In Ruby 2100 is an exact integer. In JavaScript, it is promoted to a poor floating point approximation. If you want to try to preserve Ruby's numeric semantics, you can't use native math.
Gah, the website ruined what I was typing. I meant 2 to the power of 100. Which is a 100 bit integer, and certainly not correctly calculated in JavaScript.
While your general point is correct, the details aren't. Powers of 2 are represented exactly in floating point (for any number, only the 53 most significant bits can be stored). So 2^100 + 1 is an example of something that's non-representable in JS.
We could have a philosophical debate about what number a finite precision representation represents when you exceed its precision. But on a practical level, if you print it, what do you get? An exact integer, or a floating point representation?