I've heard of `isolatedDeclarations` in TS 5.5, but this post left me with more questions than answers because it conflates so many distinct things (TS 5.5, Deno, JSR).
I'll try my best to break it down succinctly:
1) Historically, .d.ts generation is slow because it requires the full TypeScript type checker in order to correctly handle all cases (e.g. exported functions with inferred return types)
2) However, if types were to be fully explicit, then .d.ts generation could be performed without a full type checker (i.e. by a faster compiler via mere syntax transformation)
3) The isolatedDeclarations flag causes the TS compiler to trigger errors when any exports would require inference in order to generate .d.ts
4) Thus, the isolatedDeclarations flag can be used to guarantee compatibility with faster, simpler .d.ts generation using syntax-based transformation
5) A consequence of simpler .d.ts generation is that it can be trivially done in parallel, which can be helpful for large monorepos