Using toFixed(2) and math round to get correct rounding

Question

I would like to find a function that will return this kind of formatted values :

1.5555 => 1.55
1.5556 => 1.56
1.5554 => 1.55
1.5651 => 1.56

toFixed() and math round return this value :

1.5651.fixedTo(2) => 1.57

This will be usefull for money rounding.

You have the fourth decimal affect the second decimal? When ever would you use this? — Matti Virkkunen, May 18 '10 at 20:50
Why would you want to round in such a strange way? How would this be useful for money rounding? — sth, May 18 '10 at 20:55
Your examples don't really seem to be getting your point across. What's the rule you're trying to enforce, exactly? Round down on a final "5" digit, unless the next digit is > 5? — Mark Bessey, May 19 '10 at 00:22
Indeed, this is a strange way of rounding. Doesn't really make any sense to me. — Sasha Chedygov, May 19 '10 at 00:51
Isn't that how Richar Pryor made all that money in that Superman movie? — kennebec, May 19 '10 at 01:33
@adrien334: Thanks for ignoring all our comments completely. — Matti Virkkunen, May 21 '10 at 21:32

Mark Bessey · Accepted Answer · 2011-04-04T19:58:27.160

6

How about this?

function wacky_round(number, places) {
    var multiplier = Math.pow(10, places+2); // get two extra digits
    var fixed = Math.floor(number*multiplier); // convert to integer
    fixed += 44; // round down on anything less than x.xxx56
    fixed = Math.floor(fixed/100); // chop off last 2 digits
    return fixed/Math.pow(10, places);
}

1.5554 => 1.55

1.5555 => 1.55

1.5556 => 1.56

1.5651 => 1.56

So, that works, but I think you'll find that it's not a generally-accepted way to round. http://en.wikipedia.org/wiki/Rounding#Tie-breaking

edited Apr 04 '11 at 19:58

answered May 19 '10 at 00:43

Mark Bessey

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Sorry, copy and paste error while formatting for the site. Oh, and you're clear on why you shouldn't use this rounding method, right? – Mark Bessey Apr 04 '11 at 20:04
it was not for my consumption ;) however the one [asking](http://stackoverflow.com/questions/5535394/round-off-decimal-using-javascript) wanted to round amounts from .181 to .18 and .185 to .19 which I am never so sure of. toFixed(2) feels wrong in these cases. – mplungjan Apr 05 '11 at 06:00

score 3 · Answer 2 · edited Aug 28 '14 at 04:06

3

And standard function

fixedTo = function (number, n) {
  var k = Math.pow(10, n);
  return (Math.round(number * k) / k);
}

and then call

fixedTo(1.5555, 2)  // 1.56
fixedTo(1.5555, 2)  // 1.556
fixedTo(0.615, 2)   // 0.62

edited Aug 28 '14 at 04:06

dperish

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answered May 18 '10 at 21:09

Jaroslav Moravec

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This solution doesn't work for `1.005`. I recommend using [this function instead](http://stackoverflow.com/a/23204425/573634) – Hanna Mar 29 '16 at 16:53

score 0 · Answer 3 · answered Mar 26 '12 at 22:41

0

You can use the native Number.toFixed() method.

parseFloat(10.159.toFixed(2))

The above returns 10.16. Number.toFixed() returns a string, so I use parseFloat to convert it to a number.

answered Mar 26 '12 at 22:41

Johnny Oshika

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Using toFixed(2) and math round to get correct rounding

3 Answers3

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