🟢 Node.js Lesson 01 — Intro & First Script

1. Node.js ဆိုတာ

2. Install & First Run

# 1. Download: nodejs.org → LTS version (Long Term Support)
# 2. Install → restart terminal

# Version check
node --version     # v20.x.x
npm --version      # 10.x.x

# Run a JS file
node app.js

# REPL (Read-Eval-Print Loop) — interactive
node
> 2 + 2
4
> "Hello".toUpperCase()
'HELLO'
> .exit    # quit REPL

3. First Node.js Script

// app.js
console.log("Hello from Node.js!");
console.log("Node version:", process.version);
console.log("Platform:", process.platform);
console.log("Current dir:", process.cwd());

// run: node app.js
// Output:
// Hello from Node.js!
// Node version: v20.11.0
// Platform: win32
// Current dir: C:\projects\myapp

4. Global Objects (Browser vs Node)

// Browser: window, document, navigator, location
// Node.js: process, __dirname, __filename, global

// process — current process info
process.env.NODE_ENV   // "development" | "production"
process.argv           // command line arguments
process.exit(0)        // exit (0 = success)
process.env.PORT       // environment variable

// __dirname / __filename
console.log(__dirname);    // C:\projects\myapp
console.log(__filename);   // C:\projects\myapp\app.js

// setTimeout, setInterval — same as browser
setTimeout(() => console.log("After 1s"), 1000);

// setImmediate — after current event loop iteration
setImmediate(() => console.log("Immediate"));

// Command line args
// run: node app.js --name=Ko --age=25
const args = process.argv.slice(2);
console.log(args);   // ['--name=Ko', '--age=25']

5. Event Loop — Node.js core concept

// Node.js is SINGLE-THREADED but NON-BLOCKING
// Event loop = handle async operations without multiple threads

console.log("1. Start");

setTimeout(() => {
  console.log("4. Timeout (after 0ms — after current loop)");
}, 0);

setImmediate(() => {
  console.log("3. Immediate");
});

Promise.resolve().then(() => {
  console.log("2. Promise (microtask — runs first!)");
});

console.log("5. End");

// Output order:
// 1. Start
// 5. End
// 2. Promise (microtask)
// 3. Immediate
// 4. Timeout