MergeSort implementation in JavaScript

merge-sort.js

function mergeSort(input) {
	const merge = (left, right) => {
		const merged = [];
		while (left.length && right.length) {
			if (left[0] < right[0]) {
				merged.push(left.shift());
			} else {
				merged.push(right.shift());
			}
		}
		return [...merged, ...left, ...right];
	};
	const sort = source => {
		if (source.length === 1) {
			return source;
		}
		const mid = Math.floor(source.length / 2);
		const left = source.slice(0, mid);
		const right = source.slice(mid);
		return merge(sort(left), sort(right));
	};
	return sort(input);
}