@[email protected] @[email protected] decorators are not in the standard. field initializers were only standardized recently. `constructor(readonly foo: string) {}` shorthand has no alternative in the standard. a lot of ts-specific stuff like type aliases just don't exist at runtime and is just removed, leading to a different number of lines *a lot* of code is targeting commonjs, which leads to ugly generated imports/exports code. just off the top of my head