The code to reverse an array in-place iteratively is a matter of swapping elements from one end to the other. The algorithm iterates from one end of the array to the middle of the array, swapping characters $i$ and $(length-1-i)$ for a zero indexed array. This algorithm has time complexity $O(n)$ and space complexity $O(1)$.

The javascript built-in function `a.reverse()`

returns a new array and does not perform an in-place reverse.

The recursive version of this algorithm is a useful study of recursion, but would not be used due to its worse performance. This algorithm has time complexity $O(n)$ and space complexity $O(n)$.