You've definitely googled js random number. I know I have. This here is an explainer for me, myself, when I forget what each part of the equation does.
Getting a random number
Math.random() returns a number that looks like this: 0.7212660732474856. It's a decimal, it's long, and it's always between 0 and 1. It's inclusive of 0 but not of 1., which means that the number returned by Math.random() is between 0 and .9999999999999999.
Adding a maximum
If I want an integer, I take the returned number and multiply it by 10 or 100 or 322 - whatever I want the upper limit of my range to be. Say I want a number between 0 and 128.
Math.random() * 128The largest number possible is .9999999999999999 * 128. So, 127.99999999999999. The smallest possible will be 0 * 128. So, 0.
Cleaning up the number, two ways
The return from Math.random() looks all gnarly: 0.7212660732474856. That number times the top of my range isn't gonna look great. It's gonna be 92.32205737567816, for example. So I'm gonna wanna clean it up. Two ways to do that:
Math.ceil()
This function rounds a number up to the next largest integer. When using Math.ceil(), both 9.001 and 9.9 round up to 10.
Math.round()rounds the way I was taught in school (i.e. 2.5 rounds up tp 3 and 2.3 rounds down to 2).
So, calling Math.ceil() with an argument of anything larger than zero returns 1. Calling it with an argument of any non-decimal (e.g. 3, 37288, 81239817381273980000) returns that integer (albeit in exponential notation if it's larger than 20 digits).
Math.ceil(Math.random()) will almost always be 1 except on the very, very rare occasion that Math.random() returns exactly 0. What is Math.ceil() if Math.random() returns .00028311? .28883999? .998778668?
If I want a number between 0 and 128, Math.ceil(Math.random() * 128) will give me that. If I want a number between 1 and 128, I can tack a 1 on to the Math.ceil(), and take 1 off the top of the range: Math.ceil(Math.random() * 127) + 1
Math.ceil(Math.random() * 127) + 1
// say Math.random returns .00028311
Math.ceil(.00028311 * 127) + 1
// .00028311 * 127 = 0.035954969999999996
Math.ceil(0.035954969999999996) + 1
// the argument is larger than 0, so Math.ceil()is 1
1+1 Math.ceil(Math.random() * 127) + 1
// say Math.random returns .28883999
Math.ceil(.28883999 * 127) + 1
// .28883999 * 127 = 36.68267873
Math.ceil(36.68267873) + 1
// the closest integer, rounding up, from 36.68267873 is 37
37+1 Math.ceil(Math.random() * 127) + 1
// say Math.random returns .998778668
Math.ceil(.998778668 * 127) + 1
// .998778668 * 127 = 126.844890836
Math.ceil(126.844890836 ) + 1
// the closest integer, rounding up, from 126.844890836 is 127
127+1 Math.floor()
This function returns the largest integer less than or equal to a given number.
Say I want a number between 0 and 128 again. So, Math.random() * 128. Great. I get a number like 47.56094752691706 which I can then clean up using Math.floor().
Math.ceil(Math.random() * 128)
// say Math.random returns .00028311
Math.floor(.00028311 * 128)
// .00028311 * 127 = 0.035954969999999996
Math.floor(.03623808)
// rounding toward zero, Math.floor(.03623808) is 0 To get a number between 1 and 128, I've got to add a 1, just like when working with Math.ceil().
Math.ceil(Math.random() * 128) + 1
// say Math.random() returns .28883999
Math.floor(.00028311 * 128) + 1
// .00028311 * 128 = 36.97151872
Math.floor(36.97151872) + 1
// the largest integer less than 36.97151872 is 36
36 + 1 Math.floor(Math.random() * 128) + 1
// say Math.random() returns .998778668
Math.floor(.998778668 * 128) + 1
// .998778668 * 128 = 127.843669504
Math.floor(127.843669504) + 1
// the largest integer less than 127.843669504 is 127
127 + 1TL;DR
For a number between 0 and 128, inclusive:
Math.ceil(Math.random() * 128)
Math.floor(Math.random() * 129)
For a number between 1 and 128, inclusive:
Math.ceil(Math.random() * 127) + 1
Math.floor(Math.random() * 128) + 1
The 1 is outside the Math. method to account for the rare occasions that Math.random() returns exactly 0.