Generate random number between two numbers in JavaScript

Question

Is there a way to generate a random number in a specified range (e.g. from 1 to 6: 1, 2, 3, 4, 5, or 6) in JavaScript?

Answer 1

TL;DR

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

To get the random number generateRandomInteger(-20, 20);

EXPLANATION BELOW

We need to get a random integer, say X between min and max.

Right?

i.e min <= X <= max

If we subtract min from the equation, this is equivalent to

0 <= (X - min) <= (max - min)

Now, lets multiply this with a random number r which is

0 <= (X - min) * r <= (max - min) * r

Now, lets add back min to the equation

min <= min + (X - min) * r <= min + (max - min) * r

Now, lets chose a function which results in r such that it satisfies our equation range as [min,max]. This is only possible if 0<= r <=1

OK. Now, the range of r i.e [0,1] is very similar to Math.random() function result. Isn't it?

The Math.random() function returns a floating-point, pseudo-random number in the range [0, 1); that is, from 0 (inclusive) up to but not including 1 (exclusive)

For example,

Case r = 0

min + 0 * (max-min) = min

Case r = 1

min + 1 * (max-min) = max

Random Case using Math.random 0 <= r < 1

min + r * (max-min) = X, where X has range of min <= X < max

The above result X is a random numeric. However due to Math.random() our left bound is inclusive, and the right bound is exclusive. To include our right bound we increase the right bound by 1 and floor the result.

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

To get the random number

generateRandomInteger(-20, 20);