Here are some tests you can run to check out probable.js's probability distribution. These tests run the rollDie method, which uses Math.floor(Math.random() * sides. That does have some bias, but not enough to affect the applications for which probable is used. See for yourself.

If you have an interest in the fairness of Smidgeo Dice, it's best to run these tests in Chrome. Chrome executes JavaScript with V8, the same JavaScript engine that runs Node, which is what Smidgeo Dice runs probable on.