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Without jQuery, how can I round a float number to 2 non-zero decimals (but only when needed - 1.5 instead of 1.50)?

Just like this:

2.50000000004 -> 2.5 2.652 -> 2.65 2.655 -> 2.66 0.00000204 -> 0.000002 0.00000205 -> 0.0000021

I tried this code:

var r = n.toFixed(1-Math.floor(Math.log10(n)));

but n=0.00000205 implies r=0.0000020, which is in conflict with conditions above. But n=0.0000020501 implies r=0.0000021, which is OK, so the error is only for 5 as a last decimal, which should be rounded up.

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  • 5
    Possible duplicate of Round to at most 2 decimal places in JavaScript Commented Aug 8, 2016 at 20:06
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    @PrasadShinde This will round the last 2 examples to 0, which is not what the OP wants. Commented Aug 8, 2016 at 20:10
  • It is not a duplicate. I don't want to round to 2 decimal places, I want to round to 2 NON-ZERO decimal places. Commented Aug 8, 2016 at 20:42
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    The term you're looking for is "rounding to precision". See developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… Commented Aug 8, 2016 at 20:49
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    The reason you get 0.000020 for that is related to the imprecision of the floating point representation. 0.0000205 cannot be represented as exactly that value in floating point. For numbers that can, the result is as you would expect. NB: Ad-hoc coded functions that try to do this will probably all suffer from the same problem. Commented Aug 8, 2016 at 21:40

2 Answers 2

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This should do what you want:

function twoDecimals(n) { var log10 = n ? Math.floor(Math.log10(n)) : 0, div = log10 < 0 ? Math.pow(10, 1 - log10) : 100; return Math.round(n * div) / div; } var test = [ 2.50000000004, 2.652, 2.655, 0.00000204, 0.00000205, 0.00000605 ]; test.forEach(function(n) { console.log(n, '->', twoDecimals(n)); });

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4 Comments

n = 0.00000305 is rounded correctly to 0.0000031, but n = 0.00000605 is rounded to 0.000006, not to 0.0000061.
Yeah, a bit better. Idk why, but there are still some values that ain't rounded properly, like 0.00000105 and 0.00000805.
Alas, these ones are the result of the inevitable approximation performed by IEEE 754 encoding. console.log(0.00000105 * 1E7) -> 10.499999999999998
Yeah, I can see it. 0.000008050000000000001 is rounded correctly....several very particular values just don't matter. Thank you!
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Thank's for the @Arnauld answer. Just added decimals parameter.

function roundToDecimals(n, decimals) { var log10 = n ? Math.floor(Math.log10(n)) : 0, div = log10 < 0 ? Math.pow(10, decimals - log10 - 1) : Math.pow(10, decimals); return Math.round(n * div) / div; } var numDecimals = 2 var test = [ 2.50000000004, 2.652, 2.655, 0.00000204, 0.00000205, 0.00000605 ]; test.forEach(function(n) { console.log(n, '->', roundToDecimals(n, numDecimals)); });

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