File System in Node.js

fs Module

The fs module provides an API for interacting with the file system.

const fs = require('fs');
const fsPromises = require('fs').promises;

Reading Files

Asynchronous

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

Synchronous

try {
  const data = fs.readFileSync('file.txt', 'utf8');
  console.log(data);
} catch (err) {
  console.error(err);
}

Promise-based

async function readFile() {
  try {
    const data = await fsPromises.readFile('file.txt', 'utf8');
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

Writing Files

// Async
fs.writeFile('output.txt', 'Hello World', (err) => {
  if (err) throw err;
  console.log('File written');
});

// Sync
fs.writeFileSync('output.txt', 'Hello World');

// Promise
await fsPromises.writeFile('output.txt', 'Hello World');

// Append
fs.appendFile('log.txt', 'New log entry\n', (err) => {
  if (err) throw err;
});

File Operations

// Check if file exists
fs.access('file.txt', fs.constants.F_OK, (err) => {
  console.log(err ? 'File does not exist' : 'File exists');
});

// Get file stats
fs.stat('file.txt', (err, stats) => {
  console.log(stats.size);
  console.log(stats.isFile());
  console.log(stats.isDirectory());
});

// Rename file
fs.rename('old.txt', 'new.txt', (err) => {
  if (err) throw err;
});

// Delete file
fs.unlink('file.txt', (err) => {
  if (err) throw err;
});

// Copy file
fs.copyFile('source.txt', 'dest.txt', (err) => {
  if (err) throw err;
});

Directory Operations

// Create directory
fs.mkdir('newdir', (err) => {
  if (err) throw err;
});

// Read directory
fs.readdir('.', (err, files) => {
  console.log(files);
});

// Remove directory
fs.rmdir('dir', (err) => {
  if (err) throw err;
});

// Remove directory recursively
fs.rm('dir', { recursive: true }, (err) => {
  if (err) throw err;
});

Streams

// Read stream
const readStream = fs.createReadStream('large-file.txt', 'utf8');

readStream.on('data', (chunk) => {
  console.log('Chunk:', chunk);
});

readStream.on('end', () => {
  console.log('Reading complete');
});

// Write stream
const writeStream = fs.createWriteStream('output.txt');
writeStream.write('Hello ');
writeStream.write('World');
writeStream.end();

// Pipe streams
fs.createReadStream('input.txt')
  .pipe(fs.createWriteStream('output.txt'));

Watch Files

fs.watch('file.txt', (eventType, filename) => {
  console.log(`${filename} ${eventType}`);
});

fs.watchFile('file.txt', (curr, prev) => {
  console.log('File modified');
});

Path Module

const path = require('path');

// Join paths
const filePath = path.join(__dirname, 'files', 'data.txt');

// Resolve absolute path
const absolute = path.resolve('file.txt');

// Get directory name
const dir = path.dirname('/users/john/file.txt'); // /users/john

// Get file name
const file = path.basename('/users/john/file.txt'); // file.txt

// Get extension
const ext = path.extname('file.txt'); // .txt

Best Practices

1. Use async methods for better performance
2. Handle errors properly
3. Use streams for large files
4. Use path module for cross-platform compatibility
5. Close file descriptors when done

Interview Tips

• Explain fs module: File system operations
• Show async vs sync: Performance differences
• Demonstrate streams: Large file handling
• Discuss path module: Cross-platform paths
• Mention promises: Modern async approach

Summary

The fs module provides file system operations. Use async methods for non-blocking I/O, streams for large files, and path module for cross-platform file paths. Available in callback, synchronous, and promise-based APIs.