How do bun, deno, and other typescript-built-in languages keep up with language features?
bun, deno, esbuild, swc etc. can parse the syntax, but they chuck the TS (they probably don't even add it to the AST, but I haven't checked).
Keeping up with syntax is very doable. It doesn't change often, and updating the parser when it does isn't much work.
There are some past/ongoing projects[1][2] to create type checkers faster than tsc, but they aren't going to reach full parity and probably don't plan on keeping up with language features.