gistfile1.js

function merge(ray1, ray2) {
	var out = [];
	var index1 = 0;
	var index2 = 0;
	var length1 = ray1.length;
	var length2 = ray2.length;
	while (index1 < length1 && index2 < length2) {
		var item1 = ray1[index1];
		var item2 = ray2[index2];
		if (item1 <= item2) {
			out.push(item1);
			++index1;
		} else {
			out.push(item2);
			++index2;
		}
	}
	return out.concat(index1 < length1
	  ? ray1.slice(index1, length1)
	  : ray2.slice(index2, length2)
	);
}

function mergesort(arr) {
  return arr.length <= 1
    ? arr
	: merge(mergesort(arr.slice(0, arr.length / 2)),
	    mergesort(arr.slice(arr.length / 2, arr.length)));
};

console.log(mergesort([]))
console.log(mergesort([1]))
console.log(mergesort([2, 1]))
console.log(mergesort([3, 2, 1]))