This should be more than enough for most money amounts, but when we start using cents we immediately need to remove 2 orders of magnitude of precision. In fact, the number type has 53 bits set aside for the integer part of the number, which is roughly 10¹⁵. The second issue is that integers in javascript are not infinitely big. We can somewhat get around this, but we soon get into the next issue… The reason is that our multiplier is still a float. Take the example we saw earlier, but replace the money amount with an integer number of cents: 209050 * 8.61 => 1799920.499999999.ĭamnit. The first issue is that multiplication is still a problem even with integers. This sounds pretty reasonable, but comes with some major pitfalls. It is often said that money should be handled by using integer amounts of cents. Rounding this number gives the wrong result! Why integers are not good enough There’s an inherrent lack of precision in this number type that just isn’t meant for precision calculations. If you didn’t already know, floating point numbers are not to be trusted with money. Pitfalls of floating point numbers in javascript It’s useful then, to handle price and money as two separate things, or at least be explicit about when an amount is money and when it is a price. It is only when we sum all the partial amounts and round the result that we arrive at the money data type. It is given with a higher precision than a cent, even though less than a cent is not a valid amount of money in and of itself. Think of the price of gasoline for example. A price, however, may be given with a higher precision. There is no such thing as half a cent in the money data type. and more Money is not PriceĪs described already, money has a fixed number of decimals. Let’s define our money datatype like this: number number of decimals currencyĪnd some operations we can perform on money: Money.add(Money) Money.subtract(Money) Money.multiply(Factor) Money.divide(Divisor) Money.toCurrency(Currency, Factor, Unit) Money.abs() Money.equals(Money) pare(Money) // compare can be lte, lt, gte, gt, eq, etc. How money is rounded after operations such as multiplication is not always up to you! There may be different regulations in different domains.Each step of a calculation needs to handle this in a thoughtful way. The number of decimals in money is a standardized thing, and varies between different currencies. The precision of money should be explicit.Money can only be multiplied with a unit-less factor. It is not correct to multiply two money objects (it is maybe obvious to you now, but if we think about money as just a number, it is very easy to make this mistake).It is not correct to add together money of two different currencies.Any operation on money should take this into account. This data type has a number with a specific number of decimals, and a specific currency. It’s better to think of money as a separate data type. Money as a data typeįirst of all, we need to acknowledge that money is not just a plain old number. In this article we’ll look at all the pitfalls and solutions to handling money in javascript.
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