If you store integer amounts of the smallest currency subdivision, you're already ready for foreign currencies. You just need to specify how each currency converts from a representation like `$60` to an integer count like `6000`. This even accounts for non-decimal currencies (which basically aren't in modern use) - just set a conversion factor of 500 or 60 or whatever instead of 100.

Not every currency has a decimal amount. For example, there are no decimals in the Japanese yen.

Not to mention then you need to translate that for all the currencies, {pounds: 60, <whatever a decimal of a pound is>: 00 }

Or just use strings. That's what we do for basic webshop math - any naive attempt at doing eg a multiplication will break, requiring you to use the supplied math library (just like with objects...)

But: with strings you still have pass-by-value. So you can pass the values anywhere without worrying that whoever received the values will be modifying your object. And we still have the infinite range that integers can't give, which is especially useful when doing the multiplications for VAT and coupon codes etc..

The only remaining danger is that '+' works on strings, but we should notice those fast enough - it hasn't been a real issue yet.

(And if we need to start optimizing things at some point, I'd expect to have a better starting point when the data is already in a string than if we have to copy around objects all the time)