yes, you showed this earlier https://news.ycombinator.com/item?id=30306256 by mutating the array parameter of the map, but I made my wrong statement before reading your example.

I guess it's a failure of imagination on my part not to have realized that I could modify the array length in that way, or really the failure of imagination was not seeing any good reason for sending the original array in as a parameter to map - when I saw that in documentation I thought what a weird idea!