## 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);
}
```

## Prime links

http://io9.com/5819325/the-bizarre-mathematical-conundrum-of-ulams-spiral

http://scienceblogs.com/goodmath/2010/06/the_surprises_never_eend_the_u.php

http://img529.imageshack.us/img529/615/ulam3.png

http://imprompt.us/2010/ulam-spiral-take-2/

http://home.manhattan.edu/~peter.boothe/ulam.pdf

http://mathworld.wolfram.com/PrimeSpiral.html

http://blog.morphism.com/2010/05/building-numbers.html

http://blog.morphism.com/2010/07/pdfs-from-building-numbers.html

http://math.ucr.edu/home/baez/roots/

http://math.ucr.edu/home/baez/roots/polynomialrootssmall.png

http://incubator.quasimondo.com/flash/wheel_of_primes.php

http://incubator.quasimondo.com/flash/Primeverse_1024.html

http://rob.tarrfamily.com/share/Triangle.gif

http://www.numberspiral.com/p/p006.html

http://www.numberspiral.com/p/common_diff.html

http://punarvak.blogspot.com/2010/07/ulam-spirals-and-its-variants.html

btw, via the last link I came across to the very interesting “On-Line Encyclopedia of Integer Sequences” http://oeis.org/Seis.html (a service of http://oeisf.org/). Their search facility allows you to enter a series of numbers separated by space or comma and check if they exist in some known integer number series.