Advanced Routing with Express Router 🛣️

As your application grows, keeping all your routes in a single file (index.js or app.js) becomes unmanageable. Express Router is the solution to this problem. It’s a “mini-app” that allows you to group related routes into their own files, making your application more modular, organized, and easier to maintain.

Using `express.Router` for Modular Routes

The workflow involves creating a router in a separate file, defining routes on it, and then “mounting” that router in your main application file.

Create a Router File

First, create a new file for a specific part of your application, like products. Here, you define routes relative to the path where the router will be mounted. You create a router instance with express.Router() and define routes on it (e.g., router.get('/')). Finally, you export the router.

products.js


// routes/productRoutes.js
const express = require("express");
 
// 1. Create a new router object
const router = express.Router();
 
// A route for GET /products/
router.get("/", (req, res) => {
  res.send("Get a list of all products");
});
 
// A route for GET /products/123
router.get("/:productId", (req, res) => {
  res.send(`Get details for product with ID: ${req.params.productId}`);
});
 
// 2. Export the router
module.exports = router;

Mount the Router in Your Main App

In your main server file, you import the router file and tell your Express app to use it for a specific base path using app.use('/base-path', routerObject). Any request starting with that base path will then be handled by the corresponding router.

index.js


// app.js
const express = require("express");
const app = express();
 
// 1. Import the router file
const productRoutes = require("./routes/productRoutes");
 
// 2. Mount the router on the '/products' path
// Any request starting with /products will be handled by productRoutes.
app.use("/products", productRoutes);
 
app.listen(3000, () => {
  console.log("Server is running on port 3000");
});

Now, a request to GET /products is handled by the router.get(’/’) handler, and a request to GET /products/123 is handled by the router.get(’/:productId’) handler.

Pre-loading Data with `router.param()`

router.param() is a special middleware that runs only when a specific route parameter is present in a route. It’s perfect for running pre-condition logic, like fetching a user or product from a database before the main route handler runs. This helps you avoid duplicating code across multiple route handlers that share the same parameter.

💻 Code Example

Let’s add router.param() to our productRoutes.js file to simulate fetching a product.

products.js


// routes/productRoutes.js (updated)
// ... (keep the existing router setup)
 
// This middleware will run for any route that contains the ':productId' parameter
router.param("productId", (req, res, next, id) => {
  console.log(`Fetching data for product ID: ${id}`);
  // In a real app, you would fetch the product from a database here.
  // For this example, we'll just attach some data to the request object.
  req.product = { id: id, name: "Sample Product" };
  next(); // Pass control to the next handler
});
 
// The route handler for GET /products/123
router.get("/:productId", (req, res) => {
  // The product data is already available on req.product! No need to fetch it here.
  res.send(`Details for ${req.product.name} (ID: ${req.product.id})`);
});
 
// ... (export the router)

Complex Matching with Regular Expressions

For even more control, you can use regular expressions (RegExp) in your route paths. This is useful for enforcing a specific format for route parameters, ensuring that a route only matches if the parameter conforms to a certain pattern (e.g., consists only of digits).

💻 Code Example

This route will only match if the :id parameter consists of exactly 5 digits.