use of slice and splice add elements array

1function frankenSplice(arr1, arr2, n) {
2  // Create a copy of arr2.
3  let combinedArrays = arr2.slice()
4  //                   [4, 5, 6]
5
6  // Insert all the elements of arr1 into arr2 beginning
7  // at the index specified by n. We're using the spread
8  // operator "..." to insert each individual element of 
9  // arr1 instead of the whole array.
10  combinedArrays.splice(n, 0, ...arr1)
11  //                   (1, 0, ...[1, 2, 3])
12  //                   [4, 1, 2, 3, 5, 6]
13
14  // Return the combined arrays.
15  return combinedArrays
16}
17
18frankenSplice([1, 2, 3], [4, 5, 6], 1);