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Comment by tylergetsay | Hacker News Reader

For simple accounting I've always used integers and done all operations in cents, only converting on the frontend. what's my downside here? I guess it wouldn't support unit prices less than a penny

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If you have different currencies you need to keep track of the number of decimals used, e.g. YEN has 0 decimals, bitcoin has 6, etc. It could even change over time like Icelandic ISK did in 2007. If you have different services with different knowledge about this you're in big trouble. Also prices can have an arbitrary number of decimals up until you round it to an actual monetary amount. And if you have enough decimals, the integer solution might not have enough bits anymore, so make sure you use bigints (also when JSON parsing in javascript).

Example in js: Number(9999999.999999999).toString() // => 9999999.999999998

And make sure you're not rounding using Math.round

Math.round(-1.5) // => -1

or toFixed

(2090.5 * 8.61).toFixed(2) // => 17999.20 should have been 17999.21 8.165.toFixed(2) // => 8.16 should be 8.17

The better solution is to use arbitrary precision decimals, and transport them as strings. Store them as arbitrary precision decimals in the database when possible.

Also many types of operations could give you the wrong result from incorrect rounding. E.g. let's say you're calculating 10% of $1.01 ten times and adding the result together. The correct result is $1.01, but with your method you will get $1.00.

The correct answer will depend on the specifics of your environment. In some places, tax is calculated per line item. If you go to a dollar store and buy 10 items with 7.3% sales tax, it adds up without those 0.3¢ bits. In other places, the tax is supposed to be calculated on the total for the tax category in the sale. If you wanted to keep it by line item you'd need the extra digits of precision.

Well, yes, which is why you need to be in control of your rounding and not just let the width of data type you chose for the implementation dictate that.

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