The best answers to the question “Generating random whole numbers in JavaScript in a specific range?” in the category Dev.
QUESTION:
How can I generate random whole numbers between two specified variables in JavaScript, e.g. x = 4
and y = 8
would output any of 4, 5, 6, 7, 8
?
ANSWER:
var randomnumber = Math.floor(Math.random() * (maximum  minimum + 1)) + minimum;
ANSWER:
There are some examples on the Mozilla Developer Network page:
/**
* Returns a random number between min (inclusive) and max (exclusive)
*/
function getRandomArbitrary(min, max) {
return Math.random() * (max  min) + min;
}
/**
* Returns a random integer between min (inclusive) and max (inclusive).
* The value is no lower than min (or the next integer greater than min
* if min isn't an integer) and no greater than max (or the next integer
* lower than max if max isn't an integer).
* Using Math.round() will give you a nonuniform distribution!
*/
function getRandomInt(min, max) {
min = Math.ceil(min);
max = Math.floor(max);
return Math.floor(Math.random() * (max  min + 1)) + min;
}
Here’s the logic behind it. It’s a simple rule of three:
Math.random()
returns a Number
between 0 (inclusive) and 1 (exclusive). So we have an interval like this:
[0 .................................... 1)
Now, we’d like a number between min
(inclusive) and max
(exclusive):
[0 .................................... 1)
[min .................................. max)
We can use the Math.random
to get the correspondent in the [min, max) interval. But, first we should factor a little bit the problem by subtracting min
from the second interval:
[0 .................................... 1)
[min  min ............................ max  min)
This gives:
[0 .................................... 1)
[0 .................................... max  min)
We may now apply Math.random
and then calculate the correspondent. Let’s choose a random number:
Math.random()

[0 .................................... 1)
[0 .................................... max  min)

x (what we need)
So, in order to find x
, we would do:
x = Math.random() * (max  min);
Don’t forget to add min
back, so that we get a number in the [min, max) interval:
x = Math.random() * (max  min) + min;
That was the first function from MDN. The second one, returns an integer between min
and max
, both inclusive.
Now for getting integers, you could use round
, ceil
or floor
.
You could use Math.round(Math.random() * (max  min)) + min
, this however gives a noneven distribution. Both, min
and max
only have approximately half the chance to roll:
min...min+0.5...min+1...min+1.5 ... max0.5....max
└───┬───┘└────────┬───────┘└───── ... ─────┘└───┬──┘ ← Math.round()
min min+1 max
With max
excluded from the interval, it has an even less chance to roll than min
.
With Math.floor(Math.random() * (max  min +1)) + min
you have a perfectly even distribution.
min.... min+1... min+2 ... max1... max.... max+1 (is excluded from interval)
     
└───┬───┘└───┬───┘└─── ... ┘└───┬───┘└───┬───┘ ← Math.floor()
min min+1 max1 max
You can’t use ceil()
and 1
in that equation because max
now had a slightly less chance to roll, but you can roll the (unwanted) min1
result too.
ANSWER:
function getRandomizer(bottom, top) {
return function() {
return Math.floor( Math.random() * ( 1 + top  bottom ) ) + bottom;
}
}
usage:
var rollDie = getRandomizer( 1, 6 );
var results = ""
for ( var i = 0; i<1000; i++ ) {
results += rollDie() + " "; //make a string filled with 1000 random numbers in the range 16.
}
breakdown:
We are returning a function (borrowing from functional programming) that when called, will return a random integer between the the values bottom
and top
, inclusive. We say ‘inclusive’ because we want to include both bottom and top in the range of numbers that can be returned. This way, getRandomizer( 1, 6 )
will return either 1, 2, 3, 4, 5, or 6.
(bottom is lower number, top is greater number)
Math.random() * ( 1 + top  bottom )
Math.random()
returns a random double between 0 and 1, and if we multiply it by one plus the difference between top
and bottom
, we’ll get a double somewhere between 0
and 1+ba
.
Math.floor( Math.random() * ( 1 + top  bottom ) )
Math.floor
rounds the number down to the nearest integer. So we now have all the integers between 0
and topbottom
. The 1 looks confusing, but it needs to be there because we are always rounding down, so the top number will never actually be reached without it. The random decimal we generate needs to be in the range 0
to (1+topbottom)
so we can round down and get an int in the range 0
to topbottom
Math.floor( Math.random() * ( 1 + top  bottom ) ) + bottom
The code in the previous example gave us an integer in the range 0
and topbottom
, so all we need to do now is add bottom
to that result to get an integer in the range bottom
and top
inclusive. 😀
NOTE: If you pass in a noninteger value or the greater number first you’ll get undesirable behavior, but unless anyone requests it I am not going to delve into the argument checking code as its rather far from the intent of the original question.
ANSWER:
Math.random()
Returns an integer random number between min (included) and max (included):
function randomInteger(min, max) {
return Math.floor(Math.random() * (max  min + 1)) + min;
}
Or any random number between min (included) and max (not included):
function randomNumber(min, max) {
return Math.random() * (max  min) + min;
}
Useful examples (integers):
// 0 > 10
Math.floor(Math.random() * 11);
// 1 > 10
Math.floor(Math.random() * 10) + 1;
// 5 > 20
Math.floor(Math.random() * 16) + 5;
// 10 > (2)
Math.floor(Math.random() * 9)  10;
** And always nice to be reminded (Mozilla):
Math.random() does not provide cryptographically secure random
numbers. Do not use them for anything related to security. Use the Web
Crypto API instead, and more precisely the
window.crypto.getRandomValues() method.