Javascript multiplying incorrectly, causing incorrect rounding

Question

When I pull the values I want to multiply, they're strings. So I pull them, parse them as floats (to preserve the decimal places), and multiply them together.

LineTaxRate = parseFloat(myRate) * parseFloat(myQuantity) * parseFloat(myTaxRateRound);

This has worked for 99% of my invoices but I discovered one very odd problem.

When it multiplied: 78 * 7 * 0.0725

Javascript is returning: 39.584999999999994

When you normally do the math in a calculator its: 39.585

When all is said and done, I take that number and round it using .toFixed(2)

Because Javascript is returning that number, it's not rounding to the desired value of: $39.59

I tried Math.round() the total but I still get the same number.

I have thought of rounding the number to 3 decimals then two, but that seems hacky to me.

I have searched everywhere and all I see is people mention parseFloat loses its precision, and to use .toFixed, however in the example above, that doesn't help.

Here is my test script i made to recreate the issue:

<script>
var num1 = parseFloat("78");
var num2 = parseFloat("7");
var num3 = parseFloat("0.0725");
var myTotal = num1 * num2 * num3;
var result = Math.round(myTotal*100)/100

alert(myTotal);
alert(myTotal.toFixed(2));
alert(result);
</script>

Answer 1

Floating points are represented in binary, not decimal. Some decimal numbers will not be represented precisely. And unfortunately, since Javascript only has one Number class, it's not a very good tool for this job. Other languages have decent decimal libraries designed to avoid precisely this kind of error. You're going to have to either accept one-cent errors, implement a solution server-side, or work very hard to fix this.

edit: ooh! you can do 78 * 7 * 725 and then divide by 10000, or to be even more precise just put the decimal point in the right place. Basically represent the tax rate as something other than a tiny fraction. Less convenient but it'll probably take care of your multiplication errors.

Answer 2

You might find the Accounting.js library useful for this. It has an "improved" toFixed() method.