**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Testing and Debugging Express Applications **Topic:** Write unit tests for routes and controllers in an Express.js application and debug using built-in tools.(Lab topic) **Objective:** By the end of this topic, you will be able to write unit tests for routes and controllers in an Express.js application using Jest and understand how to debug your application using built-in tools. **Prerequisites:** - Familiarity with Express.js and Node.js - Understanding of the basics of testing and debugging **Materials Needed:** - Node.js (latest version) - Express.js (latest version) - Jest (latest version) - A code editor or IDE of your choice **Step 1: Setting up the Project** Create a new directory for your project and initialize a new Node.js project using the following command: ```bash npm init -y ``` Install the required dependencies, including Express.js and Jest: ```bash npm install express jest ``` Create a new file called `app.js` and add the following code to get started with Express.js: ```javascript const express = require('express'); const app = express(); app.get('/', (req, res) => { res.send('Hello World!'); }); app.listen(3000, () => { console.log('Server started on port 3000'); }); ``` **Step 2: Writing Unit Tests for Routes** Create a new file called `test/app.test.js` and add the following code to write unit tests for the routes: ```javascript const request = require('supertest'); const app = require('./app'); describe('GET /', () => { it('should return Hello World!', async () => { const response = await request(app).get('/'); expect(response.status).toBe(200); expect(response.text).toBe('Hello World!'); }); }); ``` In this example, we're using the `supertest` library to make a GET request to the root route (`/`) and asserting that the response status is 200 and the response text is 'Hello World!'. **Step 3: Writing Unit Tests for Controllers** Create a new file called `test/controllers.test.js` and add the following code to write unit tests for the controllers: ```javascript const express = require('express'); const app = express(); const userController = require('./controllers/userController'); app.use('/users', userController); describe('GET /users', () => { it('should return a list of users', async () => { const response = await request(app).get('/users'); expect(response.status).toBe(200); expect(response.body).toBeInstanceOf(Array); }); }); ``` In this example, we're creating a new Express.js app, requiring the `userController` module, and mounting it at the `/users` route. We're then writing a unit test to assert that the response status is 200 and the response body is an array. **Step 4: Running the Tests** Run the tests using the following command: ```bash jest ``` You should see the test results in your terminal, indicating whether the tests passed or failed. **Step 5: Debugging Using Built-in Tools** To debug your application, you can use the built-in Node.js debugger or the Chrome DevTools. To use the Node.js debugger, add the following code to your `app.js` file: ```javascript const debug = require('debug')('app'); app.get('/', (req, res) => { debug('Request received'); res.send('Hello World!'); }); ``` Then, run the application with the following command: ```bash node app.js ``` You can then use the `debug` command to inspect the application's variables and functions. Alternatively, you can use the Chrome DevTools to debug your application. To do this, open the Chrome browser and navigate to `http://localhost:3000`. Then, click on the "Sources" tab and select the `app.js` file. You can then set breakpoints and inspect the application's variables and functions. **Conclusion:** In this topic, you learned how to write unit tests for routes and controllers in an Express.js application using Jest and how to debug your application using built-in tools. You also learned how to use the `supertest` library to make HTTP requests to your application and how to use the Chrome DevTools to debug your application. **Exercise:** Write unit tests for the following routes: - GET / - GET /users - POST /users **Additional Resources:** - [Jest Documentation](https://jestjs.io/docs/en/getting-started) - [Supertest Documentation](https://supertest.vcpc.io/) - [Chrome DevTools Documentation](https://developer.chrome.com/docs/devtools/) **Leave a comment or ask for help if you have any questions or need further clarification on any of the topics covered in this topic.**