Flattening nested lists

You'll run into nested lists everywhere, like comments that have replies and sections that have subsections, and what you need most often is to go through all of them as one flat list.

arr.flat(Infinity) is enough for nested arrays, but a real tree is made of objects with children, and those need recursion: the function calls itself on each branch.

Recursion always needs a base case where it stops, and here that's a node with no children.