Possibly wrong result after multiplication

[magnusjt]

Multiplying these two numbers together seems to yield the wrong result:

Money.fromDecimal(2090.50, Money.USD).multiply(Money.fromDecimal(8.61, Money.USD)).toDecimal() // = 17999.2

Should be 17999.21.

Internally I think this is what happens:

209050 * 8.61 // = 1799920.499999999.... Math.round(1799920.4999999998) // = 1799920

[tastypackets]

It tests if you passed in a rounder, if not it defaults to regular old Math.Round: if (!isFunction(fn)) fn = Math.round;

If you add the rounder of your choice to the formula you should get what you want. For example: Money.fromDecimal(2090.50, Money.USD).multiply(Money.fromDecimal(8.61, Money.USD),Math.ceil).toDecimal()

Result: 17999.21

[magnusjt]

The thing is, I don't want to use ceil here since every other number would also be rounded up. The issue is that 1799920.499999999.... is actually 1799920.5 in precise arithmetic, and should be round up.

[tastypackets]

Hmm perhaps someone can review and or write up some code to better handle rounding after the currency decimal place. I believe MDN had some documentation on this or maybe it was stack exchange I read about it.

At the end of the day though if financial precision is important down to the fraction of a cent it may be something that should be shifted off JS and to a language that better supports decimal values.

[magnusjt]

Maybe use something like https://github.com/MikeMcl/big.js ? I think you get most of the precision related stuff for free with a library like that.

Edit: And another bonus is that you get correct rounding for negative numbers.