Node fs.open() Method

Last Updated : 19 Mar, 2025

To create the file, to write to a file, or to read a file fs.open() method is used. fs.readFile() is only for reading the file and similarly fs.writeFile() is only for writing to a file, whereas fs.open() method does several operations on a file. First, we need to load the fs class which is a module to access the physical file system. For it require method is used. For example: var fs = require('fs');

Syntax:

fs.open( filename, flags, mode, callback )

Parameter: This method accepts four parameters as mentioned above and described below:

filename: It holds the name of the file to read or the entire path if stored at another location.
flag: The operation in which the file has to be opened.
mode: Sets the mode of the file i.e. r-read, w-write, r+ -readwrite. It sets to default as readwrite.
callback: It is a callback function that is called after reading a file. It takes two parameters:
- err: If any error occurs.
- data: A file descriptor, used by subsequent file operations. A file descriptor is a handle used to access a file. It is a non-negative integer uniquely referencing a specific file.

The fs.open() method is used to open files in Node.js.

All the types of flags are described below:

Flag	Description
r	To open the file to read and throw an exception if the file doesn't exist.
r+	Open the file to read and write. Throws an exception if the file doesn't exist.
rs+	Open files in synchronous mode to read and write.
w	Open file for writing. A file is created if it doesn't exist.
wx	It is the same as 'w' but fails if the path exists.
w+	Open the file to read and write. A file is created if it doesn't exist.
wx+	It is the same as 'w+' but fails if the path exists.
a	Open the file to append. A file is created if it doesn't exist.
ax	It is the same as a but fails if the path exists.
a+	Open the file for reading and appending. A file is created if it doesn't exist.
ax+	It is the same as 'a+' but fails if the path exists.

Example 1: Opening a File in Read Mode (r)

JavaScript

// Include the fs module const fs = require('fs');  // Open file demo.txt in read mode fs.open('demo.txt', 'r', function (err, fd) {     if (err) {         return console.error("Error opening file:", err);     }     console.log("File Descriptor:", fd); });

Opens the file demo.txt in read-only mode.
Prints the file descriptor (e.g., 10), which is used for further file operations.

Output

File Descriptor: 3

Example 2: Opening a File in Read & Write Mode (w+)

JavaScript

// Include the fs module const fs = require('fs');  console.log("Opening file...");  // Open file in write and read mode fs.open('demo.txt', 'w+', function (err, fd) {     if (err) {         return console.error("Error opening file:", err);     }     console.log("File Descriptor:", fd); });

Opens the file demo.txt in read and write mode (creates the file if it doesn't exist).
Prints the file descriptor (e.g., 12), which can be used for reading and writing the file.

Output

Opening file...
File Descriptor: 3

We have a Cheat Sheet on Nodejs fs modules where we covered all the fs methods to check those please go through Node File System Module Complete Reference this article.