Customize chances of picking element randomly

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?