दिन 2: Variables, Data Types और Template Literals

const shopName = 'Sharma Sweets';   // will never change → const
let cartItems = 0;                   // will change → let

cartItems = cartItems + 1;
cartItems = cartItems + 2;
console.log(shopName, 'cart:', cartItems);

// shopName = 'Other';   ← TypeError: Assignment to constant variable.

const name = 'Kaju Katli';     // string
const price = 900;              // number (integers and decimals)
const rating = 4.8;             // number
const inStock = true;           // boolean
let discountCode;               // undefined – declared, no value yet
const gstNumber = null;         // null – intentionally empty
const bigOrderId = 9007199254740993n;  // bigint
const product = { name, price }; // object
const sizes = ['250g', '500g', '1kg']; // array (also an object)

console.log(typeof name, typeof price, typeof inStock);
console.log(typeof discountCode, typeof gstNumber, typeof bigOrderId);
console.log(typeof product, typeof sizes, Array.isArray(sizes));

const qtyFromInput = '2';          // everything from an <input> is a STRING
const price = 900;

console.log('Wrong:', qtyFromInput + price);           // joins text!
console.log('Wrong:', qtyFromInput * price);           // works by luck (* converts)
console.log('Right:', Number(qtyFromInput) * price);   // convert first ✅

console.log(Number('2.5'), Number(''), Number('abc'));
console.log(parseInt('250g'), parseFloat('1.5kg'));
console.log(String(1800), (1800).toString(), Boolean(0), Boolean('hi'));

const customer = 'Aarti';
const item = 'Kaju Katli';
const qty = 2;
const price = 900;

// Old way – hard to read
console.log('Hi ' + customer + ', ' + qty + ' kg ' + item + ' = Rs ' + qty * price);

// Template literal – backticks and ${ }
console.log(`Hi ${customer}, ${qty} kg ${item} = ₹${qty * price}`);

// Multi-line text is easy too
const sms = `Sharma Sweets 🍬
Order confirmed for ${customer}.
Total: ₹${qty * price}`;
console.log(sms);

<label>Quantity (kg) <input id="qty" type="number" value="2" min="0.25" step="0.25"></label>
<button id="calc">Calculate</button>
<pre id="bill"></pre>

<script>
const PRICE_PER_KG = 900;
const GST_RATE = 0.05;   // 5% GST on sweets

document.querySelector('#calc').addEventListener('click', function () {
  const qty = Number(document.querySelector('#qty').value);   // string → number
  const amount = qty * PRICE_PER_KG;
  const gst = amount * GST_RATE;
  const total = amount + gst;
  document.querySelector('#bill').textContent =
    `Kaju Katli  ${qty} kg × ₹${PRICE_PER_KG}
Amount      ₹${amount.toFixed(2)}
GST 5%      ₹${gst.toFixed(2)}
Total       ₹${total.toFixed(2)}`;
});
</script>