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 operating system:

JS
const fs = require('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:

  • r+ open the file for reading and writing, if file doesn't exist it won't be created.
  • w+ open the file for reading and writing, positioning the stream at the beginning of the file. The file is created if not existing.
  • a open the file for writing, positioning the stream at the end of the file. The file is created if not existing.
  • a+ open the file for reading and writing, positioning the stream at the end of the file. The file is created if not existing.

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

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

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.

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.

JS
const fs = require('fs/promises');
// Or const fs = require('fs').promises before v14.
async function example() {
let filehandle;
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();
}
}
example();

Here is an example of util.promisify:

JS
const fs = require('fs');
const util = require('util');
async function example() {
const open = util.promisify(fs.open);
const fd = await open('/Users/joe/test.txt', 'r');
}
example();

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

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