What you are misunderstanding is that, arr is only the array [1, 2, 3, 1, 2, 3].
Which means, first parameter is the value which is passed in first.
What I mean is, arr will be the first argument which comes before the first , .
In this case that the first argument is an array [1, 2, 3, 1, 2, 3] so arr is it.
But if I call the function like destroyer([[1, 2, 3], [1, 2, 3]], [1, 2, 3]); , arr will be [[1, 2, 3], [1, 2, 3]].
Here for more: Can someone explain a line of code in intermediate-algorithm-scripting 3rd challenge("Seek and destroy") - #5 by AndrewAung11