mergesort.js

function mergeSort(array) {
    if(array.length <= 1) {
        return array;
    }
    const mid = array.length / 2 | 0;

    const left = mergeSort(array.slice(0, mid));
    const right = mergeSort(array.slice(mid));
    return merge(left, right);
};

function merge(left, right) {
    const merged = [];
    while(left.length && right.length) {
        if(left[0] < right[0]) {
            merged.push(left.shift());
        } else {
            merged.push(right.shift());
        }
    }
    const rem = left.length ? left : right;
    merged.push(...rem);
    return merged;
}