Randomization using Math.random()

├─ index.html
└─ script.js

Math.random() gives you a decimal between 0 and 1. That’s the only random function JavaScript has ~ everything else is built out of it.

Math.random()   // 0.7263849...

It never returns exactly 1, which matters more than you’d think.

A whole number in a range

Math.floor(Math.random() * 10)      // 0 to 9

Multiply to set the range, then Math.floor() to chop off the decimal. Note that * 10 gives you 0 to 9, not 1 to 10.

For a range with both ends included:

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

The + 1 is what makes max reachable. Without it you can never roll a six, and you won’t notice for ages.

A random item from a list

function pick(list) {
	return list[Math.floor(Math.random() * list.length)];
}

This is the one I actually use. Random colour, random fortune, random layout ~ all the same three lines.

Not really random

Math.random() is pseudorandom: predictable if you know enough about it. Completely fine for a fortune generator or a colour picker. Not fine for passwords or anything involving security ~ that needs crypto.getRandomValues().