Javascript rounding to the desired number of decimal places issue

Question

I use a function which rounds to the desired number of decimal places:

function modRound(value, precision)
{
    var precision_number = Math.pow(10, precision);
    return Math.round(value * precision_number) / precision_number;
}

It works correct, but not with 0.565. modRound(0.575, 2) gives 0.58 (correct), but modRound(0.565, 2) gives 0.56; I expect 0.57:

function modRound(value, precision) {
  var precision_number = Math.pow(10, precision);
  return Math.round(value * precision_number) / precision_number;
}
function test(num) {
  console.log(num, "=>", modRound(num, 2));
}
test(0.575);
test(0.565);

Why is this happening and what to do?

`.565 * 100` results in `56.49999999999999` see http://stackoverflow.com/questions/588004/is-floating-point-math-broken — VLAZ, Oct 27 '16 at 08:06
@JEY No, a `.5` will be rounded UP `Math.round(56.5) // -> 57` — VLAZ, Oct 27 '16 at 08:13
Actually, `modRound(0.575, 2)` also exhibits this behavior, because `0.575 * 100` is `57.49999999999999`. — T.J. Crowder, Oct 27 '16 at 08:14
@T.J.Crowder well, `.585` would work as expected. Is it worth changing the example in the OP? — VLAZ, Oct 27 '16 at 08:16
@T.J.Crowder fair point, I wasn't sure if it'd be good practice to do that. — VLAZ, Oct 27 '16 at 08:18
Alexey - See [this answer](http://stackoverflow.com/questions/1726630/formatting-a-number-with-exactly-two-decimals-in-javascript/21323330#21323330). — T.J. Crowder, Oct 27 '16 at 09:11

score -1 · Answer 1 · answered Oct 27 '16 at 08:09

-1

You can add a very small number like this:

function modRound(value, precision)
{
    var precision_number = Math.pow(10, precision);
    return Math.round((value+0.00000000001) * precision_number) / precision_number;
}
alert(modRound(0.565, 2))

here is the JSFiddle

answered Oct 27 '16 at 08:09

O_Z

1,515
9
11

This is just bad. This breaks other test cases, rather than fixing the root issue. – JosephGarrone Oct 27 '16 at 09:34

Javascript rounding to the desired number of decimal places issue

1 Answers1