Node.js fs File System Module

What is fs File System Module in Node.js?

The fs module reads and writes files. It offers three APIs: promise-based under fs/promises (the modern default), callback-based, and synchronous methods suffixed with Sync that block the event loop.

fs File System Module example

import { readFile, writeFile, appendFile, mkdir, readdir, stat } from 'node:fs/promises';

const text = await readFile('data.json', 'utf8');
const data = JSON.parse(text);

await writeFile('out.json', JSON.stringify(data, null, 2));
await appendFile('app.log', 'started\n');
await mkdir('uploads', { recursive: true });

const files = await readdir('./uploads');
const info = await stat('data.json');
console.log(info.size, info.isDirectory());

How this works

Passing "utf8" returns a string; without it you get a Buffer. recursive: true on mkdir creates intermediate directories and does not throw when the directory already exists.

Key points to remember

• Prefer fs/promises with async/await in new code.
• Sync methods are acceptable at startup, never inside a request handler.
• Use streams for large files instead of reading them entirely into memory.
• Handle ENOENT explicitly — a missing file is a normal condition, not a crash.

Common mistakes with fs File System Module

• readFileSync inside an HTTP handler, which blocks every other request.
• Reading a multi-gigabyte file with readFile and exhausting memory.
• Checking existence with fs.exists before opening, which races — just open and catch.