Most of these WTF examples basically boil down to JS doing type coercion willy-nilly. This 'feature' makes writing conditionals slightly shorter, in exchange for introducing the possibility of massive bugs everywhere in your code at any moment. Seriously, f* JS type coercion.

The other misfeature I hate is that accessing undefined properties doesn't raise an error (then, but you can be sure it will make your program blow up a bit later).

Typescript helps to solve both.