How does the File System (fs) module work in Node.js?

The fs Module in Node.js

The fs (File System) module is a core Node.js module that provides an API for interacting with the file system. It allows you to read, write, update, delete, and watch files and directories.

Three API Styles

Node.js fs provides three styles for every operation:

1. Callback-based (original)

const fs = require('fs');

fs.readFile('data.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

2. Synchronous (blocking)

const fs = require('fs');

const data = fs.readFileSync('data.txt', 'utf8');
console.log(data);

3. Promise-based (recommended)

const fs = require('fs/promises');

async function readData() {
  const data = await fs.readFile('data.txt', 'utf8');
  console.log(data);
}

Common Operations

Reading Files

const fs = require('fs/promises');

// Read as string
const text = await fs.readFile('file.txt', 'utf8');

// Read as Buffer (binary)
const buffer = await fs.readFile('image.png');

Writing Files

// Write (creates or overwrites)
await fs.writeFile('output.txt', 'Hello World');

// Append to file
await fs.appendFile('log.txt', 'New log entry\n');

Working with Directories

// Create directory
await fs.mkdir('new-folder', { recursive: true });

// Read directory contents
const files = await fs.readdir('src');

// Read with file types
const entries = await fs.readdir('src', { withFileTypes: true });
entries.forEach(entry => {
  console.log(`${entry.name} - ${entry.isDirectory() ? 'dir' : 'file'}`);
});

File Information

const stats = await fs.stat('file.txt');
console.log({
  size: stats.size,           // bytes
  isFile: stats.isFile(),
  isDir: stats.isDirectory(),
  created: stats.birthtime,
  modified: stats.mtime
});

Delete and Rename

// Delete file
await fs.unlink('old-file.txt');

// Delete directory (recursive)
await fs.rm('old-folder', { recursive: true, force: true });

// Rename / move
await fs.rename('old-name.txt', 'new-name.txt');

// Copy file
await fs.copyFile('source.txt', 'dest.txt');

Streaming Large Files

For large files, use streams instead of readFile to avoid loading the entire file into memory:

const fs = require('fs');

const readStream = fs.createReadStream('large-file.csv', 'utf8');
const writeStream = fs.createWriteStream('output.csv');

readStream.pipe(writeStream);

readStream.on('end', () => console.log('Done!'));
readStream.on('error', (err) => console.error(err));

Watching Files

const fs = require('fs');

// Watch for file changes
const watcher = fs.watch('config.json', (eventType, filename) => {
  console.log(`${filename} changed: ${eventType}`);
});

// Or use fs.watchFile for polling-based watching
fs.watchFile('data.txt', { interval: 1000 }, (curr, prev) => {
  console.log(`File modified at: ${curr.mtime}`);
});

Important Considerations

Topic	Best Practice
Encoding	Always specify `'utf8'` for text files
Error handling	Always handle errors (try/catch or callback)
Large files	Use streams, not `readFile`
Sync methods	Avoid in server code (blocks event loop)
Paths	Use `path.join()` for cross-platform paths
Security	Validate user-provided paths to prevent directory traversal

const path = require('path');

// ✅ Safe path construction
const safePath = path.join(__dirname, 'uploads', path.basename(userInput));

// ❌ Vulnerable to directory traversal
const unsafePath = './uploads/' + userInput;
// userInput = '../../etc/passwd'

Tip: Always prefer fs/promises for modern async code. Use streams for files larger than ~50MB. Never use sync methods in server request handlers.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.

Finished reading?

Practice Problems