Pretty numbers in Javascript

pretty_numbers.js

function pretty_number(num, opts)
{
	var defaultOpts = {
		short: true,
		lowerCase: false,
		addCommas: true,
		round: 2
	};
	
	if (typeof num != "number")
	{
		return "";
	}
	
	function round(num, dec)
	{		
		num = num * Math.pow(10, dec);

		num = Math.round(num);

		num /= Math.pow(10, dec);

		return num;
	}

	if (typeof opts == 'undefined')
	{
		opts = {};
	}

	for (var i in defaultOpts)
	{
		opts[i] = (typeof opts[i] != 'undefined')? opts[i] : defaultOpts[i];
	}

	if (opts.short)
	{
		var decimal_places = Math.floor(Math.log(num) / Math.log(10));
	
		var dec = [{
			'suffix': 'T',
			'divisor': 12
		},{
			'suffix': 'B',
			'divisor': 9
		},{
			'suffix': 'M',
			'divisor': 6			
		},{
			'suffix': 'K',
			'divisor': 3
		},{
			'suffix': '',
			'divisor': 0
		}];
		
		for (var i in dec)
		{
			if (decimal_places > dec[i].divisor)
			{
				num = round((num / Math.pow(10, dec[i].divisor)), 2 - (decimal_places - dec[i].divisor));
				
				if (num >= 1000 && i > 0)
				{
					decimal_places -= 3;
					num = round(num / 1000, 2 - (decimal_places - dec[i - 1].divisor));
					num += dec[i - 1].suffix;
				}
				else
				{
					num += dec[i].suffix;
				}
				
				break;
			}
		}
		
		num = '' + num;

		if (opts.lowerCase)
		{
			num = num.toLowerCase();
		}
	}
	else if (opts.addCommas)
	{
		var decnum = ('' + (round(num, opts.round) - Math.floor(num))).substr(2);	
		
		var tempnum = '' + Math.floor(num);
		num = '';
		for (i = tempnum.length - 1, j = 0; i >= 0; i--, j++)
		{
			if (j > 0 && j % 3 == 0)
			{
				num = ',' + num;
			}
			num = tempnum[i] + num;
		}
		
		if (decnum > 0)
		{
			num = num + '.' + decnum;
		}
	}

	return num;
}

function commify(num)
{
	return pretty_number(num, {'short': false});
}

seems like it doesn't work well for negative margins (the addition of the comma)