Javascript arithmetic error

Question

Today I went through a strange problem, a simple calculation of the sum of the difference according to the ordination.

If in the browser console run this calculation below:

2.3+2.3+2.1

Would the expected value be 6.7 correct? Yes, but from 6.699999999999, so far so good, it is understandable....

But.. if I change the order of the values by putting the lowest first.

2.1+2.3+2.3

Surprise "mothafoca"! 6.7

Note: I can not use toFixed because I do not want to harness this number, where can fix me a problem now, but result me in something worse there in front.

A solution that happened in my head but I refuse to use it would be.

soma = 0;
[2.3, 2.3, 2.1].sort().forEach( valor => {
   soma += valor
});

Answer 1

The arithmetic of fractional numbers is never exact (at least for nonmultiple numbers of 1/2 powers). The exact solution was pure coincidence.

One way to solve the problem is to convert the problem into an integer arithmetic (or fixed point, not available in Javascript).

Thus, assuming one decimal place is always used, the sum would be:

   23+21+23 = 67 // Exato!

Still, by showing the result, 67/10 could result in something like 6,7000001 or 6,999999...