... or equal _to_ key is found

What does Search return? Also, ordinarily I think methods should start with lowercase letters rather than uppercase, which should be reserved for classes... But it's just a style preference.

I've written in lowercase before but there was a conflict with the js keyword `delete`. Switching to uppercase sounded the nearest workaround to me.

What is a skip list? I think you should give a very short description about what it is and what are the advantages with regard to, say, red-black trees or other deterministic data structures...

This looks weird, since Search seemingly does not return anything...

It was a problem with the lazy (topological) order of execution in this environment. The search was executed before the insertions. I've included the same line in the previous cell for a working example.

Good! Now you can remove this cell.

I would start with an empty list defined in the block itself. Like so (see below).