Node fs.open() Method

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:

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

// 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+)

// 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.