Testing API routes in NextJS - Mini Project: Build & Apply

Testing API routes

📖 Scenario: You are building a Next.js app that has API routes to handle user data. To make sure your API routes work correctly, you want to write simple tests for them.

🎯 Goal: Create a test for a Next.js API route that returns a JSON response. You will set up the test data, configure the test environment, write the test logic, and complete the test file.

📋 What You'll Learn

Create a mock request and response object

Set a test configuration variable

Write a test function that calls the API route handler

Complete the test file with the necessary export or setup

💡 Why This Matters

🌍 Real World

Testing API routes ensures your backend logic works correctly before deploying your Next.js app.

💼 Career

Writing tests for API routes is a key skill for frontend and full-stack developers to maintain reliable web applications.

1

Set up mock request and response objects

Create two variables called req and res. Set req to an empty object {}. Set res to an object with a method json that is a jest mock function.

NextJS

# Create mock request and response objects
# Your code here

Hint

Think of req as the request coming into your API, and res as the response you send back. Use jest.fn() to mock the json method.

2

Add a test configuration variable

Create a constant called testUserId and set it to the string 'user123'.

NextJS

const req = {};
const res = { json: jest.fn() };

# Create a test user ID
# Your code here

Hint

This variable will be used to simulate a user ID in your test.

3

Write the test function calling the API handler

Write an async function called testApiRoute that calls the imported handler function with req and res. Then check that res.json was called.

NextJS

const req = {};
const res = { json: jest.fn() };

const testUserId = 'user123';

# Write the test function
# Your code here

Hint

Use await to wait for the handler to finish. Use Jest's expect to check the mock function call.

4

Complete the test file with export

Export the testApiRoute function as the default export.

NextJS

const req = {};
const res = { json: jest.fn() };

const testUserId = 'user123';

async function testApiRoute() {
    await handler(req, res);
    expect(res.json).toHaveBeenCalled();
}

# Export the test function
# Your code here

Hint

This makes the test function available for running by your test runner.

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