mergeSort.js

const mergeSort = (arr) => {

	if (arr.length < 2) return arr;

	const middle = Math.floor(arr.length / 2);
	const left = arr.slice(0, middle);
	const right = arr.slice(middle);

	return merge(mergeSort(left), mergeSort(right));
}

const merge = (left, right) => {
	const result = [];
	while (left.length && right.length) {
		if (left[0] <= right[0]) {
			result.push(left.shift())
		} else {
			result.push(right.shift())
		}
	}
	return result.concat(left, right);
}

console.log(mergeSort([4, 2, 7, 1, 3, 5, 9]));