Numbers and Math Object

One number type for everything

Many languages separate whole numbers from decimals. JavaScript has just Number for both.

const whole = 42;
const decimal = 3.14;
const negative = -7;
const scientific = 1.5e6;
console.log(scientific);

You can also write numbers in other bases, and use underscores to make long ones readable:

const million = 1_000_000;
const binary = 0b1010;
const hex = 0xff;
console.log(million, binary, hex);

The famous floating point problem

console.log(0.1 + 0.2);
console.log(0.1 + 0.2 === 0.3);

That prints 0.30000000000000004 and false. This is not a JavaScript bug. Computers store decimals in binary, and some values simply cannot be represented exactly, in the same way that one third cannot be written exactly in decimal.

How to deal with it

For display, round it:

console.log((0.1 + 0.2).toFixed(2));

For comparison, check that the difference is tiny:

const a = 0.1 + 0.2;
console.log(Math.abs(a - 0.3) < Number.EPSILON);

For money, work in the smallest unit. Store paise or cents as whole numbers, and divide only when you show the value.

const priceInPaise = 19999;
console.log('Rs ' + (priceInPaise / 100).toFixed(2));

Rounding

console.log(Math.round(4.5));
console.log(Math.round(4.4));
console.log(Math.ceil(4.1));
console.log(Math.floor(4.9));
console.log(Math.trunc(-4.9));
console.log(Math.floor(-4.9));

Watch the last two. trunc chops the decimal off, while floor always moves down, which for negatives means away from zero.

toFixed returns a string

const price = 19.999;
const rounded = price.toFixed(2);
console.log(rounded, typeof rounded);
console.log(Number(rounded) + 1);

This catches people out constantly. If you need to keep calculating, wrap it in Number().

Useful Math methods

console.log(Math.abs(-7));
console.log(Math.max(3, 9, 1));
console.log(Math.min(3, 9, 1));
console.log(Math.pow(2, 10));
console.log(Math.sqrt(144));
console.log(Math.cbrt(27));

To use Math.max on an array, spread it:

const scores = [45, 92, 78];
console.log(Math.max(...scores));

Random numbers

Math.random() gives a decimal from 0 up to, but not including, 1.

console.log(Math.random());

To get a whole number in a range:

function randomBetween(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}
console.log(randomBetween(1, 6));

Picking a random array item is a common use:

const colours = ['red', 'green', 'blue'];
const pick = colours[Math.floor(Math.random() * colours.length)];
console.log(pick);

Checking numbers

console.log(Number.isInteger(42));
console.log(Number.isInteger(4.2));
console.log(Number.isNaN(Number('abc')));
console.log(Number.isFinite(1 / 0));

Use the Number. versions rather than the older global isNaN() and isFinite(), which convert their argument first and give misleading answers.

Special values

console.log(1 / 0);
console.log(-1 / 0);
console.log(Number.MAX_SAFE_INTEGER);
console.log(Number.MIN_SAFE_INTEGER);

Beyond MAX_SAFE_INTEGER, whole numbers start losing precision. If you need bigger, use BigInt:

const big = 9007199254740993n;
console.log(big + 1n);

Formatting for humans

toLocaleString adds separators and currency symbols properly.

const amount = 1234567.891;
console.log(amount.toLocaleString('en-IN'));
console.log(amount.toLocaleString('en-US'));
console.log(amount.toLocaleString('en-IN', { style: 'currency', currency: 'INR' }));

Note that India groups digits differently from the US, and this handles it for you.

Quick recap

• One Number type covers whole numbers and decimals.
• 0.1 + 0.2 is not exactly 0.3, so round for display and compare with a tolerance.
• Store money as whole paise or cents.
• round, ceil, floor and trunc each round differently for negatives.
• toFixed returns a string, so convert back before doing maths.
• Math.random() plus Math.floor gives whole numbers in a range.
• toLocaleString formats numbers and currency for a region.