Why not use Math.ceil()?

While it's highly unlikely, Math.random() can theoretically return zero, so your formula would return 1-6 with approximately equal likelihood, but perhaps also 0 with a far lower probability.

That's why Math.floor() is generally used instead of Math.ceil() with random formulae, since the result can be relied on to remain in the range and give each value approximately equal probability.

What Steven has said above in simpler terms.

Going to break it down into steps.

Math.random(); provides values from 0(including zero) to 0.99999999999 but never 1.

Math.floor will round it down to the nearest whole number.

so if we take it mathematically. even if you get 0.9999999 as above and you * 6 you will get 5.999999 whichMath.floor will drop to 5

so your range is 0-5 the "+1" just increases the minimum from 0 to 1 and the max from 5 to 6.

with Math.ceil if you random 0 it will leave the value as zero.

which will mean that your code var randomNumber = Math.ceil(Math.random() * 6); gives you a range of 0 to 6 instead of 1 to 6 like a dice.