I thought you were allowed to assume functions you needed in the interest of a modular solution. Array.flatten is an obvious “assume I have this, I would write it anyways.”
I think most decent interviews will give you a pass on an enhanced standard library.
When doing an interview at Google in C, I asked if I could assume I had a hashtable implementation with so-and-so interface, and the interviewer said no ¯\_(ツ)_/¯
I think most decent interviews will give you a pass on an enhanced standard library.