Home > Posts > HowTo: round a number up to N decimal digits in Javascript

HowTo: round a number up to N decimal digits in Javascript

Was just trying to round-off some Google Maps coordinates for display in Javascript up to 3 decimal digits and that was a bit like a blast from the past (the end of the ‘90s to be more accurate)…

So here’s my contributed answer at:
https://stackoverflow.com/questions/2221167/javascript-formatting-a-rounded-number-to-n-decimals

This works for rounding to N digits (if you just want to truncate to N digits remove the Math.round call and use the Math.trunc one):

function roundN(value, digits) {
   var tenToN = 10 ** digits;
   return /*Math.trunc*/(Math.round(value * tenToN)) / tenToN;
}

Had to resort to such logic at Java in the past when I was authoring data manipulation E-Slate components. That is since I had found out that adding 0.1 many times to 0 you’d end up with some unexpectedly long decimal part (this is due to floating point arithmetics).

A user comment at Format number to always show 2 decimal places calls this technique scaling.

Some mention there are cases that don’t round as expected and at http://www.jacklmoore.com/notes/rounding-in-javascript/ this is suggested instead:

function round(value, decimals) {
  return Number(Math.round(value+'e'+decimals)+'e-'+decimals);
}

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