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 express module:

1. You can visit the link to Install 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 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

Example 1: Filename: index.js

var express = require('express');
var app = express();
var 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:

1. The project structure will look like this:
   
2. Make sure you have installed express module using the following command:

   npm install express

3. Run index.js file using below command:

   node index.js

   Output:

   Server listening on PORT 3000
   

4. 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

var express = require('express');
var app = express();
var 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);
});

Run index.js file using below command:

node index.js

Output:
Now open your browser and make 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