Thank you, David!

Man, this is such an excellent solution! I wish I would learn to destructure like you:

const [ left,right ] = arr.etc

If you don't mind, how does the reduce function work here?

This is so succinct! Very nice :)

Well, I mean, the instructions say to return a variable 'l', and it's part of the code that's automatically put into the solution. I know my comment seems pedantic, but that wasn't my intention. I really do think this is the best answer, just that it doesn't follow that specific instruction.