A readable merge sort implementation in JS

algorithm.txt

Ref: https://en.wikipedia.org/wiki/Merge_algorithm#Merging_two_lists

algorithm merge(A, B) is
    inputs A, B : list
    returns list

    C := new empty list
    while A is not empty and B is not empty do
        if head(A) ≤ head(B) then
            append head(A) to C
            drop the head of A
        else
            append head(B) to C
            drop the head of B

    // By now, either A or B is empty. It remains to empty the other input list.
    while A is not empty do
        append head(A) to C
        drop the head of A
    while B is not empty do
        append head(B) to C
        drop the head of B

    return C

mergeSort.js

function mergeSort(arr, low, high) {
	if (low < high) {
		const mid = low + Math.floor((high - low) / 2);
		mergeSort(arr, low, mid);
		mergeSort(arr, mid + 1, high);
		merge(arr, low, mid, high);
	}
}

function merge(arr, low, mid, high) {
	const temp = new Array(high - low + 1);
	let c = 0;			// temp pointer
	let a = low;		// list A pointer
	let b = mid + 1;	// list B pointer

	while (a <= mid && b <= high) {
		if (arr[a] < arr[b]) {
			temp[c++] = arr[a++];
		} else {
			temp[c++] = arr[b++];
		}
	}
	// By now, either A or B is empty.
	// add elements left in the list A
	while (a <= mid) {
		temp[c++] = arr[a++];
	}
	// add elements left in the list B
	while (b <= high) {
		temp[c++] = arr[b++];
	}

	// copy temp to original list
	for(let k = 0; k < temp.length; k++) {
		arr[k + low] = temp[k]
	}
}

let arr = [ 4, 5, 3, 2, 4, 1 ];
console.log(arr); // [ 4, 5, 3, 2, 4, 1 ]
mergeSort(arr, 0, arr.length - 1);
console.log(arr); // [ 1, 2, 3, 4, 4, 5 ]