order

3. Given this Express GET endpoint and test code, what will the test output?

const express = require('express');
const app = express();
app.get('/hello', (req, res) => res.json({ message: 'Hi!' }));

const request = require('supertest');

test('GET /hello returns message', async () => {
  const response = await request(app).get('/hello');
  return response.body.message;
});

medium

A. "Hi!"

B. undefined

C. Error: No response

D. "hello"

Solution

1. Step 1: Analyze the endpoint response

   The GET /hello endpoint sends JSON { message: 'Hi!' } as response.

2. Step 2: Check the test return value

   The test awaits the GET request and returns response.body.message, which is 'Hi!'.

3. Final Answer:

   "Hi!" -> Option A

4. Quick Check:

   GET /hello returns message = "Hi!" [OK]

Hint: Response body message matches endpoint JSON key [OK]

Common Mistakes:

• Expecting response.body instead of response.body.message
• Forgetting to await the request
• Assuming string 'hello' instead of 'Hi!'

4. Identify the error in this test code for an Express GET endpoint:

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

test('GET /users returns 200', () => {
  request(app)
    .get('/users')
    .expect(200);
});

medium

A. Missing import of Express

B. Incorrect HTTP method used

C. Missing return or await to handle async test

D. Wrong path '/users' does not exist

Solution

1. Step 1: Check async test handling

   The test function is async but does not return or await the request promise, so Jest won't wait for it.

2. Step 2: Confirm correct fix

   Adding return before request(...) or making the test async and awaiting fixes the issue.

3. Final Answer:

   Missing return or await to handle async test -> Option C

4. Quick Check:

   Async test needs return or await [OK]

Hint: Always return or await supertest calls in tests [OK]

Common Mistakes:

• Not returning or awaiting async calls in tests
• Assuming path or method is wrong without checking
• Confusing import errors with test async errors

5. You want to test an Express GET endpoint that returns a list of users, but only those with active status true. Which test code correctly checks this filtering?

app.get('/active-users', (req, res) => {
  const users = [
    { id: 1, active: true },
    { id: 2, active: false },
    { id: 3, active: true }
  ];
  res.json(users.filter(u => u.active));
});

// Test code options:
A) await request(app).get('/active-users').expect(200).expect(res => {
  if (!res.body.every(u => u.active)) throw new Error('Inactive user found');
});

B) await request(app).get('/active-users').expect(200).expect(res => {
  if (res.body.length !== 3) throw new Error('Wrong number of users');
});

C) await request(app).get('/active-users').expect(404);

D) await request(app).get('/active-users').expect(200).expect(res => {
  if (!res.body.some(u => u.active === false)) throw new Error('Inactive user found');
});

hard

A. Incorrectly throws error when no inactive users found

B. Incorrectly expects all users, including inactive

C. Wrong status code expectation

D. Checks for inactive users but throws error if found

Solution

1. Step 1: Understand endpoint filtering

   The endpoint returns only users with active: true, so no inactive users should appear.

2. Step 2: Analyze test options

   Checks for inactive users but throws error if found uses every(u => u.active) and throws if any inactive, correctly verifying the filter.
   Incorrectly expects all users, including inactive expects length !== 3 (wrong, it's 2).
   Wrong status code expectation expects 404 (wrong).
   Incorrectly throws error when no inactive users found incorrectly throws error when no inactive users found.

3. Step 3: Verify A logic

   !every(u => u.active) detects any inactive user, matching the requirement.

4. Final Answer:

   Checks for inactive users but throws error if found -> Option D

5. Quick Check:

   Test filters out inactive users = no inactive found [OK]

Hint: Check response has no inactive users with some() or every() [OK]

Common Mistakes:

• Expecting all users instead of filtered active only
• Using wrong status code in test
• Confusing every() and some() logic in tests