Math.random and Math.floor

In short: Math.random() returns a random number in JavaScript from 0 (inclusive) to 1 (exclusive). Math.floor(x) rounds down to the nearest whole number. Together they create integer random values.

In more detail: The random number is a decimal such as 0.7342. To get a number in a range you scale it with the size of the range and round down.

In detail

// integer from 0 to 5 (die minus 1)
const r = Math.floor(Math.random() * 6);
// die 1 to 6
const die = Math.floor(Math.random() * 6) + 1;
// range min..max (inclusive)
const random = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;

• floor, ceil, round, trunc: floor down, ceil up, round to nearest, trunc cuts off decimals. For negative numbers floor and trunc differ (floor(-1.5) is -2, trunc(-1.5) is -1).
• Not secure: Math.random() is unsuitable for passwords and tokens, use crypto.getRandomValues there, see Random numbers: pseudo-random and true random.
• Other languages: see Random and rounding in many programming languages.

See also: Random values, Mathematical functions