merge.js

mergeSort = (arr) => {
	if (arr.length < 2) return arr;
	var middle = Math.floor(arr.length / 2);
	var left = arr.slice(0, middle);
	var right = arr.slice(middle, arr.length);
	return merge(mergeSort(left), mergeSort(right));
}
merge = (left, right) => {
	var result = [];
	while (left.length && right.length) {
		if (left[0] <= right[0]) {
			result.push(left.shift());
		} else {
			result.push(right.shift());
		}
	}
	while (left.length) result.push(left.shift());
	while (right.length) result.push(right.shift());
	return result;
}

console.log(mergeSort([3, 1, 6, 2, 7]));