How can I find the length of a number?

Ok, so many answers, but this is a pure math one, just for the fun or for remembering that Math is Important:

var len = Math.ceil(Math.log(num + 1) / Math.LN10);

This actually gives the "length" of the number even if it's in exponential form. num is supposed to be a non negative integer here: if it's negative, take its absolute value and adjust the sign afterwards.

Update for ES2015

Now that Math.log10 is a thing, you can simply write

const len = Math.ceil(Math.log10(num + 1));

Could also use a template string:

const num = 123456
`${num}`.length // 6

var x = 1234567;

x.toString().length;

This process will also work forFloat Number and for Exponential number also.