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Both of these are valid notation. Json spec also says that numbers are floats, so it's perfectly acceptable to denote them this way. This is why currency should never use a number notation, and instead use a string. If it must be precise, use a string.
Or, use the smallest denomination. In the US, that’s typically tenths of a penny. So, $1 = 1000. Then everything uses integer arithmetic.
Of course, JS doesn’t actually have integers, so yeah, strings are probably best.
But the second you go international that goes out the window. There are currencies with 3 and 4 digits of precision. There are currencies that are integers. Keeping track of that is a nightmare using a numerical value. It's safest just to represent it as a string.
Every currency has a smallest denomination.
Right but then you have to pass that information around so people know how to deserialize it, and it means things like the UI need to do exact currency conversions on their side that must match the server too. So if you are doing USD you would not only need to pass 1000 to denote $10 but also the currency name, USD, and it's precision value of 2. However if you are using the Dinar, and they pass the same 1000 they would need to make sure they pass the precision of 3, and the UI would need to calculate that. (And remember JS is floats, so you run the risk there that the value may not be the same as what the server would see)
The best course of action is to format it on the server. I've found that passing the currency code is good along with the stringified value of "10.00" or in the Dinar case "1.000". That way the UI knows exactly what it should show. Its also extremely rare that someone needs to modify a value on the frontend, but if they do most currency libraries prefer string anyway.
Source: I've done FinTech for trading companies, banks, and payment processors. There are a lot of gotchas with money
If you’re passing a string, and you don’t know what currency it is, you have the exact same problem as passing an int and not knowing what currency it is. USD’s smallest denomination is 1/10 cent (gas stations usually charge in tenths of a cent, and half pennies are still legal tender, even though they’re not minted anymore), so the string representation in your examples would be exactly the same for USD and Dinar.
I agree that the best way to represent it between server and client is as a string, but that doesn’t work when you need to perform calculations, so in that case, the best way to do the calculations is to use the smallest denomination, then use banker’s rounding on the result, then use the int for storage and turn it back into a string representing the default denomination for transit. Or, just use ints representing the smallest denomination everywhere except displaying to the user. Even JavaScript can handle integer arithmetic.
Source: I’ve also done fintech for loan and retail companies. Yes, there are definitely a lot of gotchas, so using integers is best when you need to do calculations.