Node.js チートシート

# start the REPL (interactive shell)
node

# run a script
node app.js

# run with ES modules (type: module in package.json)
node --input-type=module app.js

# execute inline code
node -e "console.log(process.version)"

# watch mode (auto-restart on change)
node --watch app.js

# check version
node --version    # v20.11.0
node -p "process.versions.v8"

# enable warning stack traces
node --trace-warnings app.js

# pass NODE_OPTIONS (max ~128KB string)
NODE_OPTIONS="--max-old-space-size=4096" node app.js

# inspect memory usage at runtime
node --inspect app.js

# scaffold a project (answer prompts)
npm init

# non-interactive defaults
npm init -y

# "type" controls module system
# "commonjs" (default) -> require/module.exports
# "module"              -> import/export
{
  "name": "my-app",
  "version": "1.0.0",
  "type": "module",
  "main": "index.js",
  "scripts": { "start": "node index.js" }
}

# CommonJS (.cjs or type:commonjs) ----
const fs = require('fs');                 // import
module.exports = { greet: () => 'hi' };   // export

# ES Modules (.mjs or type:module) -------
import fs from 'fs';                      // import
export function greet() { return 'hi'; }  // named export
export default { greet };                 // default export

# .mjs is always ESM, .cjs is always CJS
node file.mjs   # forced ESM
node file.cjs   # forced CJS

# available everywhere, no import needed
global;        // global namespace object
process;       // env, argv, stdin/stdout
console;       // log, error, warn, table
setTimeout;    // schedule a callback
setInterval;   // schedule repeating callback
Buffer;        // binary data (Uint8Array)
queueMicrotask;// schedule microtask
structuredClone; // deep clone

# CommonJS-only globals (not in ESM)
__dirname;     // current directory
__filename;    // current file path

#!/usr/bin/env node
// cli.js - make a file directly runnable
const name = process.argv[2] || 'world';
console.log(`Hello, ${name}!`);

# make it executable (Unix) and run directly
chmod +x cli.js
./cli.js Alice        # Hello, Alice!

# or wire it up in package.json
{
  "bin": { "mycli": "./cli.js" }
}

// math.js
function add(a, b) { return a + b; }
const PI = 3.14;

// export via module.exports
module.exports = { add, PI };
// or: exports.add = add; exports.PI = PI;

// app.js
const math = require('./math');      // .js optional
console.log(math.add(1, 2));          // 3
console.log(math.PI);                 // 3.14

// destructuring
const { add } = require('./math');
console.log(add(5, 7));               // 12

// math.mjs (or type:module)
export function add(a, b) { return a + b; }
export const PI = 3.14;
export default function square(x) { return x * x; }

// app.mjs
import square, { add, PI } from './math.mjs';
console.log(add(1, 2));     // 3
console.log(square(5));     // 25

// import all as namespace
import * as math from './math.mjs';
console.log(math.PI);       // 3.14

// dynamic import returns a Promise
async function loadPlugin(name) {
  const mod = await import(`./plugins/${name}.mjs`);
  return mod.default;
}

// lazy-load heavy modules on demand
if (needPdf) {
  const { exportPdf } = await import('./pdf.mjs');
  exportPdf(data);
}

// works in both CommonJS and ESM
import('./optional.mjs')
  .then(mod => mod.run())
  .catch(err => console.error('load failed', err));

// core modules - no install, just require/import
const fs = require('fs');            // file system
const path = require('path');        // path handling
const http = require('http');        // HTTP server/client
const os = require('os');            // OS info
const crypto = require('crypto');    // hashing/encryption
const { EventEmitter } = require('events');
const { Buffer } = require('buffer');
const stream = require('stream');    // streams
const net = require('net');          // TCP
const child_process = require('child_process');

// some are promise-based (Node 10+)
const fsp = require('fs/promises');  // async/await fs

// modules are cached after first load
// a.js
require('./b');                 // b loads, sets module.exports
const b = require('./b');       // returns cached b (same object)
console.log(b === require('./b')); // true

// circular dependency: a -> b -> a
// a.js: const b = require('./b'); b.f();
// b.js: const a = require('./a'); // a is PARTIAL here!
//       module.exports = { f: () => a.x };

// inspect the cache
console.log(require.cache);     // map of loaded modules
delete require.cache[require.resolve('./b')]; // force reload

// CommonJS - synchronous path globals
console.log(__dirname);   // /app/src
console.log(__filename);  // /app/src/index.js

// ES Modules - no __dirname, use import.meta
import { fileURLToPath } from 'url';
import { dirname, join } from 'path';

const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);
const dataPath = join(__dirname, 'data.json');

// import.meta also carries the module URL
console.log(import.meta.url);  // file:///app/src/index.mjs

# create package.json interactively
npm init

# accept all defaults
npm init -y

# pnpm equivalent
pnpm init

# set author/license globally
npm set init-author-name "Alice"
npm set init-license MIT

# scope packages with @username
npm init --scope=@myorg

# install everything in package.json
npm install            # alias: npm i
pnpm install           # alias: pnpm i

# add a runtime dependency -> dependencies
npm install express
pnpm add express

# add a dev-only dependency -> devDependencies
npm install --save-dev jest     # npm i -D jest
pnpm add -D jest

# install a specific version
npm install [email protected]
npm install lodash@^4.17.0      # caret: 4.x
npm install lodash@~4.17.0      # tilde: 4.17.x

# global install (CLI tools)
npm install -g pm2
pnpm add -g pm2

// package.json
{
  "scripts": {
    "start": "node index.js",
    "dev": "node --watch index.js",
    "test": "jest",
    "build": "tsc && vite build"
  }
}

# run a script
npm run dev          # npm run-script dev
pnpm dev             # pnpm has no "run" requirement

# "start" and "test" have shortcuts
npm start
npm test

# pre/post hooks run automatically
// "pretest": "npm run lint",
// "postbuild": "npm run deploy"

# pass extra args with --
npm run build -- --mode production

# run a one-off command without installing globally
npx create-vite my-app
npx cowsay "hello"

# pin a version
npx prettier@3 --write .

# pnpm equivalent
pnpm dlx create-vite my-app

# execute a local bin from node_modules
npx jest            # runs ./node_modules/.bin/jest

# never prompt, always download latest
npx --yes create-react-app my-app

# semantic versioning: MAJOR.MINOR.PATCH
# 1.4.2 -> 1:breaking 4:feature 2:fix
"^1.4.2"  # >=1.4.2 <2.0.0  (caret, default)
"~1.4.2"  # >=1.4.2 <1.5.0  (tilde)
"1.4.2"   # exact 1.4.2
">=1.4.0 <2.0.0" # range

# lockfiles pin the installed tree
npm install       # writes/updates package-lock.json
pnpm install      # writes pnpm-lock.yaml
yarn install      # writes yarn.lock

# install exactly as locked (CI)
npm ci            # clean install from lockfile

# find outdated deps
npm outdated
npm update        # update within ^ ranges

# login once
npm login

# publish to the registry
npm publish
npm publish --access public   # scoped default restricted

# bump version (updates package.json + tag + commit)
npm version patch   # 1.0.0 -> 1.0.1
npm version minor   # 1.0.0 -> 1.1.0
npm version major   # 1.0.0 -> 2.0.0

# lifecycle scripts (in package.json)
// "prepublishOnly": "npm test && npm run build",
// "prepublish": "npm run build",
// "postpublish": "git push --tags"

# unpublish within 72h
npm unpublish [email protected]

const fs = require('fs');

// synchronous (blocks the event loop)
const text = fs.readFileSync('data.txt', 'utf8');
const buf = fs.readFileSync('image.png');  // Buffer

// asynchronous with callback (error-first)
fs.readFile('data.txt', 'utf8', (err, data) => {
  if (err) return console.error(err);
  console.log(data);
});

// async with fs/promises (preferred)
const fsp = require('fs/promises');
async function read() {
  const text = await fsp.readFile('data.txt', 'utf8');
  return text;
}

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

// sync
fs.writeFileSync('out.txt', 'hello');

// async callback
fs.writeFile('log.txt', 'line1\n', err => {
  if (err) throw err;
});

// promise
await fsp.writeFile('config.json', JSON.stringify(cfg, null, 2));

// flags: 'w' write (default), 'a' append, 'wx' fail if exists
await fsp.writeFile('new.txt', 'x', { flag: 'wx' }); // throws if exists

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

// create (recursive like mkdir -p)
await fsp.mkdir('a/b/c', { recursive: true });

// list entries
const entries = await fsp.readdir('.');
// ['app.js', 'data', 'package.json']

// list with type info
const detailed = await fsp.readdir('.', { withFileTypes: true });
detailed.forEach(e => console.log(e.name, e.isDirectory()));

// remove
await fsp.rmdir('empty');             // empty dir only
await fsp.rm('folder', { recursive: true, force: true }); // rm -rf

// sync versions exist too: mkdirSync, readdirSync, rmSync

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

const stats = await fsp.stat('data.txt');
stats.isFile();          // true
stats.isDirectory();     // false
stats.size;              // bytes
stats.mtime;             // Date modified
stats.birthtime;         // Date created
stats.mode;              // permission bits

// check existence (avoid existsSync for races)
try {
  await fsp.access('config.json');
  console.log('exists');
} catch {
  console.log('missing');
}

// watch for changes
const watcher = fs.watch('.');
watcher.on('change', (eventType, filename) => {});

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

// full async/await file operations
async function loadConfig() {
  const file = path.join(__dirname, 'config.json');
  const raw = await fsp.readFile(file, 'utf8');
  return JSON.parse(raw);
}

async function saveConfig(cfg) {
  const file = path.join(__dirname, 'config.json');
  await fsp.writeFile(file, JSON.stringify(cfg, null, 2));
}

// iterate directory tree
for await (const entry of await fsp.opendir('.')) {
  console.log(entry.name);
}

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

// append to a file (creates if missing)
await fsp.appendFile('log.txt', new Date() + ' started\n');

// atomic rename (also for moving)
await fsp.rename('old.txt', 'new.txt');
// move across dirs
await fsp.rename('a/x.txt', 'b/x.txt');

// delete a file
await fsp.unlink('temp.txt');

// copy
await fsp.copyFile('src.txt', 'dest.txt');
// or fsp.cp('src', 'dest', { recursive: true })

// truncate to a length
await fsp.truncate('data.bin', 1024);

const path = require('path');

// join segments with the OS separator
path.join('src', 'utils', 'file.js');
// 'src/utils/file.js' (Linux)  or 'src\utils\file.js' (Windows)

// resolve to an ABSOLUTE path (from cwd)
path.resolve('src', 'file.js');
// '/current/dir/src/file.js'

// resolve with absolute segment resets
path.resolve('/a', 'b', '/c', 'd');   // '/c/d'

// normalize '..' and '.' segments
path.normalize('a/b/../c/./d');        // 'a/c/d'

// never hardcode '/' — use path.join for portability

const path = require('path');
const file = '/app/src/utils/helper.js';

path.dirname(file);    // '/app/src/utils'
path.basename(file);   // 'helper.js'
path.basename(file, '.js'); // 'helper' (strip ext)
path.extname(file);    // '.js'
path.extname('archive.tar.gz'); // '.gz' (last ext)

// for the current file (CommonJS)
path.dirname(__filename);  // current dir
path.basename(__filename); // current file name

const path = require('path');

// decompose a path into parts
const p = path.parse('/app/src/helper.test.js');
// { root: '/', dir: '/app/src',
//   base: 'helper.test.js', name: 'helper.test', ext: '.js' }

// rebuild from a parsed object
const rebuilt = path.format({
  dir: '/app/dist',
  name: 'bundle',
  ext: '.min.js'
});
// '/app/dist/bundle.min.js'

// get the relative path from A to B
path.relative('/a/b/c', '/a/x');   // '../../x'

const path = require('path');

// platform-specific values
path.sep;           // '/' (POSIX) or '\\' (Windows)
path.delimiter;     // ':' (POSIX) or ';' (Windows)

// force a specific platform's behavior
path.win32.join('a', 'b');   // 'a\\b'
path.posix.join('a', 'b');   // 'a/b'

// parse PATH env var portably
process.env.PATH.split(path.delimiter).forEach(dir => {
  console.log(dir);
});

// check if absolute
path.isAbsolute('/etc');     // true
path.isAbsolute('a/b');      // false
path.win32.isAbsolute('C:\\'); // true

const path = require('path');
const url = require('url');

// convert a path to a file:// URL (ESM import)
const fileUrl = url.pathToFileURL('/app/src/mod.mjs');
// URL { href: 'file:///app/src/mod.mjs' }

// and back
const p = url.fileURLToPath('file:///app/src/mod.mjs');
// '/app/src/mod.mjs'

// in ESM, import.meta.url is already a file URL
// convert to a normal path:
import { fileURLToPath } from 'url';
const __filename = fileURLToPath(import.meta.url);

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello, World!\n');
});

// listen on a port
server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

// or with a host and backlog
server.listen(3000, '127.0.0.1', 511, () => {});

const http = require('http');

http.createServer((req, res) => {
  // request info
  console.log(req.method);   // 'GET'
  console.log(req.url);      // '/users?id=5'
  console.log(req.headers);  // { host, 'user-agent', ... }

  // response helpers
  res.statusCode = 200;
  res.setHeader('Content-Type', 'application/json');
  res.write('partial chunk\n');
  res.end(JSON.stringify({ ok: true }));
}).listen(3000);

// streaming a large response
res.writeHead(200);
readStream.pipe(res);   // pipe file -> response

const http = require('http');
const { URL } = require('url');

http.createServer((req, res) => {
  const url = new URL(req.url, 'http://localhost');
  const path = url.pathname;
  const q = url.searchParams.get('id');

  if (req.method === 'GET' && path === '/') {
    res.end('home');
  } else if (path === '/users') {
    res.end('users');
  } else {
    res.writeHead(404);
    res.end('not found');
  }
}).listen(3000);

const http = require('http');

http.createServer(async (req, res) => {
  if (req.method === 'POST') {
    // collect the streamed body
    const chunks = [];
    for await (const chunk of req) chunks.push(chunk);
    const body = JSON.parse(Buffer.concat(chunks).toString());

    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify({ received: body }));
  }
}).listen(3000);

// test: curl -X POST localhost:3000 -d '{"a":1}'

const server = http.createServer(handler);

server.on('listening', () => console.log('ready'));
server.on('connection', socket => console.log('client connected'));
server.on('error', err => {
  if (err.code === 'EADDRINUSE') {
    console.error('port 3000 already in use');
  }
});
server.on('close', () => console.log('server closed'));

// graceful shutdown
process.on('SIGTERM', () => {
  server.close(() => process.exit(0));
});

server.listen(3000);

// HTTPS server (needs TLS certs)
const https = require('https');
const fs = require('fs');

https.createServer({
  key: fs.readFileSync('key.pem'),
  cert: fs.readFileSync('cert.pem')
}, handler).listen(443);

// HTTP client: simple GET
http.get('http://example.com', res => {
  let data = '';
  res.on('data', c => data += c);
  res.on('end', () => console.log(data));
});

// modern fetch (Node 18+, global, promise-based)
const res = await fetch('https://api.example.com');
const json = await res.json();

// install: npm install express
import express from 'express';

const app = express();
const PORT = 3000;

app.get('/', (req, res) => {
  res.send('Hello, World!');
});

app.listen(PORT, () => {
  console.log(`Server on http://localhost:${PORT}`);
});

app.get('/users', (req, res) => res.send('list users'));
app.post('/users', (req, res) => res.send('create user'));

// URL parameters
app.get('/users/:id', (req, res) => {
  res.send(`user ${req.params.id}`);
});

// query strings: /search?q=node
app.get('/search', (req, res) => {
  res.send(`searching: ${req.query.q}`);
});

// chain handlers for one path
app.route('/book')
  .get((req, res) => res.send('get book'))
  .post((req, res) => res.send('add book'));

// middleware: (req, res, next) => {}
app.use(express.json());          // parse JSON bodies
app.use(express.static('public'));// serve static files
app.use((req, res, next) => {     // custom logger
  console.log(req.method, req.url);
  next();                         // pass control forward
});

// mounted on a path
app.use('/api', (req, res, next) => {
  req.apiCall = true;
  next();
});

// error-handling middleware (4 args)
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Server Error');
});

// JSON body (needs express.json() middleware)
app.post('/api', (req, res) => {
  console.log(req.body);          // parsed JSON object
});

// URL params and query
app.get('/u/:id', (req, res) => {
  const { id } = req.params;
  const { tab } = req.query;
});

// form data (urlencoded)
app.use(express.urlencoded({ extended: true }));
app.post('/form', (req, res) => res.json(req.body));

// file uploads: use multer middleware
// const upload = multer({ dest: 'uploads/' });
// app.post('/upload', upload.single('file'), handler);

// headers & cookies
req.headers['user-agent'];
req.cookies;        // needs cookie-parser

const path = require('path');

// serve files from /public at /
app.use(express.static('public'));
// GET /style.css -> public/style.css

// mount at a prefix
app.use('/static', express.static('public'));
// GET /static/style.css -> public/style.css

// multiple directories (fallthrough)
app.use(express.static('public'));
app.use(express.static('uploads'));

// absolute path + options
app.use(express.static(path.join(__dirname, 'public'), {
  maxAge: '1d',
  setHeaders: (res, filePath) => {
    if (filePath.endsWith('.html')) res.setHeader('Cache-Control', 'no-cache');
  }
}));

// throw to jump to error middleware
app.get('/broken', (req, res, next) => {
  const err = new Error('boom');
  err.status = 400;
  next(err);              // or throw err (Express 4+)
});

// centralized error handler (last middleware)
app.use((err, req, res, next) => {
  const status = err.status || 500;
  res.status(status).json({ error: err.message });
});

// modular routing with express.Router()
const router = express.Router();
router.get('/', (req, res) => res.send('api root'));
router.get('/users', (req, res) => res.send('users'));
app.use('/api', router);  // mount at /api

const fs = require('fs');

// create a readable stream from a file
const rs = fs.createReadStream('big.log', { highWaterMark: 64 * 1024 });

// 'data' mode (flowing)
rs.on('data', chunk => {
  console.log('got', chunk.length, 'bytes');
});
rs.on('end', () => console.log('done'));
rs.on('error', err => console.error(err));

// paused mode: pull on demand
rs.on('readable', () => {
  let chunk;
  while ((chunk = rs.read()) !== null) {
    process(chunk);
  }
});

const fs = require('fs');
const ws = fs.createWriteStream('out.log');

// write returns false when the buffer is full (backpressure)
const ok = ws.write('line\n');
if (!ok) ws.once('drain', () => writeMore());

// signals completion
ws.end('final line\n');
ws.on('finish', () => console.log('all data flushed'));
ws.on('error', err => console.error(err));

// process.stdout is a writable stream
process.stdout.write('hello');

const fs = require('fs');
const { pipeline } = require('stream/promises');

// pipe: readable -> writable (auto backpressure)
fs.createReadStream('in.txt')
  .pipe(fs.createWriteStream('out.txt'));

// pipeline: chain transforms, with error handling
const { Transform } = require('stream');
const upper = new Transform({
  transform(chunk, enc, cb) { cb(null, chunk.toString().toUpperCase()); }
});

await pipeline(
  fs.createReadStream('in.txt'),
  upper,
  fs.createWriteStream('out.txt')
);
// pipeline rejects on error, unlike .pipe()

const { Transform } = require('stream');

// a Transform is both Readable and Writable
const csvToJson = new Transform({
  objectMode: false,
  transform(chunk, encoding, callback) {
    // convert each chunk
    const lines = chunk.toString().trim().split('\n');
    const json = lines.map(l => JSON.stringify(l.split(','))).join('\n');
    callback(null, json + '\n');
  },
  flush(callback) {
    callback(null, ']\n'); // finalize
  }
});

process.stdin.pipe(csvToJson).pipe(process.stdout);

// built-in transforms: zlib, crypto streams
const { createGzip } = require('zlib');
fs.createReadStream('log').pipe(createGzip()).pipe(fs.createWriteStream('log.gz'));

const rs = fs.createReadStream('huge.bin');

// switch between modes
rs.pause();
rs.resume();
rs.setEncoding('utf8');

// respect backpressure manually
function writeChunk(ws, data) {
  if (!ws.write(data)) {
    rs.pause();
    ws.once('drain', () => rs.resume());
  }
}

// async iteration (Node 10+) — cleanest pattern
async function consume() {
  for await (const chunk of rs) {
    await handle(chunk);  // naturally respects backpressure
  }
}
consume();

const { Readable, Writable, Duplex } = require('stream');

// custom Readable: generate a counter
const counter = new Readable({
  read() {
    if (++this._n > 10) this.push(null);  // end
    else this.push(String(this._n));
  }
});
counter._n = 0;
counter.pipe(process.stdout);

// from an array / async iterable
Readable.from(['a', 'b', 'c']).pipe(process.stdout);

// a Duplex: both readable and writable (e.g. a socket)
class Echo extends Duplex {
  _read() {}
  _write(chunk, enc, cb) { this.push(chunk); cb(); }
}

// allocate zeroed bytes (SAFE)
const b1 = Buffer.alloc(8);            // <Buffer 00 00 00 00 00 00 00 00>

// allocate WITHOUT zeroing (faster, may contain old memory)
const b2 = Buffer.allocUnsafe(8);      // random old data!

// from a string with encoding
const b3 = Buffer.from('hello', 'utf8');
// <Buffer 68 65 6c 6c 6f>

// from an array of bytes
const b4 = Buffer.from([0x48, 0x49]);  // 'HI'

// from another buffer (copy)
const b5 = Buffer.from(b3);

// NEVER use new Buffer() — deprecated and unsafe

const buf = Buffer.alloc(8);

// write numeric values at an offset
buf.writeUInt8(255, 0);       // 1 byte
buf.writeUInt16BE(0x1234, 1); // 2 bytes, big-endian
buf.writeInt32LE(1000, 3);    // 4 bytes, little-endian

// read them back
buf.readUInt8(0);             // 255
buf.readUInt16BE(1);          // 0x1234
buf.readInt32LE(3);           // 1000

// string read/write
buf.write('Hi', 0, 'utf8');
buf.toString('utf8', 0, 2);   // 'Hi'

const a = Buffer.from('foo');
const b = Buffer.from('bar');

// concatenate buffers
const c = Buffer.concat([a, b]);   // 'foobar'
Buffer.concat([a, b], 6);          // total length hint

// compare
Buffer.compare(a, b);        // -1 (a < b), 0, or 1
a.equals(b);                  // false

// find a sub-buffer
const big = Buffer.from('hello world');
big.indexOf(Buffer.from('world'));   // 6
big.includes(Buffer.from('hello'));  // true

// copy bytes
const target = Buffer.alloc(5);
big.copy(target, 0, 0, 5);   // copy first 5 bytes

const text = 'Hello, 世界';

// string -> buffer (encode)
const utf8 = Buffer.from(text, 'utf8');
const base64 = Buffer.from(text).toString('base64');
const hex = Buffer.from(text).toString('hex');
const latin1 = Buffer.from(text, 'latin1');

// buffer -> string (decode)
utf8.toString('utf8');     // 'Hello, 世界'
Buffer.from(base64, 'base64').toString('utf8');
Buffer.from(hex, 'hex').toString('utf8');

// common encodings: utf8, utf16le, latin1, ascii,
//   base64, base64url, hex
Buffer.byteLength('世界', 'utf8');  // 6 bytes (not 2)

// Buffer is a subclass of Uint8Array
const buf = Buffer.from([1, 2, 3]);
buf instanceof Uint8Array;  // true

// share memory with TypedArrays without copying
const u8 = new Uint8Array(buf.buffer, buf.byteOffset, buf.byteLength);
const view = new DataView(buf.buffer, buf.byteOffset, buf.byteLength);
view.getInt32(0);

// Buffer from an ArrayBuffer
const ab = new ArrayBuffer(8);
const buf2 = Buffer.from(ab);

// note: a Buffer's underlying buffer may be larger
// (allocUnsafe pools) -> always use byteOffset

const buf = Buffer.from('abcdef');

// slice shares memory (no copy) — like subarray
const s = buf.subarray(1, 4);   // 'bcd'
s[0] = 88;                      // mutates buf too -> 'aXcdef'

// fill with a value or pattern
Buffer.alloc(4).fill(0xff);            // <Buffer ff ff ff ff>
Buffer.alloc(6).fill('ab');            // 'ababab'

// reverse byte order in place (for endianness swaps)
const n = Buffer.from([0x01, 0x02, 0x03, 0x04]);
n.swap16();  // swaps pairs: <Buffer 02 01 04 03>

// iterate with for...of
for (const byte of buf) console.log(byte);  // 97, 98, ...

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

class Clock extends EventEmitter {
  start() {
    this.emit('start', Date.now());          // emit with args
    setInterval(() => this.emit('tick', new Date()), 1000);
  }
}

const clock = new Clock();
clock.on('tick', date => console.log('tick', date));
clock.on('start', ts => console.log('started at', ts));
clock.start();

// on() == addListener; once() fires only once

const ee = new EventEmitter();

// once: auto-removed after first fire
ee.once('init', () => console.log('init fired'));

// remove a specific listener (need the same function ref)
const handler = () => console.log('hi');
ee.on('msg', handler);
ee.off('msg', handler);          // off() == removeListener

// remove all listeners for an event (or all events)
ee.removeAllListeners('msg');
ee.removeAllListeners();         // everything

// inspect
ee.listenerCount('msg');
ee.eventNames();                 // ['msg', 'init']

const ee = new EventEmitter();

// 'error' is special: if unhandled, it CRASHES the process
ee.on('error', err => {
  console.error('caught:', err.message);
});

// later, emit an error safely
ee.emit('error', new Error('something broke'));

// guard against crashes from unhandled errors
// (monitor with the static capture)
EventEmitter.captureRejections = true; // for async handlers

// always register an error listener on streams/http

const ee = new EventEmitter();

// listeners run synchronously; await doesn't pause the chain
ee.on('save', async () => {
  await fs.promises.writeFile('x', 'y');
  console.log('saved');
});
ee.emit('save');  // returns before the await finishes

// wait for async listeners in sequence
const { once } = require('events');
async function waitForReady() {
  await once(ee, 'ready');   // promise resolves on first 'ready'
  console.log('now ready');
}

// handle async errors properly
EventEmitter.captureRejections = true;
ee.on('job', async () => { throw new Error('boom'); });
ee.on('error', err => console.error(err));

const ee = new EventEmitter();

// special events fired by EventEmitter itself
ee.on('newListener', (event, listener) => {
  console.log('listener added for', event);
});
ee.on('removeListener', (event, listener) => {
  console.log('listener removed for', event);
});

// limit listener count to catch leaks
ee.setMaxListeners(20);
ee.getMaxListeners();          // 20
// MaxListenersExceededWarning fires if exceeded

// convention: camelCase event names, 'error' reserved
// prefer symbols/const strings to avoid typos
const READY = Symbol('ready');
ee.emit(READY);

const { exec } = require('child_process');

// runs a command in a shell, buffers all output
exec('ls -la', (err, stdout, stderr) => {
  if (err) { console.error('failed:', err); return; }
  console.log(stdout);    // full stdout as a string
  console.error(stderr);
});

// promisified (Node 12+)
const { promisify } = require('util');
const execP = promisify(exec);
const { stdout } = await execP('git rev-parse HEAD');

// WARNING: shell interpolation = injection risk!
// NEVER: exec(`rm ${userInput}`)

const { spawn } = require('child_process');

// no shell by default -> safe, stream-based
const child = spawn('ls', ['-la', '/tmp']);

child.stdout.on('data', chunk => process.stdout.write(chunk));
child.stderr.on('data', chunk => process.stderr.write(chunk));
child.on('close', code => console.log('exit code', code));
child.on('error', err => console.error('failed to spawn', err));

// pipe streams directly
const gzip = spawn('gzip');
process.stdin.pipe(gzip.stdin);
gzip.stdout.pipe(process.stdout);

const { execFile, fork } = require('child_process');

// execFile: like exec but no shell (safer, slight overhead saving)
execFile('node', ['--version'], (err, stdout) => {
  console.log(stdout.trim());   // v20.x.x
});

// fork: spawn another Node process with an IPC channel
const worker = fork('./worker.js');
worker.send({ task: 'compute', n: 100 });
worker.on('message', msg => console.log('result:', msg));
worker.on('exit', code => console.log('worker exited', code));

// in worker.js:
// process.on('message', msg => { process.send(result); });

const { spawn } = require('child_process');

// control stdio: 'pipe' (default), 'inherit', 'ignore', or fd
const child = spawn('npm', ['test'], {
  stdio: 'inherit'     // child uses parent's stdin/stdout/stderr
});

// per-stream: [stdin, stdout, stderr]
spawn('cat', [], { stdio: ['pipe', process.stdout, 'ignore'] });

// set env, cwd, and detached background process
const bg = spawn('node', ['server.js'], {
  cwd: '/app',
  env: { ...process.env, NODE_ENV: 'production' },
  detached: true          // runs independently of parent
});
bg.unref();               // let parent exit without waiting

// pass arguments safely as array (no shell parsing)
spawn('echo', ['hello world']);  // one arg, no quotes needed

const child = spawn('sleep', ['60']);

// 'exit' fires with code + signal
child.on('exit', (code, signal) => {
  if (code === 0) console.log('success');
  else if (signal) console.log('killed by', signal);
  else console.log('failed with code', code);
});

// send a signal to the child (SIGTERM by default)
setTimeout(() => child.kill('SIGTERM'), 1000);
child.kill();             // default SIGTERM
child.kill('SIGKILL');    // force-kill (cannot be caught)

// 'close' vs 'exit': 'close' fires after streams are closed
child.on('close', code => console.log('streams closed', code));

// check if process is still running
child.killed;             // true after kill() called

const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isPrimary) {            // old: isMaster
  // fork one worker per CPU core
  for (let i = 0; i < os.cpus().length; i++) cluster.fork();

  cluster.on('exit', (worker, code) => {
    console.log(`worker ${worker.process.pid} died`);
    cluster.fork();                 // restart it
  });
} else {
  // workers share the same port
  http.createServer((req, res) => res.end('hi'))
    .listen(3000);
  console.log('worker started', process.pid);
}

cluster.on('online', worker => {
  console.log('worker responsive', worker.id);
});

cluster.on('listening', (worker, address) => {
  console.log('worker bound to', address.port);
});

cluster.on('disconnect', worker => {
  console.log('worker disconnected', worker.id);
});

// gracefully shut down a worker
const w = cluster.workers[1];
w.disconnect();            // stop accepting connections, then exit
w.send('shutdown');        // custom message

// iterate all workers
for (const id in cluster.workers) {
  cluster.workers[id].send({ cmd: 'reload' });
}

const cluster = require('cluster');
const http = require('http');

if (cluster.isPrimary) {
  cluster.schedulingPolicy = cluster.SCHED_RR;  // round-robin (default on non-Windows)
  cluster.fork();
  cluster.fork();
} else {
  // all workers call listen(3000) — primary load-balances
  http.createServer((req, res) => {
    res.end(`handled by ${process.pid}`);
  }).listen(3000);
}

// SCHED_RR: primary distributes connections (default)
// SCHED_NONE: OS balances (may be uneven)

// primary -> worker messaging
if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.on('message', msg => console.log('from worker:', msg));
  worker.send({ task: 'heavy-compute' });
} else {
  process.on('message', msg => {
    console.log('from primary:', msg);
    // do work, then report back
    process.send({ done: true, result: 42 });
  });
}

// graceful shutdown signal
process.on('message', msg => {
  if (msg === 'shutdown') {
    server.close(() => process.exit(0));
  }
});

// rolling restart: reload workers one at a time
function reload() {
  const workers = Object.values(cluster.workers || {});
  let i = 0;
  const next = () => {
    if (i >= workers.length) return;
    const w = workers[i++];
    w.send('shutdown');                 // tell worker to drain
    w.disconnect();                     // stop accepting connections
    w.once('exit', () => {
      cluster.fork();                   // replacement ready
      next();                           // move to the next
    });
  };
  next();
}

// trigger reload on SIGHUP
process.on('SIGHUP', reload);

// in the worker: finish in-flight requests then exit
process.on('message', msg => {
  if (msg === 'shutdown') server.close(() => process.exit(0));
});

// Node callbacks: (err, result) => {}
const fs = require('fs');

fs.readFile('data.txt', 'utf8', (err, data) => {
  if (err) return console.error('read failed', err);
  console.log(data);
});

// custom error-first function
function divide(a, b, cb) {
  if (b === 0) return cb(new Error('divide by zero'));
  cb(null, a / b);
}

divide(10, 2, (err, result) => {
  if (err) return console.error(err);
  console.log(result);   // 5
});

// create a promise
const p = new Promise((resolve, reject) => {
  setTimeout(() => resolve('done'), 100);
});

// consume with then/catch/finally
p.then(val => console.log(val))      // 'done'
 .catch(err => console.error(err))
 .finally(() => console.log('settled'));

// chain transforms
fetch(url)
  .then(r => r.json())
  .then(data => render(data));

// convert callback APIs
const { promisify } = require('util');
const readFile = promisify(fs.readFile);
const text = await readFile('x.txt', 'utf8');

// async functions always return a Promise
async function loadUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error('not found');
  return res.json();
}

// consume with try/catch
try {
  const user = await loadUser(5);
  console.log(user);
} catch (err) {
  console.error('failed:', err);
}

// top-level await works in ESM and CommonJS (Node 14.8+)
const config = await loadConfig();

// all: run in parallel, reject fast on first failure
const [a, b, c] = await Promise.all([
  fetch('/a').then(r => r.json()),
  fetch('/b').then(r => r.json()),
  fetch('/c').then(r => r.json())
]);

// allSettled: wait for ALL, never rejects
const results = await Promise.allSettled([
  fetch('/x'),
  fetch('/y'),
  fetch('/z')
]);
results.forEach(r => {
  if (r.status === 'fulfilled') console.log(r.value);
  else console.error(r.reason);   // rejected reason
});

// race: first to SETTLE (resolve or reject) wins
const fastest = await Promise.race([
  fetch('/primary'),
  fetch('/mirror'),
  timeout(5000)
]);

// any: first to FULFILL wins; rejects only if ALL reject
const alive = await Promise.any([
  fetch('/node1'),
  fetch('/node2'),
  fetch('/node3')
]);

// implement a timeout with race
function withTimeout(p, ms) {
  return Promise.race([
    p,
    new Promise((_, reject) =>
      setTimeout(() => reject(new Error('timeout')), ms))
  ]);
}

// SEQUENTIAL: one after another (slow)
const results = [];
for (const url of urls) {
  results.push(await fetch(url).then(r => r.json()));
}

// PARALLEL: fire all, await together (fast)
const results = await Promise.all(
  urls.map(u => fetch(u).then(r => r.json()))
);

// THROTTLED: limit concurrency with a pool
async function mapLimit(items, limit, fn) {
  const ret = [];
  const executing = [];
  for (const item of items) {
    const p = Promise.resolve().then(() => fn(item));
    ret.push(p);
    if (executing.push(p) >= limit) {
      await Promise.race(executing);
      executing.splice(executing.findIndex(x => x === p), 1);
    }
  }
  return Promise.all(ret);
}

function parse(str) {
  try {
    return JSON.parse(str);
  } catch (err) {
    console.error('parse failed:', err.message);
    return null;
  } finally {
    console.log('cleanup always runs');
  }
}

// async errors need async catch
async function load() {
  try {
    const data = await fetchData();
    return data;
  } catch (err) {
    if (err.code === 'ENOENT') return defaultData();
    throw err;              // re-throw unknown errors
  }
}

const fs = require('fs');

// ALWAYS check err first, return early
fs.readFile('missing.txt', 'utf8', (err, data) => {
  if (err) {
    console.error('could not read:', err.message);
    return;            // don't use data
  }
  console.log(data);
});

// propagate the error to your own caller
function readConfig(cb) {
  fs.readFile('config.json', 'utf8', (err, raw) => {
    if (err) return cb(err);
    try { cb(null, JSON.parse(raw)); }
    catch (e) { cb(e); }
  });
}

// last-resort handler for sync exceptions
process.on('uncaughtException', err => {
  console.error('UNCAUGHT:', err);
  // the process is in an unknown state — restart it
  server.close(() => process.exit(1));
});

// rejected promises nobody awaited
process.on('unhandledRejection', (reason, promise) => {
  console.error('UNHANDLED REJECTION:', reason);
});

// Node 15+: unhandled rejections terminate the process
// always add .catch() or wrap in try/await

// subclass Error to add context
class ValidationError extends Error {
  constructor(field, message) {
    super(message);
    this.name = 'ValidationError';
    this.field = field;
    this.code = 'VALIDATION_FAILED';
  }
}

function validate(email) {
  if (!email.includes('@')) {
    throw new ValidationError('email', 'invalid email');
  }
}

// branch on error type with instanceof
try {
  validate('no-at-sign');
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(err.field, err.message);
  } else {
    throw err;
  }
}

// built-in Error props
const e = new Error('something broke');
e.message;    // 'something broke'
e.stack;      // stack trace string
e.name;       // 'Error'

// add a code for machine-readable handling
const err = new Error('file missing');
err.code = 'ENOENT';
err.errno = -2;

// Error.cause (Node 16.9+) — chain underlying errors
try {
  JSON.parse(bad);
} catch (original) {
  throw new Error('config parse failed', { cause: original });
}

// inspect the cause
console.log(err.cause);            // the original SyntaxError

// modern cleanup: 'beforeExit' for async, 'exit' for sync
process.on('beforeExit', async () => {
  await closeDbConnections();
});

process.on('exit', code => {
  // ONLY sync code allowed here — no async, no promises
  console.log('exiting with', code);
});

// graceful signal handling
const shutdown = sig => {
  console.log('got', sig);
  server.close(() => process.exit(0));
  setTimeout(() => process.exit(1), 5000).unref(); // force exit
};
process.on('SIGTERM', () => shutdown('SIGTERM'));
process.on('SIGINT', () => shutdown('SIGINT'));

console.log('basic');              // stdout
console.error('oops');           // stderr
console.warn('careful');         // stderr

// formatted table
console.table([{ id: 1, n: 'A' }, { id: 2, n: 'B' }]);

// inspect an object in depth
console.dir(obj, { depth: null, colors: true });

// timing a block
console.time('loop');
for (let i = 0; i < 1e6; i++) {}
console.timeEnd('loop');         // loop: 3.2ms

// stack trace
console.trace('where am I');

// count calls
for (let i = 0; i < 3; i++) console.count('hit');

# open Chrome DevTools debugger
node --inspect app.js
# visit chrome://inspect, click "inspect"

# pause on first line (break before anything runs)
node --inspect-brk app.js

# wait for a debugger client to attach
node --inspect-wait app.js

# the legacy CLI debugger (still works, no browser)
node inspect app.js
# > cont, step, next, breakpoints, repl

function findBug(arr) {
  let sum = 0;
  for (const n of arr) {
    debugger;        // pauses only when a debugger is attached
    sum += n;
  }
  return sum;
}

// run with --inspect-brk to hit the breakpoint
// node --inspect-brk app.js

// conditional breakpoints in DevTools/VSCode:
//   if (n > 100) { debugger; }

// .vscode/launch.json
{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Debug App",
      "program": "${workspaceFolder}/app.js",
      "skipFiles": ["<node_internals>/**"]
    },
    {
      "type": "node",
      "request": "attach",
      "name": "Attach by PID",
      "processId": "${command:PickProcess}"
    }
  ]
}

// F5 to start; set breakpoints in the editor
// "restart": true, "nodemon" for auto-restart on change

# show stack traces for process warnings
node --trace-warnings app.js

# deprecation warnings (use --throw-deprecation to make them errors)
node --trace-deprecation app.js

# capture a stack trace manually
const e = new Error('here');
console.log(e.stack);

# show async stack traces (Node 12+ enabled by default)
Error.stackTraceLimit = 50;   // deeper traces

# profile CPU usage
node --cpu-prof app.js        # writes CPU.*.cpuprofile
# then load the file in Chrome DevTools

const crypto = require('crypto');

// common hashes: sha256, sha512, md5 (insecure!), sha1
const hash = crypto.createHash('sha256');
hash.update('hello');
hash.update(' world');        // can chain updates
console.log(hash.digest('hex'));
// b94d...  (64 hex chars)

// one-shot hashing
const h = crypto.createHash('sha256').update('data').digest('hex');
console.log(h);

// file hash (streaming)
const fs = require('fs');
const fhash = crypto.createHash('sha256');
fs.createReadStream('big.iso').on('data', c => fhash.update(c))
  .on('end', () => console.log(fhash.digest('hex')));

const crypto = require('crypto');

// HMAC = keyed hash (authenticity + integrity)
const secret = process.env.SECRET_KEY;
const hmac = crypto.createHmac('sha256', secret);
hmac.update('payload to sign');
const signature = hmac.digest('hex');

// verify a webhook signature (constant-time compare)
function verify(payload, receivedSig) {
  const expected = crypto.createHmac('sha256', secret)
    .update(payload).digest('hex');
  // timingSafeEqual prevents timing attacks
  return crypto.timingSafeEqual(
    Buffer.from(expected),
    Buffer.from(receivedSig)
  );
}

const crypto = require('crypto');

// async (preferred — doesn't block the event loop)
crypto.randomBytes(16, (err, buf) => {
  const token = buf.toString('hex');   // 32-char hex
});

// sync (blocks; fine for small sizes)
const id = crypto.randomBytes(8).toString('hex');

// random integer in [min, max)
const n = crypto.randomInt(1, 101);     // 1..100

// random UUID (Node 14.17+)
const uuid = crypto.randomUUID();
// '1b9d6bcd-bbfd-4b2d-9b5d-ab8dfbbd4bed'

// use crypto, NEVER Math.random() for secrets!

const crypto = require('crypto');
const algorithm = 'aes-256-gcm';

// AES-256-GCM (authenticated encryption) ----------------
const key = crypto.randomBytes(32);   // 256-bit key
const iv = crypto.randomBytes(12);    // 96-bit nonce

function encrypt(text) {
  const cipher = crypto.createCipheriv(algorithm, key, iv);
  const enc = Buffer.concat([cipher.update(text, 'utf8'), cipher.final()]);
  const tag = cipher.getAuthTag();    // GCM auth tag
  return { enc, tag };
}

function decrypt(enc, tag) {
  const decipher = crypto.createDecipheriv(algorithm, key, iv);
  decipher.setAuthTag(tag);           // verify integrity
  return Buffer.concat([decipher.update(enc), decipher.final()]).toString('utf8');
}

const crypto = require('crypto');

// PBKDF2: tuned with iterations
function hashPbkdf2(password) {
  const salt = crypto.randomBytes(16);
  const iterations = 100000;
  const hash = crypto.pbkdf2Sync(password, salt, iterations, 64, 'sha256');
  return `${iterations}.${salt.toString('hex')}.${hash.toString('hex')}`;
}

// scrypt: memory-hard, resistant to GPU/ASIC attacks
function hashScrypt(password) {
  const salt = crypto.randomBytes(16);
  const hash = crypto.scryptSync(password, salt, 64, { N: 16384 });
  return salt.toString('hex') + '.' + hash.toString('hex');
}

// verify: re-hash with the same salt and compare
function verify(password, stored) {
  const [iter, salt, hash] = stored.split('.');
  const computed = crypto.pbkdf2Sync(password, Buffer.from(salt, 'hex'),
    +iter, 64, 'sha256');
  return crypto.timingSafeEqual(Buffer.from(hash, 'hex'), computed);
}

const crypto = require('crypto');

// NORMAL compare leaks via timing (returns early on first diff)
// 'secret' === userInput  // BAD for secrets

// timingSafeEqual takes equal-length Buffers
function safeEqual(a, b) {
  const bufA = Buffer.from(a);
  const bufB = Buffer.from(b);
  if (bufA.length !== bufB.length) return false;
  return crypto.timingSafeEqual(bufA, bufB);
}

// common use: API key check
function auth(req) {
  const provided = req.headers['x-api-key'] || '';
  return safeEqual(provided, process.env.API_KEY);
}

const os = require('os');

os.platform();    // 'linux', 'darwin', 'win32'
os.arch();        // 'x64', 'arm64'
os.type();        // 'Linux', 'Darwin', 'Windows_NT'
os.release();     // kernel version
os.hostname();    // machine hostname
os.uptime();      // seconds since boot
os.EOL;           // '\n' (POSIX) or '\r\n' (Windows)

// detect platform portably
const isWin = os.platform() === 'win32';
const isMac = os.platform() === 'darwin';

const os = require('os');

// logical core count (threads, not physical)
os.cpus().length;        // 8

// detailed per-core info
os.cpus().forEach((cpu, i) => {
  console.log(`core ${i}: ${cpu.model} @ ${cpu.speed}MHz`);
});

// 1/5/15-minute load averages (NOT on Windows)
os.loadavg();            // [0.45, 0.32, 0.28]

// set process scheduling priority
os.setPriority(os.constants.priority.PRIORITY_BELOW_NORMAL);

const os = require('os');

// system memory (bytes)
os.totalmem();          // e.g. 17179869184
os.freemem();           // currently free bytes

// process memory usage (bytes)
const m = process.memoryUsage();
m.rss;          // resident set size (total RAM held)
m.heapTotal;    // V8 heap allocated
m.heapUsed;     // V8 heap actually used
m.external;     // C++ objects bound to JS
m.arrayBuffers; // memory in ArrayBuffers

// human-readable
console.log(`heap: ${(m.heapUsed / 1048576).toFixed(1)} MB`);

const os = require('os');

const nets = os.networkInterfaces();
for (const name of Object.keys(nets)) {
  for (const net of nets[name]) {
    console.log(`${name} ${net.family}: ${net.address} (${net.mac})`);
  }
}
// en0 IPv4: 192.168.1.10 (aa:bb:cc:...)
// lo IPv4: 127.0.0.1 (:::...)

// find this machine's LAN IPv4
function getLanIp() {
  for (const nets of Object.values(os.networkInterfaces())) {
    for (const n of nets) {
      if (n.family === 'IPv4' && !n.internal) return n.address;
    }
  }
  return '127.0.0.1';
}

const os = require('os');

os.homedir();      // /home/alice  or C:\Users\alice
os.tmpdir();       // /tmp         or C:\Users\...\Temp
os.userInfo();     // { username, uid, gid, homedir, shell }
os.constants;      // signal/errno/priority constants

// errno constants map
os.constants.errno.ENOENT;   // -2
// signal constants
os.constants.signals.SIGTERM; // 15

// priority constants
os.constants.priority.PRIORITY_HIGH;

// build a portable temp path
const path = require('path');
const tmpFile = path.join(os.tmpdir(), 'app-' + Date.now() + '.log');

// argv[0] = node path, argv[1] = script path, argv[2+] = args
// node app.js --port 3000 users.csv
console.log(process.argv);
// ['/usr/bin/node', '/app/app.js', '--port', '3000', 'users.csv']

const args = process.argv.slice(2);
console.log(args);  // ['--port', '3000', 'users.csv']

// minimal flag parsing
function parseArgs(arr) {
  const out = { _: [] };
  for (let i = 0; i < arr.length; i++) {
    if (arr[i].startsWith('--')) out[arr[i].slice(2)] = arr[++i];
    else out._.push(arr[i]);
  }
  return out;
}
// for real CLI use commander, yargs, or minimist

// read environment variables
const port = process.env.PORT || 3000;
const dbUrl = process.env.DATABASE_URL;
const nodeEnv = process.env.NODE_ENV || 'development';

const isProd = process.env.NODE_ENV === 'production';

// set an env var (affects child processes too)
process.env.MY_VAR = 'value';

// load .env files (install: npm i dotenv)
// require('dotenv').config();

// enumerate everything
for (const [k, v] of Object.entries(process.env)) {
  console.log(k, '=', v);
}

// exit immediately with a status code
process.exit(0);    // success
process.exit(1);    // general failure

// exit code is also settable
process.exitCode = 2;   // used if the loop ends naturally

// prefer setting exitCode + returning over process.exit()
// so async work can finish

// common conventions:
// 0  success
// 1  general error
// 2  misuse (bad args)
// 124 timeout (from the 'timeout' command)
// 128+N  killed by signal N (e.g. 143 = SIGTERM)

// stdout/stderr are writable streams
process.stdout.write('no newline');
process.stdout.write('line\n');
console.log === process.stdout.write.bind(...); // roughly

// stdin is a readable stream
process.stdin.setEncoding('utf8');
process.stdin.on('data', chunk => console.log('got:', chunk));

// read all of stdin (async iteration)
async function readStdin() {
  let data = '';
  for await (const chunk of process.stdin) data += chunk;
  return data;
}

// ask a yes/no question
process.stdin.resume();
process.stdin.once('data', input => process.exit(input[0] === 'y'.charCodeAt(0) ? 0 : 1));

// lifecycle events
process.on('exit', code => {
  // sync only here — no async, no event loop
  console.log('bye, code', code);
});
process.on('beforeExit', () => {
  // async OK; can postpone exit
});

// signal handlers (Unix + Windows for SIGINT/SIGTERM)
process.on('SIGINT', () => {       // Ctrl-C
  console.log('graceful shutdown...');
  server.close(() => process.exit(0));
});
process.on('SIGTERM', () => {      // kill / docker stop
  server.close(() => process.exit(0));
});
process.on('SIGHUP', () => reloadConfig());

// process info
process.pid;             // OS process ID
process.ppid;            // parent PID
process.platform;        // 'linux', 'darwin', 'win32'
process.version;         // 'v20.11.0'
process.versions;        // { v8, openssl, modules, ... }
process.cwd();           // current working directory
process.uptime();        // seconds since process started

// change directory
process.chdir('/tmp');

// memory snapshot
const m = process.memoryUsage();
console.log(m.rss, m.heapUsed);

// CPU usage of this process
const cpu = process.cpuUsage();
console.log(cpu.user, cpu.system);  // microseconds

const net = require('net');

const server = net.createServer(socket => {
  console.log('client connected', socket.remoteAddress);
  socket.write('welcome\n');

  socket.on('data', data => {
    socket.write('echo: ' + data);     // echo back
  });

  socket.on('end', () => console.log('client left'));
  socket.on('error', err => console.error('socket err', err));
});

server.listen(5000, () => {
  console.log('TCP server on port 5000');
});

// test with: nc localhost 5000  or  telnet localhost 5000

const net = require('net');

// connect to a TCP server
const client = net.connect(5000, 'localhost', () => {
  console.log('connected');
  client.write('hello server\n');
});

client.setEncoding('utf8');
client.on('data', data => console.log('server said:', data));
client.on('end', () => console.log('disconnected'));
client.on('error', err => console.error(err));

// or with an options object + timeout
const c = net.createConnection({
  port: 5000,
  host: 'localhost',
  timeout: 5000
});

const net = require('net');

net.createServer(socket => {
  socket.setEncoding('utf8');
  socket.setKeepAlive(true, 30000);   // enable TCP keepalive
  socket.setTimeout(60000);           // idle timeout

  socket.on('data', chunk => {
    // TCP is a STREAM: chunks may split or merge messages.
    // Frame your protocol (length prefix, delimiter, etc.)
    console.log('received', chunk.length, 'bytes');
  });

  socket.on('timeout', () => socket.end());
  socket.on('close', hadError => console.log('closed', hadError));
  socket.on('drain', () => console.log('buffer drained'));
}).listen(5000);

const net = require('net');

net.createServer(socket => {
  // write() returns false when the buffer is full (backpressure)
  const ok = socket.write(bigPayload);
  if (!ok) socket.once('drain', writeMore);

  // half-close: send FIN, keep reading
  socket.end();   // writes any pending data, then closes our side
  // socket.end('final message\n'); // write + close

  // fully destroy the socket immediately
  socket.destroy();

  // pause/resume reading
  socket.pause();
  socket.resume();

  // address info
  socket.remoteAddress;  // '192.168.1.5'
  socket.remotePort;     // 54321
}).listen(5000);

const net = require('net');

const server = net.createServer();
server.on('connection', socket => {});   // same as createServer cb
server.on('listening', () => console.log('bound'));
server.on('error', err => {
  if (err.code === 'EADDRINUSE') console.error('port taken');
});
server.on('close', () => console.log('server closed'));

server.listen(5000);

// Unix domain socket / named pipe (fast local IPC)
server.listen('/tmp/app.sock');
// Windows: server.listen('\\\\.\\pipe\\app');

// stop accepting, close idle connections
server.close(() => console.log('all done'));

// get bound address
server.address();   // { port: 5000, family: 'IPv4', address: '::' }

// run once after a delay (ms)
const t = setTimeout(() => console.log('hi'), 1000);

// run repeatedly every interval
const i = setInterval(() => console.log('tick'), 2000);

// run right after the current event loop turn
setImmediate(() => console.log('immediate'));

// run before the next event loop turn (microtask)
queueMicrotask(() => console.log('microtask'));

// order inside one loop:
// microtasks -> timers -> I/O -> check (setImmediate)

const t = setTimeout(() => {}, 1000);
clearTimeout(t);     // cancel

const i = setInterval(() => {}, 1000);
clearInterval(i);

const imm = setImmediate(() => {});
clearImmediate(imm);

// keep the event loop alive?
// timers do by default; unref() lets the process exit if
// only this timer is pending
const timer = setInterval(() => {}, 1000);
timer.unref();       // won't keep Node alive
timer.ref();         // keep alive again

// pass args to the callback
setTimeout((a, b) => console.log(a, b), 500, 'x', 'y');

const util = require('util');

// format like printf
util.format('%s:%d', 'port', 3000);   // 'port:3000'
util.format('%j', { a: 1 });           // JSON

// deep inspect an object (what console.dir uses)
util.inspect(obj, { depth: null, colors: true, compact: false });

// pretty-print a function (its source)
util.inspect(function f() { return 1; });

// deprecate an API with a warning
const oldFn = util.deprecate(
  () => doThingOldWay(),
  'oldFn() is deprecated, use newFn() instead'
);

const util = require('util');
const fs = require('fs');

// turn an error-first callback fn into a promise fn
const readFile = util.promisify(fs.readFile);
const text = await readFile('data.txt', 'utf8');

// works on custom functions too
function delay(ms, cb) { setTimeout(() => cb(null, ms), ms); }
const delayP = util.promisify(delay);
await delayP(100);

// custom promisify symbol (controls the promise behavior)
// const { promisify } = util;
// obj[util.promisify.custom] = () => Promise.resolve(42);

// many core modules ship promise versions directly:
// require('fs/promises'), require('dns/promises')

const util = require('util');

// type checks
util.isPromise(Promise.resolve());    // true
util.isFunction(() => {});            // true
util.types.isMap(new Map());          // true
util.types.isAsyncFunction(async () => {}); // true
util.types.isRegExp(/x/);             // true

// reverse of promisify: promise -> error-first callback
const readFileCb = util.callbackify(require('fs/promises').readFile);
readFileCb('x.txt', 'utf8', (err, data) => {
  if (err) console.error(err);
  else console.log(data);
});

// MIME type utilities (Node 19+)
util.MIMEType.parse('text/html; charset=utf-8');

// process.nextTick: runs BEFORE I/O, after the current op
process.nextTick(() => console.log('tick'));

// setImmediate: runs AFTER I/O events, in the 'check' phase
setImmediate(() => console.log('immediate'));

// setTimeout(0): runs in the timers phase, after nextTick
setTimeout(() => console.log('timeout'), 0);

// typical order in one loop turn:
// 1. current sync code
// 2. nextTick callbacks  (can starve I/O!)
// 3. timers
// 4. I/O callbacks
// 5. setImmediate

// prefer setImmediate to yield to I/O; use nextTick for
// immediate post-sync cleanup