File System (fs) Module

🌿 Node.js Lesson 3 Beginner

The built-in fs (File System) module provides APIs for reading, writing, updating, and deleting files and directories. It offers both synchronous and asynchronous variants of every operation.

1 Sync vs Async File Operations

Node.js strongly prefers asynchronous file operations to avoid blocking the event loop:

Synchronous (Sync): Blocks execution until the operation completes. Only suitable for startup scripts or CLI tools.
Asynchronous (Callback): Non-blocking. Passes the result to a callback function when ready.
Promise-based (fs/promises): Modern async/await style, cleanest approach for new code.

2 Reading & Writing Files

JavaScript — fs/promises

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

async function fileOperations() {
  const filePath = path.join(__dirname, 'data.txt');

  // Write a file (creates if not exists, overwrites if exists)
  await fs.writeFile(filePath, 'Hello from Node.js!
Line 2 of data.', 'utf8');
  console.log('File written successfully.');

  // Read the file back
  const content = await fs.readFile(filePath, 'utf8');
  console.log('File content:
', content);

  // Append to an existing file
  await fs.appendFile(filePath, '
Appended line.');

  // Get file metadata
  const stats = await fs.stat(filePath);
  console.log('File size:', stats.size, 'bytes');
  console.log('Modified:', stats.mtime);

  // Delete the file
  await fs.unlink(filePath);
  console.log('File deleted.');
}

fileOperations().catch(console.error);

3 Working with Directories

JavaScript — Directory Operations

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

async function dirOps() {
  // Create directory (recursive creates parent dirs too)
  await fs.mkdir('./output/reports', { recursive: true });

  // List directory contents
  const entries = await fs.readdir('./output', { withFileTypes: true });
  entries.forEach(entry => {
    console.log(entry.name, entry.isDirectory() ? '[DIR]' : '[FILE]');
  });

  // Remove empty directory
  await fs.rmdir('./output/reports');
}

dirOps();

4 Code Challenge

Challenge: Write a script that reads all .txt files from a directory, concatenates their contents, and writes the result to a single merged.txt output file.