Express.js res.headersSent Property

The res.headersSent property is a boolean property that indicates if the app sent HTTP headers for the response. 

Syntax:

res.headersSent

Parameter: No parameters. 

Return Value: This property returns a Boolean value either True or False. 

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;

app.get('/', function (req, res) {

    // Before res.send()
    console.log(res.headersSent);
    res.send('OK');
});

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:

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

Server listening on PORT 3000
false

Example 2: Filename: index.js

javascript

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

app.get('/', function (req, res) {
    res.send('OK');

    // After res.send()
    console.log(res.headersSent);
});

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 your browser and make a GET request to http://localhost:3000, now you can see the following output on your console:

Server listening on PORT 3000
true

Reference: https://expressjs.com/en/4x/api.html#res.headersSent