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.)

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