Core Modules & Global Objects

fs - the File System Module

`fs` covers reading, writing, and watching files and directories. Nearly every function has three forms: an asynchronous callback version (`fs.readFile`), a synchronous version that blocks the event loop (`fs.readFileSync`), and, in modern Node.js, a Promise-based version under `fs.promises` (or `require('fs/promises')`). Reach for the sync versions only at startup/config-loading time, never inside a request handler, since they block the entire event loop while they run.

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

// Callback style
fs.readFile('config.json', 'utf8', (err, data) => {
  if (err) throw err;
  console.log('callback:', JSON.parse(data).name);
});

// Sync - blocks the event loop, fine at startup only
const raw = fs.readFileSync('config.json', 'utf8');
console.log('sync:', JSON.parse(raw).name);

// Promise-based - plays nicely with async/await
async function loadConfig() {
  const data = await fsPromises.readFile('config.json', 'utf8');
  console.log('promise:', JSON.parse(data).name);
}
loadConfig();

path - Working with File Paths Safely

`path` lets you build and manipulate file paths without hardcoding path separators, which differ between Windows (`\`) and POSIX systems (`/`). `path.join()` combines segments and normalizes the result, while `path.resolve()` produces an absolute path anchored at the current working directory (or an earlier absolute segment you pass in).

const path = require('path');

console.log(path.join('data', 'users', 'file.json'));
// 'data/users/file.json' (or 'data\\users\\file.json' on Windows)

console.log(path.resolve(__dirname, '..', 'config.json'));
// an absolute path, one directory above this file

console.log(path.extname('report.PDF')); // '.PDF'
console.log(path.basename('/a/b/c.txt')); // 'c.txt'

os and crypto

`os` reports information about the machine Node.js is running on - CPU count, total/free memory, platform, home directory - useful for scaling decisions (like sizing a `cluster` worker pool) or diagnostics. `crypto` provides hashing, HMACs, and random-byte generation built directly into Node.js, with no external dependency needed for common security primitives.

const crypto = require('crypto');

const hash = crypto.createHash('sha256').update('my-password').digest('hex');
console.log('sha256:', hash);

const token = crypto.randomBytes(16).toString('hex');
console.log('random token:', token);

The global Object and Avoiding Pollution

Node.js has a `global` object analogous to `window` in a browser - anything attached to it is reachable from every module without an explicit `require`. Built-ins like `console`, `process`, `setTimeout`, and `Buffer` are effectively available everywhere without importing them, but they are properties Node.js exposes deliberately, not things you should be adding to casually yourself.

Attaching your own app data to `global` (e.g. `global.currentUser = ...`) creates implicit, hard-to-trace coupling between unrelated files and risks name collisions with other modules or future Node.js globals. Prefer exporting values explicitly from a dedicated module (or a small dependency-injection pattern) so it is obvious where shared state comes from.