⚡ JavaScript Lesson 04 — Functions

1. Function Declaration

// Basic function
function greet(name) {
  return `Hello, ${name}!`
}

// Call / invoke
greet("Min")      // "Hello, Min!"
greet("Aye")      // "Hello, Aye!"

// Multiple parameters
function add(a, b) {
  return a + b
}
add(3, 4)   // 7

// No return = undefined
function sayHi() {
  console.log("Hi!")
  // no return
}
let result = sayHi()  // result = undefined

2. Default Parameters

function greet(name = "Guest", greeting = "Hello") {
  return `${greeting}, ${name}!`
}

greet()                    // "Hello, Guest!"
greet("Min")               // "Hello, Min!"
greet("Min", "Mingalaba")  // "Mingalaba, Min!"

// Real example
function createUser(name, role = "user", active = true) {
  return { name, role, active }
}
createUser("Ko Min")
// { name: "Ko Min", role: "user", active: true }

3. Function Expression & Arrow Functions

// Function Expression — variable မှာ သိမ်း
const multiply = function(a, b) {
  return a * b
}
multiply(4, 5)   // 20

// Arrow Function — modern, shorter
const multiply = (a, b) => a * b

// Arrow function styles
const double = x => x * 2                   // 1 param, no ()
const square = (x) => x * x                  // 1 param with ()
const add = (a, b) => a + b                  // 2 params
const greet = (name) => {                    // multi-line
  const msg = `Hello, ${name}!`
  return msg
}

// No params
const now = () => new Date().toLocaleTimeString()

4. Rest Parameters (...args)

// ...rest collects extra args into array
function sum(...numbers) {
  let total = 0
  for (const n of numbers) total += n
  return total
}

sum(1, 2, 3)           // 6
sum(1, 2, 3, 4, 5)     // 15
sum(10, 20)             // 30

// Mix with regular params
function introduce(greeting, ...names) {
  return `${greeting}: ${names.join(", ")}`
}
introduce("Hi", "Min", "Aye", "Kyaw")
// "Hi: Min, Aye, Kyaw"

5. Higher-Order Functions (Function as Argument)

// Function ကို argument အနေနဲ့ pass နိုင်တယ်
function doMath(a, b, operation) {
  return operation(a, b)
}

const add = (a, b) => a + b
const sub = (a, b) => a - b
const mul = (a, b) => a * b

doMath(10, 5, add)  // 15
doMath(10, 5, sub)  // 5
doMath(10, 5, mul)  // 50

// setTimeout — callback function
setTimeout(() => {
  console.log("3 seconds later!")
}, 3000)

// Array methods use callbacks
[1,2,3].forEach(n => console.log(n))
[1,2,3].map(n => n * 2)      // [2,4,6]
[1,2,3].filter(n => n > 1)   // [2,3]

6. Function Scope

let globalVar = "I'm global"   // ဘယ်နေရာမှာမဆို ရမယ်

function myFunc() {
  let localVar = "I'm local"   // ဒီ function ထဲမှာပဲ ရမယ်
  console.log(globalVar)       // ✅ Global ရမယ်
  console.log(localVar)        // ✅ Local ရမယ်
}

myFunc()
console.log(globalVar)         // ✅ OK
console.log(localVar)          // ❌ Error: localVar is not defined

// Block scope (let/const)
{
  let blockVar = "block"
  console.log(blockVar)        // ✅ Inside block
}
console.log(blockVar)          // ❌ Error: outside block

7. Comparison Table