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:

const module "node:fs"fs = var require: NodeJS.Require
(id: string) => anyrequire('node:fs');

module "node:fs"fs.function open(path: fs.PathLike, flags: fs.OpenMode | undefined, callback: (err: NodeJS.ErrnoException | null, fd: number) => void): void (+2 overloads)open('/Users/joe/test.txt', 'r', (err: NodeJS.ErrnoException | nullerr, fd: numberfd) => {
  // 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	File gets created if it doesn't exist
r+	This flag opens the file for reading and writing	❌
w+	This flag opens the file for reading and writing and it also positions the stream at the beginning of the file	✅
a	This flag opens the file for writing and it also positions the stream at the end of the file	✅
a+	This flag opens the file for reading and writing and it 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 module "node:fs"fs = var require: NodeJS.Require
(id: string) => anyrequire('node:fs');

try {
  const const fd: numberfd = module "node:fs"fs.function openSync(path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null): numberopenSync('/Users/joe/test.txt', 'r');
} catch (var err: unknownerr) {
  var console: Consoleconsole.Console.error(message?: any, ...optionalParams: any[]): void (+1 overload)error(var err: unknownerr);
}

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.

const module "node:fs/promises"fs = var require: NodeJS.Require
(id: string) => anyrequire('node:fs/promises');
// Or const fs = require('fs').promises before v14.
async function function example(): Promise<void>example() {
  let let filehandle: anyfilehandle;
  try {
    let filehandle: anyfilehandle = await module "node:fs/promises"fs.function open(path: PathLike, flags?: string | number, mode?: Mode): Promise<fs.FileHandle>open('/Users/joe/test.txt', 'r');
    var console: Consoleconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)log(let filehandle: fs.FileHandlefilehandle.FileHandle.fd: numberfd);
    var console: Consoleconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)log(await let filehandle: fs.FileHandlefilehandle.FileHandle.readFile(options: ({
    encoding: BufferEncoding;
} & EventEmitter<T extends EventMap<T> = DefaultEventMap>.Abortable) | BufferEncoding): Promise<string> (+2 overloads)readFile({ encoding: BufferEncodingencoding: 'utf8' }));
  } finally {
    if (let filehandle: fs.FileHandle | undefinedfilehandle) {
      await let filehandle: fs.FileHandlefilehandle.FileHandle.close(): Promise<void>close();
    }
  }
}
function example(): Promise<void>example();

Here is an example of util.promisify:

const module "node:fs"fs = var require: NodeJS.Require
(id: string) => anyrequire('node:fs');
const module "node:util"util = var require: NodeJS.Require
(id: string) => anyrequire('node:util');

async function function example(): Promise<void>example() {
  const const open: (path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null) => Promise<number>open = module "node:util"util.function promisify<(path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null) => Promise<number>>(fn: util.CustomPromisify<(path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null) => Promise<number>>): (path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null) => Promise<number> (+13 overloads)promisify(module "node:fs"fs.function open(path: fs.PathLike, flags: fs.OpenMode | undefined, mode: fs.Mode | undefined | null, callback: (err: NodeJS.ErrnoException | null, fd: number) => void): void (+2 overloads)open);
  const const fd: numberfd = await const open: (path: fs.PathLike, flags: fs.OpenMode, mode?: fs.Mode | null) => Promise<number>open('/Users/joe/test.txt', 'r');
}
function example(): Promise<void>example();

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

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