Around at 1.78e308 (1.78 x 10, 308 times) double floating point variables just... stop working. It's too large. Some games work around this, somehow, by simplifying the numbers further or changing how the number is stored. Others consider 1e308 equivalent to actual infinity, and make that the point to prestige and do it all again.
NaN stands for "Not a Number". Technically, the value it is is "INF" for infinity, but I'm putting that variable through a $variable.toLocaleString() to put the commas in and it doesn't play nice when it's given a string.
I'm glad you enjoyed the game, though! It means a lot to read comments like this!