mergeSort.js

const list = [5, 4, -3, 2, 1]

const mergeSort = (list) => {
    if (list.length <= 1) return list;
    const middle = list.length / 2;
    const left = list.slice(0, middle);
    const right = list.slice(middle, list.length);
    return merge(mergeSort(left), mergeSort(right));
}

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

console.log(mergeSort(list))