Basic JavaScript merge sort for my own understanding with a optional custom compare func

merge-sort.js

// original: https://stackabuse.com/merge-sort-in-javascript/

/**
 * Default comparison
 * @param {number} left 
 * @param {number} right 
 * @returns {number}
 */
function defaultCompare(left, right)
{
	return left - right
}

/**
 * Merge two sorted sub-arrays into a single sorted array
 * @param {any[]} left
 * @param {any[]} right
 * @param {function} compareFn
 */
function merge(left, right, compareFn)
{
	const arr = [];

	// empty sub-arrays until one is completely empty
	while(left.length > 0 && right.length > 0)
	{
		// custom comparator check
		// value < 0 assumes left < right
		// shift() removes first element and returns it
		if (compareFn(left[0], right[0]) < 0)
		{
			arr[arr.length] = left.shift();
		}
		else
		{
			arr[arr.length] = right.shift();
		}
	}

	// concat the newly merged array and any leftover elements
	return [ ...arr, ...left, ...right ];
}

/**
 * Sort an array using the MergeSort algorithm
 * 
 * Splits the array up into multiple sub-arrays and puts it back together again
 * @param {any[]} arr
 * @param {function} [compareFn]
 */
function sort(arr, compareFn = defaultCompare)
{
	// prevent arr splitting if too small
	if (arr.length < 2)
	{
		return arr;
	}

	// get half the size of arr
	const half = arr.length / 2;

	// remove the left half from arr
	// store it in a new sub-array
	const left = arr.splice(0, half);

	return merge(sort(left, compareFn), sort(arr, compareFn), compareFn);
}


const payload = [100, -1, 50, 22, 2000, -50, 67, 4, 9, 250];
const payload_adv = [
	{ test: 1, foo: "bar", num: 100 },
	{ test: 1, foo: "bar", num: -1 },
	{ test: 1, foo: "bar", num: 50 },
	{ test: 1, foo: "bar", num: 22 },
	{ test: 1, foo: "bar", num: 2000 },
	{ test: 1, foo: "bar", num: -50 },
	{ test: 1, foo: "bar", num: 67 },
	{ test: 1, foo: "bar", num: 4 },
	{ test: 1, foo: "bar", num: 9 },
	{ test: 1, foo: "bar", num: 250 },
]

console.log("BEFORE: ", payload);

const sorted = sort(payload);

console.log("AFTER: ", sorted);

console.log("BEFORE: ", payload_adv);

const sorted_adv = sort(payload_adv, (left, right) => left.num - right.num);

console.log("AFTER: ", sorted_adv);