There's been a "port" of the Java BigDecimal class (I think it's here: http://freshmeat.net/projects/js_bigdecimal/ ) for a long time. I looked at it a long time ago and it seemed kind-of cumbersome and huge, but (if that's the one I'm thinking of) it's been used as part of some cryptography tools so there's a decent chance that it works OK.

Because cryptography is a likely area to generate a need for such things, that's a good way to snoop around for such packages.

edit: Thanks @Daniel (comment to question) for this older SO question: https://stackoverflow.com/questions/744099/javascript-bigdecimal-library

var decimal = n - Math.floor(n)

Although this won't work for minus numbers so we might have to do

n = Math.abs(n); // Change to positive
var decimal = n - Math.floor(n)