Number
Toolbox

const price = 450;
const rate = 7.5;
const big = 1_000_000;          // underscores only help you read it
const tiny = 2.5e-3;            // 0.0025 in science notation

console.log(typeof price, typeof rate);  // whole or decimal, both are "number"
console.log(big, tiny);
console.log(10 / 0, -10 / 0);            // dividing by zero gives Infinity
console.log(0 / 0, "abc" * 2);           // NaN means "not a number"

console.log(Number.isInteger(5), Number.isInteger(5.5));
console.log(Number.isFinite(42), Number.isFinite(Infinity));

// NaN is the only value that is not equal to itself
console.log(NaN === NaN);
console.log(Number.isNaN(NaN));        // the reliable check
console.log(isNaN("hello"));           // older isNaN converts first: true
console.log(Number.isNaN("hello"));    // Number.isNaN does not: false

console.log(Number("42"), Number("4.5"), Number(""));  // empty text becomes 0
console.log(Number("42px"));                  // any junk gives NaN

console.log(parseInt("42px", 10));            // reads digits until it hits junk
console.log(parseInt("08", 10), parseInt("ff", 16)); // always pass the base
console.log(parseFloat("3.14 metres"));
console.log(+"7" + 1, "7" + 1);               // + in front converts; + between joins text

const qty = Number("abc");
console.log(Number.isNaN(qty) ? "please enter a number" : qty);

const n = 4.567;

console.log(Math.round(n), Math.floor(n), Math.ceil(n), Math.trunc(n));
console.log(Math.floor(-4.5), Math.trunc(-4.5));   // floor goes down, trunc cuts off

console.log(n.toFixed(2));           // "4.57": a string with 2 decimals
console.log(n.toFixed(0), typeof n.toFixed(2));
console.log(Number(n.toFixed(1)));   // back to a number

console.log((1234.5678).toPrecision(3)); // 3 significant digits

console.log(Math.max(3, 9, 1), Math.min(3, 9, 1));
console.log(Math.max(...[40, 75, 12]));      // spread an array in
console.log(Math.abs(-8), 2 ** 10, Math.sqrt(81));
console.log(17 % 5, 10 % 2 === 0);           // remainder; checks even numbers

// a whole random number between min and max, both included
const randomInt = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
const roll = randomInt(1, 6);
console.log(roll >= 1 && roll <= 6);

console.log(0.1 + 0.2);                  // tiny error from how decimals are stored
console.log(0.1 + 0.2 === 0.3);

const close = (a, b) => Math.abs(a - b) < Number.EPSILON;
console.log(close(0.1 + 0.2, 0.3));     // compare with a tiny allowed gap

// for money, count the smallest unit (paise) as whole numbers
const paise = 10 + 20;                  // ₹0.10 + ₹0.20
console.log(paise / 100);

console.log(Number.MAX_SAFE_INTEGER);
console.log(9007199254740993);           // too big: the last digit is lost
console.log(9007199254740993n + 1n);     // BigInt, with n, stays exact

const amount = 1234567.891;

console.log(amount.toLocaleString("en-IN"));   // Indian grouping: lakhs and crores
console.log(amount.toLocaleString("en-US"));

const inr = new Intl.NumberFormat("en-IN", { style: "currency", currency: "INR" });
console.log(inr.format(amount));

const usd = new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" });
console.log(usd.format(amount));

const pct = new Intl.NumberFormat("en-IN", { style: "percent", maximumFractionDigits: 1 });
console.log(pct.format(0.1834));

const short = new Intl.NumberFormat("en-US", { notation: "compact" });
console.log(short.format(1500), short.format(2_400_000));

// split a restaurant bill fairly, working in paise so nothing is lost
const items = [
  { name: "Paneer tikka", price: 349.0 },
  { name: "Dal makhani", price: 289.5 },
  { name: "Naan x4", price: 180.0 },
];
const people = 3;
const GST = 0.05;

const toPaise = (rupees) => Math.round(rupees * 100);
const subtotal = items.reduce((sum, item) => sum + toPaise(item.price), 0);
const tax = Math.round(subtotal * GST);
const total = subtotal + tax;

// share out the paise, and give any leftover to the first people
const base = Math.floor(total / people);
const leftover = total % people;
const shares = Array.from({ length: people }, (_, i) => base + (i < leftover ? 1 : 0));

const inr = new Intl.NumberFormat("en-IN", { style: "currency", currency: "INR" });
console.log("total", inr.format(total / 100));
console.log(shares.map((p) => inr.format(p / 100)).join(" | "));
console.log("adds up:", shares.reduce((a, b) => a + b) === total);