**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Routing and Middleware **Topic:** Understanding routing in Express.js (parameterized routes, query strings) ### Introduction In the previous topics, we covered the basics of Express.js and how to handle HTTP requests using routes. However, as your application grows, you'll need to handle more complex routing scenarios. In this topic, we'll explore parameterized routes and query strings in Express.js. These concepts will help you create more flexible and dynamic routes that can handle a wide range of requests. ### Parameterized Routes Parameterized routes allow you to capture values from the URL path and use them in your route handlers. This is achieved using route parameters, which are defined using colons (`:`) followed by the parameter name. Example: ```javascript const express = require('express'); const app = express(); // Define a parameterized route app.get('/users/:id', (req, res) => { const id = req.params.id; // Use the id parameter to fetch data from a database or perform some action res.send(`User ID: ${id}`); }); app.listen(3000, () => { console.log('Server started on port 3000'); }); ``` In this example, the `:id` parameter captures the value from the URL path and passes it as a property on the `req.params` object. You can also define multiple parameters by separating them with forward slashes (`/`). For example: ```javascript app.get('/users/:id/posts/:postId', (req, res) => { const id = req.params.id; const postId = req.params.postId; // Use the id and postId parameters to fetch data from a database or perform some action res.send(`User ID: ${id}, Post ID: ${postId}`); }); ``` ### Query Strings Query strings allow you to pass additional data to your route handlers using the URL. They are defined using a question mark (`?`) followed by key-value pairs. Example: ```javascript const express = require('express'); const app = express(); // Define a route with a query string app.get('/search', (req, res) => { const query = req.query; // Use the query object to fetch data from a database or perform some action res.send(`Search query: ${query.term}`); }); app.listen(3000, () => { console.log('Server started on port 3000'); }); ``` In this example, the `req.query` object contains the key-value pairs passed in the query string. ### Practical Example Let's create a simple API that handles GET requests for users and their posts. We'll use parameterized routes and query strings to capture the user ID and post ID. ```javascript const express = require('express'); const app = express(); // Define a route for fetching all users app.get('/users', (req, res) => { // Fetch all users from a database const users = [{ id: 1, name: 'John Doe' }, { id: 2, name: 'Jane Doe' }]; res.json(users); }); // Define a parameterized route for fetching a single user app.get('/users/:id', (req, res) => { const id = req.params.id; // Fetch the user from a database const user = { id: 1, name: 'John Doe' }; res.json(user); }); // Define a route for fetching all posts for a user app.get('/users/:id/posts', (req, res) => { const id = req.params.id; // Fetch the posts from a database const posts = [{ id: 1, title: 'Post 1' }, { id: 2, title: 'Post 2' }]; res.json(posts); }); // Define a route for searching for posts by keyword app.get('/search', (req, res) => { const query = req.query; // Search for posts in a database using the query term const posts = [{ id: 1, title: 'Post 1' }, { id: 2, title: 'Post 2' }]; res.json(posts); }); app.listen(3000, () => { console.log('Server started on port 3000'); }); ``` ### Key Concepts * Parameterized routes allow you to capture values from the URL path and use them in your route handlers. * Query strings allow you to pass additional data to your route handlers using the URL. * Use the `req.params` object to access route parameters. * Use the `req.query` object to access query string values. ### Practical Takeaways * Use parameterized routes to capture values from the URL path and perform actions based on those values. * Use query strings to pass additional data to your route handlers and perform actions based on those values. * Use the `req.params` and `req.query` objects to access route parameters and query string values. **Do you have any questions or need help with understanding parameterized routes and query strings in Express.js? Please leave a comment below.** **External Resources:** * Express.js documentation: [Route parameters](https://expressjs.com/en/guide/routing.html#route-parameters) * Express.js documentation: [Query string](https://expressjs.com/en/api.html#req.query) * MDN Web Docs: [URL](https://developer.mozilla.org/en-US/docs/Web/API/URL) **Next Topic:** Using middleware to handle requests and responses.