Node.js

Core Modules

Module	Purpose
fs	File system operations (sync/async)
path	Path manipulation (join, resolve, extname)
http	HTTP server/client
url	URL parsing (parse, format)
os	OS info (platform, cpus, freemem)
events	EventEmitter pattern
crypto	Hashing, encryption
child_process	Spawn subprocesses (exec, spawn)
stream	Readable/Writable streams

const fs = require('fs');
const path = require('path');
const { EventEmitter } = require('events');

Async Patterns

Pattern	Example
Callback	fs.readFile('f.txt', (err, data) => {})
Promise	fs.promises.readFile('f.txt').then(...)
async/await	const data = await fs.promises.readFile('f.txt')
EventEmitter	emitter.on('event', fn) / emitter.emit('event')

// EventEmitter
const ee = new EventEmitter();
ee.on('data', (d) => console.log(d));
ee.emit('data', 'hello');

File System Operations

Operation	Sync	Async
Read file	fs.readFileSync(path)	fs.readFile(path, callback)
Write file	fs.writeFileSync(path, data)	fs.writeFile(path, data, callback)
Read dir	fs.readdirSync(path)	fs.readdir(path, callback)
Stats	fs.statSync(path)	fs.stat(path, callback)
Watch	—	fs.watch(path, (ev, name) => {})

// Streams
const rs = fs.createReadStream('in.txt');
const ws = fs.createWriteStream('out.txt');
rs.pipe(ws);

HTTP Server Basics

const http = require('http');
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello');
});
server.listen(3000);

Method	Purpose
req.url	Request path
req.method	GET, POST, etc.
res.writeHead(status, headers)	Set response headers
res.end(body)	Send and close

Process & Environment

Item	Purpose
process.env	Environment variables
process.argv	CLI args [node, script, ...]
process.exit(code)	Exit with code (0=ok)
process.cwd()	Current working directory
process.platform	'darwin', 'win32', etc.

const [,, arg1, arg2] = process.argv;
const port = process.env.PORT || 3000;

npm Commands

Command	Purpose
npm init	Create package.json
npm init -y	Init with defaults
npm install	Install deps
npm install pkg	Add dependency
npm install -D pkg	Add devDependency
npm run <script>	Run script from package.json
npm publish	Publish to registry
npm link	Link local package globally
npm outdated	Check outdated deps
npm audit	Security audit

package.json Fields

Field	Purpose
name	Package name
version	Semver (e.g. 1.0.0)
main	Entry point
scripts	npm run scripts
dependencies	Prod deps
devDependencies	Dev deps
type	"module" for ESM
exports	Conditional exports

Module System

Style	Syntax
CommonJS	const x = require('x') / module.exports = x
ES Modules	import x from 'x' / export default x

// CommonJS
module.exports = { foo, bar };
exports.foo = foo;

// ESM (package.json: "type": "module")
export default fn;
export { a, b };

Error Handling

// Callback: err first
fs.readFile('f.txt', (err, data) => {
  if (err) return console.error(err);
  console.log(data);
});

// async/await: try/catch
try {
  const data = await fs.promises.readFile('f.txt');
} catch (err) {
  console.error(err);
}

Useful Globals

Global	Purpose
Buffer	Binary data
__dirname	Dir of current module (CJS)
__filename	Path of current module (CJS)
setTimeout(fn, ms)	Run after delay
setInterval(fn, ms)	Run repeatedly
clearTimeout(id)	Cancel timeout
console	log, error, warn, etc.
global	Global object

// Buffer
Buffer.from('hello');
Buffer.alloc(10);

// __dirname in ESM: use import.meta.url
import { fileURLToPath } from 'url';
const __dirname = path.dirname(fileURLToPath(import.meta.url));