**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Introduction to Node.js and Express.js **Topic:** Basic routing and handling HTTP requests in Express.js **Introduction** In the previous topics, we've introduced Node.js, its event-driven architecture, and the Express.js framework. We've also set up a Node.js development environment. Now, it's time to dive into one of the core features of Express.js: routing and handling HTTP requests. **What is Routing?** In Express.js, routing is the process of mapping URL paths to specific handlers that execute when a client (usually a web browser) sends an HTTP request to that path. The handler is typically a function that processes the request, interacts with a database or performs computations, and returns a response to the client. **Basic Routing in Express.js** Express.js provides several methods for defining routes, including: * `app.get()`: handles HTTP GET requests * `app.post()`: handles HTTP POST requests * `app.put()`: handles HTTP PUT requests * `app.delete()`: handles HTTP DELETE requests * `app.all()`: handles all HTTP request methods Here's an example of a simple route that responds to a GET request to the root URL (`"/"`) of the server: ```javascript const express = require('express'); const app = express(); app.get('/', (req, res) => { res.send('Hello, World!'); }); ``` In this example, when a client sends a GET request to the root URL of the server, the handler function is executed, and the server responds with the string "Hello, World!". **Handling HTTP Requests** When a client sends an HTTP request to the server, Express.js provides several objects and methods that allow you to access and manipulate the request data. The main objects are: * `req`: the request object, which contains information about the client's request, such as the URL, method, headers, and body. * `res`: the response object, which is used to send a response back to the client. Here's an example of how to access and manipulate request data: ```javascript const express = require('express'); const app = express(); app.get('/user/:id', (req, res) => { const userId = req.params.id; const userAgent = req.headers['user-agent']; res.send(`Hello, User ${userId} with User Agent ${userAgent}!`); }); ``` In this example, we're using the `req.params` object to access the `id` parameter in the URL path, and the `req.headers` object to access the `User-Agent` header in the request. **Key Concepts** * **Route Paths**: URL paths that are mapped to specific handlers. * **Request Methods**: HTTP methods such as GET, POST, PUT, and DELETE. * **Request Object**: The `req` object, which contains information about the client's request. * **Response Object**: The `res` object, which is used to send a response back to the client. **Practical Takeaways** * Use the `app.get()`, `app.post()`, `app.put()`, `app.delete()`, and `app.all()` methods to define routes for your Express.js application. * Use the `req` and `res` objects to access and manipulate request data. * Use route parameters to capture variable data in the URL path. **Recap** In this topic, we've covered the basics of routing and handling HTTP requests in Express.js. We've learned how to define routes, access and manipulate request data, and use route parameters to capture variable data in the URL path. **Next Topic** In the next topic, we'll dive deeper into routing and explore more advanced concepts such as parameterized routes and query strings. Specifically, we'll cover: * **Understanding routing in Express.js (parameterized routes, query strings)** **External Resources** * [Express.js documentation](https://expressjs.com/en/starter/basic-routing.html) * [MDN Web Docs - HTTP request methods](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods) **Do You Have Any Questions or Comments?** Please feel free to ask any questions or leave a comment below.