归并排序

Array.prototype.mergeSort = function() {
   const rec = (arr) => {
    if(arr.length === 1) { return arr };
    const mid = Math.floor(arr.length / 2);
    const left = arr.slice(0, mid);
    const right = arr.slice(mid, arr.length);
    const orderLeft =  rec(left);
    const orderRight = rec(right);
    const res = [];
    while(orderLeft.length || orderRight.length) {
        if(orderLeft.length && orderRight.length) {
            res.push(orderLeft[0] < orderRight[0] ? orderLeft.shift() : orderRight.shift())
        } else if (orderLeft.length) {
            res.push(orderLeft.shift())
        } else if (orderRight.length) {
            res.push(orderRight.shift())
        }
    }
    return res
   }
    console.log(this);
    rec(this) ;
}
const arr = [5, 4, 3, 2, 1];
arr.mergeSort()