Merge Sort in Javascript

function mergeSort(array) {

    if(array.length < 2) return array;
    
    var middle = Math.floor(array.length / 2);
    var left = array.slice(0, middle);
    var right = array.slice(middle);

    return merge(mergeSort(left), mergeSort(right));
}

function merge(left, right) {
    var sorted = [];

    while (left.length && right.length) {
        if (left[0] <= right[0]) {
            sorted.push(left.shift());
        } else {
            sorted.push(right.shift());
        }
    }
    return [...sorted, ...left, ...right];
}