gistfile1.js

function rangify(list){
	list = list.map(Number).sort(function(a, b){ return a - b; });
	list.push(0);

	var adder, i, list, number, rangeStart, ranges, _fn, _len;
	ranges = [];
	rangeStart = list[0];
	adder = 0;
	_fn = function(number) {
		if (number !== adder + 1 || i === list.length - 1) {
			ranges.push((rangeStart === adder) && ("" + adder) || ("" + rangeStart + "-" + adder));
			rangeStart = number;
		}
		return adder = number;
	};
	for (i = 0, _len = list.length; i < _len; i++) {
		number = list[i];
		_fn(number);
	}

	return ranges.slice(1);
}

rangify('1,2,3,8,9,12,15,17,18,19,20,21,22,23'.split(',')).join(',');