recursive merge sort: Average runtime O(log n)

mergeSort.js

function mergeSort(arr, low = 0, top = arr.length - 1) {
    if (low < top) {
        const mid = Math.floor((low + top) / 2);

        mergeSort(arr, low, mid);
        mergeSort(arr, mid+1, top);

        merge(arr, low, mid, top);
    }

    return arr;
}

function merge(arr, low, mid, top) {
    const lowHalf = new Array(mid - low);
    const topHalf = new Array(top - mid);

    // load elements from arr into each "half" array
    for (let k = mid; k > -1; k--) lowHalf[k] = arr[k];
    for (let k = top; k > mid; k--) topHalf[k-mid-1] = arr[k];

    let a = top;
    let l = lowHalf.length - 1;
    let t = topHalf.length - 1;

    while (~a) {
        const elmLow = lowHalf[l];
        const elmTop = topHalf[t];

        if (!~t || elmLow > elmTop) {
            arr[a--] = elmLow;
            --l;
        } else {
            arr[a--] = elmTop;
            --t;
        }
    }

    return arr;
}