Javascript does have floating point support. But anyway, for financial records, the simplest implementation would simply be storing your values in standard integers. You may either declare an integer to represent the amount in cents, or two integers one for dollars and one for cents.

So $18.57 would become 1857cents in the first technique or 18 dollars and 57 cents in the second.

This has the added advantage of being completely accurate, as integers are stored purely as unique binary representation, there would be no such thing as rounding errors.

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)

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