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Express.js res.append() Function

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The res.append() function appends the specified value to the HTTP response header field and if the header is not already set then it creates the header with the specified value. 

Syntax: 

res.append(field [, value])

Parameter: The field parameter describes the name of the field that need to be appended and the value parameter can be a string or an array.

Returns: It returns an 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:

Example 1: Filename: index.js 

javascript

const express = require('express');
const app = express();
const PORT = 3000;
 
// Without middleware
app.get('/', function (req, res) {
    res.append('Warning', '201 Warning');
    console.log(res.get('Warning')); // 201 Warning
});
 
app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

                    

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 see the following output on your console: 

Server listening on PORT 3000
201 Warning

Example 2: Filename: index.js 

javascript

const express = require('express');
const app = express();
const PORT = 3000;
 
// With middleware
app.use('/', function (req, res, next) {
    res.append('Warning', '201 Its a Warning');
    next();
})
 
app.get('/', function (req, res) {
    console.log(res.get('Warning'));
    res.send();
});
 
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:  

node index.js

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

Server listening on PORT 3000
201 Its a Warning

Reference: https://expressjs.com/en/5x/api.html#res.append



Last Updated : 16 Mar, 2023
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