it's what I was thinking, basically are 2 nested loops, idiomatically is great but not forp performance.

This is an amazing solution that also scales the best.

Time complexity by using slice and reduce instead of traditional for loop is quite costly i guess

The use of slice and reduce to get the sum is very clever.

array indexing starts at 0 as the first item and goes up to array.length - 1, thus arr[array.length] is add a new item after the last one

Contrary to vincaslt's comment, I believe n is necessary in order to catch the condition vectorjohn points out.

I wonder what the time difference is though. Its probably the same, depending on push() implementation. Using the length to determine the next index should be O(1), right? I guess if they're the same time complexity, push is a LITTLE clearer. I still love the solution though.