दिन 5: Functions, Arrow Functions और Scope

function addGst(amount, rate) {      // amount, rate = parameters
  const gst = amount * rate / 100;
  return amount + gst;                  // send a value back
}

console.log(addGst(1000, 5));           // 1000, 5 = arguments
console.log(addGst(2400, 12));
const bill = addGst(900, 5) + addGst(650, 5);
console.log('Two items with GST:', bill);

function price(kg, ratePerKg, discountPercent = 0, deliveryCharge = 60) {
  const amount = kg * ratePerKg * (1 - discountPercent / 100);
  const delivery = amount >= 1500 ? 0 : deliveryCharge;
  return Math.round(amount + delivery);
}
console.log(price(1, 900));            // defaults used
console.log(price(3, 900, 10));        // 10% off, free delivery
console.log(price(1, 500, 0, 40));     // custom delivery charge

// 1. Declaration – can be called before it appears (hoisted)
function square(n) { return n * n; }

// 2. Expression – stored in a variable
const cube = function (n) { return n * n * n; };

// 3. Arrow function – short, modern
const double = (n) => n * 2;           // implicit return for one expression
const greet = name => `Namaste, ${name}!`;
const makeOrder = (item, qty) => ({ item, qty });   // returning an object needs ( )

console.log(square(4), cube(3), double(21));
console.log(greet('Riya'));
console.log(makeOrder('Ghewar', 2));

function applyToAll(prices, rule) {      // rule is a callback
  const result = [];
  for (const p of prices) result.push(rule(p));
  return result;
}
const prices = [900, 650, 700];
console.log(applyToAll(prices, p => p * 1.05));        // add 5% GST
console.log(applyToAll(prices, p => p - 100));         // festival ₹100 off
console.log(applyToAll(prices, p => `₹${p}/kg`));      // format

const SHOP = 'Sharma Sweets';          // global – visible everywhere

function invoiceHeader(no) {
  const prefix = 'INV';                  // local – only inside this function
  return `${SHOP} · ${prefix}-${no}`;
}
console.log(invoiceHeader(1042));
console.log(typeof prefix);              // 'undefined' – prefix is not visible here

if (true) {
  const insideBlock = 'block scoped';
}
console.log(typeof insideBlock);

<label>Loan ₹ <input id="p" type="number" value="500000"></label><br>
<label>Interest % per year <input id="r" type="number" value="9.5" step="0.1"></label><br>
<label>Months <input id="n" type="number" value="60"></label><br>
<button id="go">Calculate EMI</button>
<p id="out"></p>

<script>
// EMI = P × r × (1 + r)^n / ((1 + r)^n − 1), r = monthly rate
function emi(principal, yearlyRate, months) {
  const r = yearlyRate / 12 / 100;
  if (r === 0) return principal / months;
  const f = (1 + r) ** months;
  return principal * r * f / (f - 1);
}
const rupees = n => '₹' + Math.round(n).toLocaleString('en-IN');

document.querySelector('#go').addEventListener('click', () => {
  const P = Number(document.querySelector('#p').value);
  const R = Number(document.querySelector('#r').value);
  const N = Number(document.querySelector('#n').value);
  const monthly = emi(P, R, N);
  document.querySelector('#out').innerHTML =
    `EMI <b>${rupees(monthly)}</b> · total paid ${rupees(monthly * N)} · interest ${rupees(monthly * N - P)}`;
});
</script>