Node.js: Code Outside the Browser / Built in Modules

File System (fs module)

📦 The fs Module

fs (File System) is a built-in Node.js module for reading, writing, and managing files and folders. Every method below comes in two flavors — sync and async — depending on whether you want your code to wait or keep moving.

📝 Synchronous (Blocking)

Sync methods pause your code until the file operation finishes. Nothing else runs until the write or read completes — simple to reason about, but risky in apps that need to stay responsive.

const fs = require('fs');

// Write
fs.writeFileSync('note.txt', 'This is a sync write.');
console.log('✅ File written successfully!');

// Read
const data = fs.readFileSync('note.txt', 'utf8');
console.log('📄 File content:', data);

  • writeFileSync(path, content) creates the file if it doesn't exist, or overwrites it if it does.

  • readFileSync(path, 'utf8') returns the file's content as a plain string. Skip 'utf8' and you'd get raw binary data instead.

  • Because these are sync, console.log only runs after the operation finishes — no waiting or callback needed.

⏳ Asynchronous (Non-blocking)

Async methods use a callback (or promises) so your app doesn't freeze while the disk does its work. This matters most in servers, where blocking would stall every other request.

const fs = require('fs');
// Write
fs.writeFile('async.txt', 'This is an async write.', (err) => {
  if (err) throw err;
  console.log('✅ Async file written!');
});
// Read
fs.readFile('async.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log('📄 Async file content:', data);
});

  • writeFile and readFile return immediately — the rest of your script keeps running while the file operation happens in the background.

  • The callback (err, data) fires once the operation finishes. err holds an error if something went wrong; check it before using data.

  • Since these don't block, the order things print to the console can differ from the order you wrote the code in.

➕ Appending to Files

Adds content to the end of a file without wiping out what's already there — unlike writeFileSync, which overwrites everything.

fs.appendFileSync('note.txt', '\nThis is appended.');
console.log('📝 Content appended!');

File after append:

This is a sync write.
This is appended.

  • The \n adds a line break first, so the new text starts on its own line instead of running into the existing content.

  • If the file doesn't exist yet, appendFileSync creates it — same as writeFileSync.

🗑️ Deleting Files

fs.unlinkSync('note.txt');
console.log('🗑️ File deleted!');

⚠️ unlinkSync removes the file permanently — there's no trash or undo. If the file doesn't exist, this throws an error and crashes your script unless you handle it.

✅ Delete Safely

const fs = require('fs');
const filePath = 'note.txt';

if (fs.existsSync(filePath)) {
  fs.unlinkSync(filePath);
  console.log('🗑️ File deleted!');
} else {
  console.log('❌ File not found.');
}

  • existsSync(path) checks whether the file is there first, and returns true or false.

  • Wrapping the delete in that check means your code handles a missing file gracefully instead of throwing an error.

📁 Working with Folders

Starting point — just the project file itself, no myFolder yet:

project/
└── app.js

Step 1 — Create a folder:

fs.mkdirSync('myFolder');
console.log('📁 Folder created!');

mkdirSync makes a new, empty folder at the given path. It throws an error if a folder with that name already exists.

project/
├── app.js
└── myFolder/        ← new, empty

Step 2 — Create a file inside it:

fs.writeFileSync('myFolder/info.txt', 'Hello from inside the folder!');

Same writeFileSync as before — just pointed at a path inside the folder you just made.

project/
├── app.js
└── myFolder/
    └── info.txt      ← new

Step 3 — Clean up: delete the file, then the folder:

fs.unlinkSync('myFolder/info.txt'); // delete file first
fs.rmdirSync('myFolder');           // then delete the folder
console.log('🧹 Folder cleaned up!');

  • Folders must be empty before rmdirSync can remove them — that's why the file inside gets deleted first.

  • Trying to delete the folder while it still has files in it will throw an error.

project/
└── app.js            ← back to the starting point