Numbers and Math

One number type

Many languages have separate types for whole numbers and decimals. JavaScript has just one, number, which is a 64-bit floating-point value (the IEEE 754 standard). Floating point means the decimal point can "float": the computer stores a number as digits plus a position, in binary.

Why 0.1 + 0.2 is not 0.3

Some decimals, like 0.1, cannot be written exactly in binary, just as 1/3 cannot be written exactly in decimal (0.3333…). So tiny errors appear.

console.log(0.1 + 0.2);          // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3);  // false
console.log((0.1 + 0.2).toFixed(2)); // '0.30' (a string, for display)

const pricesInPaise = [19999, 4950, 100];
const totalPaise = pricesInPaise.reduce((sum, p) => sum + p, 0);
console.log(totalPaise);              // 25049
console.log((totalPaise / 100).toFixed(2)); // '250.49'

Turning text into numbers

NaN is a special number value meaning "this is not a valid number". Any maths with NaN gives NaN, and NaN is not equal even to itself. Test it with Number.isNaN(value). Infinity appears when you divide a non-zero number by 0.

Rounding and Math

console.log(Math.round(2.5));   // 3
console.log(Math.round(-2.5));  // -2 (rounds toward +infinity at .5)
console.log(Math.floor(2.9));   // 2 (down)
console.log(Math.ceil(2.1));    // 3 (up)
console.log(Math.trunc(-2.9));  // -2 (cuts off the decimals)
console.log(Math.max(4, 9, 1)); // 9
console.log(Math.min(4, 9, 1)); // 1
console.log(Math.abs(-7));      // 7
console.log(Math.sqrt(81));     // 9

Math.random() gives a decimal from 0 (included) up to 1 (not included). To get a whole number from 1 to 6, like a dice: Math.floor(Math.random() * 6) + 1. Do not use Math.random for passwords or OTPs; it is not secure. Use crypto.getRandomValues instead.

Formatting for people: Indian grouping

const amount = 1234567.5;
console.log(amount.toFixed(2));  // '1234567.50'
console.log(new Intl.NumberFormat('en-IN', { style: 'currency', currency: 'INR' }).format(amount));
// '₹12,34,567.50' (lakh and crore grouping)

Very big whole numbers: BigInt

A number can hold whole numbers exactly only up to Number.MAX_SAFE_INTEGER, which is 9007199254740991 (about 9 quadrillion). Beyond that, adding 1 may give a wrong answer. BigInt stores any whole number exactly; write it with an n at the end. You cannot mix BigInt and number in one sum without converting.

console.log(9007199254740991 + 2);  // 9007199254740992 (wrong!)
console.log(9007199254740991n + 2n); // 9007199254740993n (exact)
// 10n + 1  → TypeError: Cannot mix BigInt and other types