Node.js fs.filehandle.fd() Method

The fs.filehandle.fd() method is an inbuilt application programming interface of class fs.filehandle within File System module which is used to provide the numeric file descriptor of this file handle object.
Syntax:

const filehandle.fd()

Parameter: This method does not accept any parameter.
Return value: This method returns the numeric file descriptor of this file handle object.
Below programs illustrates the use of fs.filehandle.fd() method in Node.js:
Example 1: Filename: index.js

javascript

// Node.js program to demonstrate the
// filehandle.fd() method
const fs = require('fs');
const fsPromises = fs.promises;

console.log("content of file before operation :- "
            + (fs.readFileSync('example.txt')));

// Initiating asynchronise function
async function funct() {

    // Initializing filehandle
    let filehandle = null;
    let value = null;

    try {

        // Creating and initiating  filehandle
        filehandle = await
            fsPromises.open('example.txt', 'r+');

        // Getting the Numeric file descriptor
        // by using fd api
        value = filehandle.fd;

    } finally {

        if (filehandle) {

            // Close the file if it is opened.
            console.log(
                "Numeric file descriptor :- " + value);
            console.log(
                "content of file after operation :- " +
                (fs.readFileSync('example.txt')));

            await filehandle.close();
        }
    }
}

funct().catch(console.error);

Directory structure before running the program: 

Directory structure after running the program: 

Run index.js file using the following command: 

node index.js

Output:

content of file before operation :- ABCD
Numeric file descriptor :- 3
content of file after operation :- ABCD

Example 2:
Filename: index.js

javascript

// Node.js program to demonstrate the
// filehandle.fd() method
const fs = require('fs');
const fsPromises = fs.promises;

// Data for the new file
let data = "This is a file containing "
            + "a collection of books.";

// Name of the file to be created
let file = "books.txt";

// Creating the new file 'books.txt'
fs.writeFile(file, data, (err) => {

    // Catching error
    if (err) {
        console.log(err);
    }
});

// Using fs.exists() method
fs.exists(file, (exists) => {
    if (exists) {
        console.log("content of file "
            + "before operation :- "
            + (fs.readFileSync(file)));
    }
});

// Initiating asynchronise function
async function funct() {

    // Initializing filehandle
    let filehandle = null;

    try {

        // Creating and initiating  filehandle
        filehandle = await
                fsPromises.open(file, 'r+');

        // Getting the Numeric file descriptor
        // by using fd api
        value = filehandle.fd;

    } finally {

        if (filehandle) {

            // Close the file if it is opened.
            console.log(
                "Numeric file descriptor :- "
                + value);

            console.log("content of file "
                + "after operation :- "
                + (fs.readFileSync(file)));

            await filehandle.close();
        }
    }
}

funct().catch(console.error);

Directory structure before running the program: 

Directory structure after running the program: 

Run index.js file using the following command: 

node index.js

Output: 

content of file before operation :- This is a file containing a collection of books.
Numeric file descriptor :- 4
content of file after operation :- This is a file containing a collection of books.

Reference: https://nodejs.org/dist/latest-v12.x/docs/api/fs.html#fs_filehandle_fd