Return an array while modifying it does not work

Here is my confusion with return statements in Javascript:

I have a challenge on FreeCodeCamp.org to write a function that takes in 2 array's, and a position. I have to return array1 copied into array2 starting at the given position.

this code works:

function insertArrayAtPostion(arr1, arr2, postion) {
  arr2.splice(n,0,...arr1);
  return arr2;
}

However, I wonder why I can't just do this:

 function insertArrayAtPostion(arr1, arr2, postion) {
  return arr2.splice(n,0,...arr1);
}

Because I've been told JavaScript evaluates everything to the right before returning