I am absolutely blown away at how good TypeScript's type checker is, given how much of an unholy mess JavaScript is, and that the TypeScript language cannot just deprecate all the awful parts.
More than once I've yelled at my screen, "HOW DO YOU KNOW THIS?!" about some inferred or narrowed type.
More than once I've yelled at my screen, "HOW DO YOU KNOW THIS?!" about some inferred or narrowed type.