Express.js res.vary() Function

Last Updated : 16 Mar, 2023

The res.vary() function is used to add the field to the Vary response header, if it is not there already. The Vary header indicates which headers it’s basically used for content negotiation.

Syntax:

res.vary( field )

Parameter: The field parameter describes the name of the field.

Return Value: It returns an Object.

Installation of the express module:

1. You can visit the link to Install the express module. You can install this package by using this command.

npm install express

2. After installing the express module, you can check your express version in the command prompt using the command.

npm version express

3. After that, you can just create a folder and add a file, for example, index.js. To run this file you need to run the following command.

node index.js

Project Structure:

Example 1: Filename: index.js

javascript

const express = require('express');
const app = express();
const PORT = 3000;
 
// Without middleware
app.get('/', function (req, res) {
    res.consty('User-Agent').send(
        "Field added to the consty response header");
});
 
app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Steps to run the program:

Run the index.js file using the below command in console:

node index.js

Output:

Console Output:

Server listening on PORT 3000

Now open the browser and go to http://localhost:3000/, now on your screen you will see the following output:

Browser Output:

Field added to the Vary response header

Example 2: Filename: index.js