合并两个数组，并降序排序

```let arr1 = [1,2,3]
   let arr2 = [4,5,6]
   
   let arr3 = arr1.concat(arr2)
   
   function numDesc(a,b) {
        return b - a;
   }
   
   console.log(arr3.sort(numDesc)) // 6,5,4,3,2,1