Slice and Splice problem doubt

JavaScript
#1

Tell us what’s happening:
Why is this code not acceptable though it prints the outputs correctly as desired?

Your code so far


function frankenSplice(arr1, arr2, n) {
  var arr = [];
  for(var i = 0;i<n;i++)
  {
    arr.push(arr2[i]);
  }
  arr.push([...arr1]);
  arr.push([...arr2.slice(n)]);
  console.log(arr);
  return arr;
}

frankenSplice([1, 2, 3], [4, 5], 1);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-algorithm-scripting/slice-and-splice

#2

The challenge requires you to return a 1-dimensional array. You are returning a multi-dimensional array.

#3