Express.js | res.format() Function

The res.format() function performs content negotiation on the Accept HTTP header on the request object if it is present. This function checks the Accept HTTP request header and then invokes the corresponding handler depending on the Accept value. 

Syntax:

res.format(object)

Installation of the express module:

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

npm install express

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

npm version express

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:

Filename: index.js 

const express = require('express');
const app = express();
const PORT = 3000;

app.get('/', function (req, res) {
    res.format({
        html: function () {
            res.send('<p>Greetings from GeeksforGeeks</p>');
        },
        text: function () {
            res.send('Greetings from GeeksforGeeks');
        },
        json: function () {
            res.send({ message: 'Greetings from GeeksforGeeks' });
        }
    });
});

app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

In the above example, the output will be { "message": "Greetings from GeeksforGeeks" } when the Accept header field is set to 'application/json' and if the Accept header field is set to 'text/plain', we will get "Greetings from GeeksforGeeks" message in response.

Steps to run the program:

Make sure you have installed the express module using the following command:

npm install express

Run the index.js file using the below command:

node index.js

Output:

Console Output:

Server listening on PORT 3000

Browser Output:

Open your browser and go to http://localhost:3000/, now you can see the following output on your screen.

Greetings from GeeksforGeeks