Working with file descriptors in Node.js

Before you're able to interact with a file that sits in your filesystem, you must get a file descriptor. A file descriptor is a reference to an open file, a number (fd) returned by opening the file using the open() method offered by the fs module. This number (fd) uniquely identifies an open file in the operating system.

Opening Files

CommonJS (CJS)

const fs = require('node:fs');
fs.open('/Users/joe/test.txt', 'r', (err, fd) => {
  // fd is our file descriptor
});

Notice the 'r' we used as the second parameter to the fs.open() call. That flag means we open the file for reading. Other flags you'll commonly use are:

Flag	Description
'w+'	This flag opens the file for reading and writing. It positions the stream at the beginning of the file.
'a+'	This flag opens the file for reading and writing and also positions the stream at the end of the file.

You can also open the file by using the fs.openSync method, which returns the file descriptor instead of providing it in a callback:

const fs = require('node:fs');

try {
  const fd = fs.openSync('/Users/joe/test.txt', 'r');
} catch (err) {
  console.error(err);
}

Performing Operations

Once you get the file descriptor in whatever way you choose, you can perform all the operations that require it, like calling fs.close() and many other operations that interact with the filesystem.

Using fsPromises

You can also open the file by using the promise-based fsPromises.open method offered by the fs/promises module. The fs/promises module is available starting only from Node.js v14. Before v14, after v10, you can use require('fs').promises instead. Before v10, after v8, you can use util.promisify to convert fs methods into promise-based methods.

ES Modules (MJS)

import fs from 'node:fs/promises';

async function run() {
  const fileHandle = await fs.open('example.txt', 'r');
  try {
    filehandle = await fs.open('/Users/joe/test.txt', 'r');
    console.log(filehandle.fd);
    console.log(await filehandle.readFile({ encoding: 'utf8' }));
  } finally {
    await fileHandle.close();
  }
}

run().catch(console.error);

Example of util.promisify

Here is an example of using util.promisify to convert fs.open into a promise-based function:

const fs = require('node:fs');
const util = require('node:util');

const open = util.promisify(fs.open);

open('test.txt', 'r')
  .then((fd) => {
    // Use file descriptor
  })
  .catch((err) => {
    // Handle error
  });

To see more details about the fs/promises module, please check the fs/promises API documentation.