This evening's academic exercise was to write your basic 1234 => 1,234 function as terse as possible, and without a loop. At first I thought I could do it with just a clever regex, and maybe I could have, but I abandoned that approach.

This is what I ended up with:

function formatNumber(n) {
    if (!isFinite(n)) {
        return n;
    }

    var s = ""+n, abs = Math.abs(n), _, i;

    if (abs >= 1000) {
        _  = (""+abs).split(/\./);
        i  = _[0].length % 3 || 3;

        _[0] = s.slice(0,i + (n < 0)) +
               _[0].slice(i).replace(/(\d{3})/g,',$1');

        s = _.join('.');
    }

    return s;
}

I could have omitted the storage of Math.abs(n) and the short circuit for numbers not needing comma injection, but eh. No need to sacrifice runtime performance for a few characters.

The basic principle is to use string replace to insert commas on only the chunk of the number that needs them. intAsString.length % 3 || 3; will return the index of the first character that needs a comma before it (for positive values, hence the abs), so everything beyond that point needs to be divided into three digit chunks, each preceded by a comma.

Update - 1/16/2009

I happened across a similar function, based on PHP's number_format function. It prompted me to adapt in the extra params to support precision and specified characters for decimal and thousands divider.

I put the result in a gist. I put together this page to confirm they were behaving comparably.

Update - 3/18/2009

Updated to account for input 1000. > should have been >= Oops! Thanks to Simon for helping me notice that :)

I also put the barebones function in a gist