namespaces are the other one, and that's a fun legacy because Typescript had to implement a half dozen module systems in the early days: no modules (jQuery-era globalThis pollution), AMD, UMD, CommonJS, SystemJS, Typescript's own which was proto-ESM inspired but not ESM, then Typescript's realignment with ESM.

Something of its own sign of Microsoft out-engineering some of the hiccups of the ecosystem as a whole. (I started using Typescript < 1 simply because it was the safest and easiest way to write AMD modules also with an eye to UMD or SystemJS output with just a compiler flag change if you needed to ship something compatible outside the house.)