How to roll a dice in JavaScript?

Let's build the ultimate dice step by step.

Last update: May 11, 2022

Math.random() as a basis

A dice is a tool providing a random integer each time you roll it. Something like that:

function rollDice() {
  return /* some randomly generated number */
}

Every programming language has a built-in random function. In JavaScript, it's Math.random:

function rollDice() {
  return Math.random();
}

That’s a good start: returning a random number. Remember Math.random is not “random enough” for serious things like cryptography, or casino games — read about Crypto.getRandomValues if that’s your business. Math.random is fair enough to roll a dice with friends. Let’s try it:

>> function rollDice() {
     return Math.random();
   }
>> rollDice();
<- 0.7367823644188911

This 0.7367823644188911 is not really what we wanted… According to documentation, Math.random returns a decimal number between 0 (inclusive) and 1 (exclusive). For a 6-sided dice, we need an integer from 1 to 6. As a first guess, you may multiply by 6:

>> function rollDice() {
     return Math.random() * 6;
   }
>> rollDice();
<- 4.3380209914241235

So we would have a random decimal number between 0 (inclusive) and 6 (exclusive). So far, so good. Next step would be to get integer values:

• If 0 ≤ Math.random() * 6 < 1, return 1
• If 1 ≤ Math.random() * 6 < 2, return 2
• If 2 ≤ Math.random() * 6 < 3, return 3
• If 3 ≤ Math.random() * 6 < 4, return 4
• If 4 ≤ Math.random() * 6 < 5, return 5
• If 5 ≤ Math.random() * 6 < 6, return 6

This can be done using Math.floor. Let's try again — with a for-loop to console.log multiple rolls:

>> function rollDice() {
     return Math.floor(Math.random() * 6);
   }
>> for(let i = 0; i < 5; i++) console.log(rollDice());
   5
   1
   4
   2
   0 // WTF?

Once again, not exactly what we wanted... What we get here is:

• If 0 ≤ Math.floor(Math.random() * 6) < 1, return 0. Not 1.
• If 1 ≤ Math.floor(Math.random() * 6) < 2, return 1. Not 2.
• If 2 ≤ Math.floor(Math.random() * 6) < 3, return 2. Not 3.
• If 3 ≤ Math.floor(Math.random() * 6) < 4, return 3. Not 4.
• If 4 ≤ Math.floor(Math.random() * 6) < 5, return 4. Not 5.
• If 5 ≤ Math.floor(Math.random() * 6) < 6, return 5. Not 6.

To get the wanted result with Math.floor, we will have to add 1 before returning:

function rollDice() {
  return 1 + Math.floor(Math.random() * 6);
}

Now we have a function to simulate our 6-sided dice :)

Yes, but… What if we want a 4-, 8-, 12- or 20-sided one?

No big deal: you can change the magic number 6 in the code for a parameter, passing the maximum value for your dice. Something like this:

function rollDice(max) {
  return 1 + Math.floor(Math.random() * max);
}

const rollDice4 = () => rollDice(4);
const rollDice6 = () => rollDice(6);
const rollDice8 = () => rollDice(8);
const rollDice12 = () => rollDice(12);
const rollDice20 = () => rollDice(20);

The ultimate dice

I was once inspired by a vision: “The Ultimate Display” by Ivan E. Sutherland, 1965. Among others, I like this quote:

There is no reason why the objects displayed by a computer have to follow the ordinary rules of physical reality with which we are familiar.

We used a parameter to replace the number of sides of our dice. Why not removing the other magic number? This ugly 1 may become an other parameter:

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

const rollDice4 = () => rollDice(1, 4);
const rollDice6 = () => rollDice(1, 6);
const rollDice8 = () => rollDice(1, 8);
const rollDice12 = () => rollDice(1, 12);
const rollDice20 = () => rollDice(1, 20);
const rollSomeUltimateDice = () => rollDice(42, 42);

This final version allows to simulate a dice which is not starting at 1. Moreover the max allows to simulate a uniform fair dice beyond “the ordinary rules of physical reality”. Imagine a 7-sided one. You can mimic your favorite dice game following its ordinary rules. But if you can imagine one, roll a dice which would never exist in reality ;)