This puts memo in the global scope, since it's not called with the new keyword, this refers to window.

Simple and clear.
I like it ;-)

You are right, I meant this one.
Thank you for help.

Since your own solution uses regex, I am assuming your question lies with the Arrow Function notation.

a => expr is roughly equivalent to function(a) {return expr;}. You can read about a few gotchas on the mdn page linked above.

Hi.

Would you tell me how does this expression calls?

Thank you.