Customize chances of picking element randomly

Question

I have two defined objects: x and y
If I do following, chances of getting either x or y are equal – 1 of 2:

var primary = [x, y];
var secondary = primary[Math.floor(Math.random() * primary.length)];

This would take a 1 of 3 (smaller) chances of getting y:

var primary = [x, x, y];
// secondary unchanged

etc.

But I believe, this is bad practice because if I'd wanted to set infinitesimal chances (e.g. 1 of 1e9) of getting y, I would have to do something extremely wasteful like this:

var primary = new Array();
for (i = 1e9 - 1; i--; i) primary.push(x);
primary.push(y);
var secondary = primary[Math.floor(Math.random() * primary.length)];

Is there a better way to do this in JavaScript?

Answer 1

Without digging too much into ECMAScript specification and its actual implementations, Math.random() appears to produce a number from the range 0..1 in a smooth fashion. This means that the number is 50% likely to be less than 0.5, 25% likely to be less than 0.25, 10% likely to be less than 0.1, etc.

To get x 17% of the time (and, conversely, y 83% of the time), one could use the corresponding number to be a gateway for Math.random()’s results:

const x = "x";
const y = "y";

function getRandom() {
  return Math.random() < 0.17 ? x : y;
}

const value = getRandom();
// 17% "x", 83% "y"

This works fine for two values, but working with lists of 3+ elements would require different thinking.