Rounding precision

[tvainika]

I modified your unit test should default rounding when parsing to use var round1 = currency(1.2349) instead of current 1.234 and that caused rouding to break (not half-up).

[scurker]

Currently, the bulk of the rounding happens inside of parse and relies on Number.toFixed() for decimal rounding.

currency.js/src/currency.js

Lines 76 to 77 in 2eaefbe

	// Handle additional decimal for proper rounding
	v = v.toFixed(1);

Part of the issue at the moment, is only half cents are taken into consideration in this calculation. So 1.2349 will get parsed internally as 123.4 which eventually rounds down to 123. Accounting for more decimals could potentially lead to a precision loss on higher numbers, but I need to do some testing around this to verify.

[charlie-s]

1.2349 appears to be getting parsed internally as 123.49, and then rounded to 124, due to Number(123.49).toFixed(1) rounding .49 to .5, and then the final round call turning 123.5 into 124.

Is there harm in using .toFixed(2)?

I'll give you my use case – 7.35% sales tax on $81.97, is resulting in $6.03 of sales tax when the actual amount is $6.02.

[scurker]

It's more of an issue of agnostically handling any arbitrary precision. Using .toFixed(2) will solve a greater amount of cases, but it feels like it's not truly addressing the issue. It's definitely something that can make the next release, but I want to ensure there's a better scalable solution long term.

[charlie-s]

I appreciate that immensely because as I was looking at the code and trying to think about this in higher and lower levels, I was coming to the same conclusion – I wish it could be solved in a more sound or proper way. If you want to discuss here or in a different issue, tag me. It's an interesting problem in general and I'm curious as to where it would land.

[scurker]

Fortunately, toFixed(x) does not exhibit the same floating point issues since it's a string so even toFixed(100) would work and still be safe, so it's possible it could be a configurable option. But mostly I haven't been able to center on a good solution yet that resolves a ton of edge cases without bloating the library a whole bunch.

[popod]

What about this to round without errors ?

let roundedPrice = +(Math.round(price + 'e2') + 'e-2')

see: https://stackoverflow.com/a/19722641/8697449

[scurker]

I bumped up the internal precision slightly to account for errors. Should handle a greater number of cases within reasonable values.