Express.js router.param() function (original) (raw)

Last Updated : 8 Jan, 2025

The parameters of **router.param() are a **name and **function. Where the **name is the actual name of the parameter and the **function is the callback function. Basically, the router.param() function triggers the callback function whenever the user routes to the parameter. This callback function will be called only a single time in the request-response cycle, even if the user routes to the parameter multiple times.

**Syntax:

router.param(name, function)

**The parameters of the callback function are:

• **req: the request object
• **res: the response object
• **next: the next middleware function
• **id: the value of the name parameter

The router.param() function is used for pre-processing route parameters in Express.js.

First, you need to install the express node module into your node js application.

**Installations of express js are as follows:

npm init
npm install express

**Example: Create a file names **app.js and paste the following code into the file.

javascript ``

// const express = require("express"); const app = express();

//import router module from route.js file const userRoutes = require("./route");

app.use("/", userRoutes);

//PORT const port = process.env.PORT || 8000;

//Starting a server app.listen(port, () => { console.log(app is running at ${port}); });

``

We have to create another file named **route.js in the same directory

Code for **route.js file

javascript `

const express = require("express"); const router = express.Router();

router.param("userId", (req, res, next, id) => { console.log("This function will be called first"); next(); });

router.get("/user/:userId", (req, res) => { console.log("Then this function will be called"); res.end(); }); // Export router module.exports = router;

`

**Steps to run the program:

Start the server by entering the following command

node app.js

**Output:

Enter the following address into the browser:

http://localhost:8000/user/343

You will see the following output in your terminal:

Express.js router.param() function Example Output