Angular JS inaccurate substraction of two decimal numbers

Question

In my angular JS application I have three amounts in one object. In Chrome browser debugger i have following:

payment.amountForClosing = payment.amountRemaining - payment.amountReserved;

payment.amountRemaining has a value of 3026.2

payment.amountReserved has a value of 2478.4

after substraction payment.amountForClosing has a value of 547.7999999999997, and 547.8 is displayed. When user tries to make another payment closing, my validation logic returns an error indicating that there is not enough money to make payment closing because of state presented above.

Those amount values come from C# WebApi 2.0 backend, as System.Decimal types.

How to deal with that nature? Im relatively new to Javascript — vpetrovic, Aug 08 '16 at 08:14
Here is an [article](http://www.2ality.com/2012/04/number-encoding.html) which will help you to deal with that issue. — Hamlet Hakobyan, Aug 08 '16 at 08:19

score 3 · Accepted Answer · answered Aug 08 '16 at 08:16

3

When dealing with currency, worst thing that you can do is use floating point numbers, as you can see. Because of way how floats are stored, some operations may provide incorrect results. Only thing that will always be correct is multiplication/division by power of 10.

Best way is to store currency as Integer/BigInteger, or whatever, and save it with e.g. 4 decimal places so 12100 should represent 1.21

Then you can do subtraction/multiplication of integer numbers and at the end divide by power of 10.

My suggestion is that you transform those numbers to integer before any operation then divide it back to float.

answered Aug 08 '16 at 08:16

Aleksandar Varicak

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Would be my suggestion to! – JoeriShoeby Aug 08 '16 at 08:16
I came back to this problem, I am trying to solve it with integers: I just want to check if I am doing this right: function add(a, b) { return ((a * 10000) + (b * 10000)) / 10000; } – vpetrovic Nov 01 '16 at 12:14
If a and b got 4 decimal digits it is fine, if they got more i would suggest that you round or floor or ceil a * 10000 and b * 10000. – Aleksandar Varicak Nov 02 '16 at 07:16

score 1 · Answer 2 · edited May 23 '17 at 12:32

Floating point precision in Javascript has been discussed before (eg. here or here). You have the following options:

Convert all your numbers to integers.
Format the result to a fixed number of significant digits using .toFixed(2)
Use a special datatype for decimals like BigDecimal

Out of all of these I agree with @PerunSS that in your case, the best option would be converting the numbers to integers before any operation, and the converting them back.

Angular JS inaccurate substraction of two decimal numbers

2 Answers2