Testing API routes in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of testing API routes in Next.js?

To ensure that the API endpoints respond correctly to requests, handle data properly, and return expected status codes and responses.

beginner

Which testing library is commonly used to test Next.js API routes?

Jest is commonly used along with libraries like Supertest to test Next.js API routes by simulating HTTP requests.

intermediate

How do you simulate a GET request to a Next.js API route in a test?

Use Supertest to send a GET request to the API handler and check the response status and body.

intermediate

Why should API route tests be isolated and not depend on external services?

To make tests reliable and fast by avoiding failures caused by network issues or changes in external services.

intermediate

What is a simple way to test a POST request with JSON data in Next.js API routes?

Use Supertest to send a POST request with a JSON body and verify the API response matches expected results.

Which library helps simulate HTTP requests to test Next.js API routes?

A Express

B React Testing Library

C Supertest

D Redux

Supertest is used to simulate HTTP requests in tests for API routes.

What should you check when testing an API route response?

A The database schema

B Response status and body

C The UI rendering

D Only the response headers

Testing API routes involves checking the response status code and the response body content.

Why is it important to mock external services in API route tests?

A To test the external service itself

B To increase test complexity

C To avoid writing tests

D To keep tests fast and reliable

Mocking external services prevents flaky tests and speeds up testing.

Which HTTP method is used to retrieve data from an API route?

A GET

B POST

C PUT

D DELETE

GET requests are used to retrieve data from an API.

In Next.js API route tests, what does the 'handler' represent?

A The function that handles the API request

B The React component

C The database connection

D The CSS styling

The handler is the function that processes incoming API requests.

Explain how you would write a test for a Next.js API route that handles a POST request with JSON data.

Think about simulating the request and checking the response.

Describe why isolating API route tests from external services is important and how you might achieve it.

Consider what happens if the external service is down during testing.

Practice

1. What is the main purpose of testing API routes in a Next.js application?

easy

A. To verify that the backend API responds correctly to requests

B. To style the API responses with CSS

C. To improve the frontend UI components

D. To deploy the API to production servers

Solution

1. Step 1: Understand API route role

   API routes handle backend logic and respond to client requests in Next.js.

2. Step 2: Purpose of testing API routes

   Testing ensures these routes respond correctly and handle inputs as expected.

3. Final Answer:

   To verify that the backend API responds correctly to requests -> Option A

4. Quick Check:

   API route testing = verify backend response [OK]

Hint: API route tests check backend responses, not frontend styles [OK]

Common Mistakes:

• Confusing API testing with frontend UI testing
• Thinking testing deploys the API
• Assuming API testing styles responses

2. Which of the following is the correct way to import the Next.js API route handler for testing with Jest?

easy

A. import handler from '../pages/api/hello';

B. require('../pages/api/hello');

C. import { handler } from '../pages/api/hello';

D. import * as handler from '../pages/api/hello';

Solution

1. Step 1: Identify default export usage

   Next.js API routes usually export a default function as handler.

2. Step 2: Correct import syntax for default export

   Use import handler from 'path' to import default export correctly.

3. Final Answer:

   import handler from '../pages/api/hello'; -> Option A

4. Quick Check:

   Default export import = import handler from [OK]

Hint: Use default import syntax for API route handlers [OK]

Common Mistakes:

• Using require() in ES module context
• Trying to destructure default export
• Importing all as an object unnecessarily

3. Given this test snippet for a Next.js API route handler, what will be the response status code if the handler returns res.status(200).json({ message: 'OK' })?

import handler from '../pages/api/hello';
import { createMocks } from 'node-mocks-http';

test('returns 200 status', async () => {
  const { req, res } = createMocks({ method: 'GET' });
  await handler(req, res);
  expect(res._getStatusCode()).toBe(200);
});

medium

A. 404

B. 200

C. 500

D. 400

Solution

1. Step 1: Understand handler response

   The handler sets status 200 and sends JSON with message 'OK'.

2. Step 2: Test checks status code

   The test expects status code 200, matching the handler's response.

3. Final Answer:

   200 -> Option B

4. Quick Check:

   Handler returns 200 status code [OK]

Hint: Match expected status with handler's res.status call [OK]

Common Mistakes:

• Assuming default status is 404
• Confusing JSON response with status code
• Not awaiting async handler

4. Identify the error in this test code for a Next.js API route:

import handler from '../pages/api/hello';
import { createMocks } from 'node-mocks-http';

test('returns 200 status', () => {
  const { req, res } = createMocks({ method: 'GET' });
  handler(req, res);
  expect(res._getStatusCode()).toBe(200);
});

medium

A. Using _getStatusCode instead of statusCode property

B. Incorrect import path for handler

C. Wrong method used in createMocks

D. Missing async/await for the handler call

Solution

1. Step 1: Check handler invocation

   API route handlers are async functions and must be awaited in tests.

2. Step 2: Identify missing async/await

   The test function is not async and does not await handler, causing test to run before response is set.

3. Final Answer:

   Missing async/await for the handler call -> Option D

4. Quick Check:

   Async handler needs await in test [OK]

Hint: Always await async API handlers in tests [OK]

Common Mistakes:

• Forgetting async keyword on test function
• Not awaiting the handler call
• Assuming synchronous execution

5. You want to test a Next.js API route that handles both GET and POST methods differently. Which approach correctly tests both methods in one test file?

hard

A. Use a single createMocks call with method 'GET,POST' to test both

B. Call handler once with method 'GET' and reuse the same response object for 'POST'

C. Create separate tests using createMocks with method set to 'GET' and 'POST', then call handler for each

D. Test only the GET method since POST is similar

Solution

1. Step 1: Understand method-specific testing

   Each HTTP method requires a separate mock request to simulate different behaviors.

2. Step 2: Use separate mocks for GET and POST

   CreateMocks should be called twice with method set to 'GET' and 'POST' respectively, calling handler each time.

3. Final Answer:

   Create separate tests using createMocks with method set to 'GET' and 'POST', then call handler for each -> Option C

4. Quick Check:

   Separate mocks per method = correct testing [OK]

Hint: Test each HTTP method separately with fresh mocks [OK]

Common Mistakes:

• Reusing response object across methods
• Testing only one method and ignoring others
• Trying to combine methods in one mock