merge-sort.js

function mergeSort(arr) {
	if (arr.length < 2) return arr;

	var mid = (arr.length - 1) >> 1;
	var l = mergeSort(arr.slice(0, mid + 1));
	var r = mergeSort(arr.slice(mid + 1));

	return merge(l, r);
}

function merge(s1, s2) {
	var result = [];
	while (s1.length && s2.length) {
		result.push(s1[0] < s2[0] ?
			s1.shift() :
			s2.shift());
	}
	return result.concat(s1.length ? s1 : s2);
}