Going deep into conditional types, and trying to contort Typescript into a dependent type system despite the fact it is not one!

Deep diving into lib.d.ts and DefinitelyTyped dealing with the weird boundary space between poorly typed JS and strongly typed TS can teach a lot these days.

Also, being able to recognize the Peano S-function on sight (and thus uses of Church-Peano numbers) is a fun part of that wizardry. (Maybe not necessary to remind the usefulness of the Lambda Calculus canon on a site named after one of the most famous combinators.)