JIT compilers can already figure out all information they need for optimization with runtime instrumentation. TypeScript can't statically eliminate the need for those checks, so it provides no extra value.
Maybe the types could give the baseline compiler an initial guess for speculative optimizations (which would be overridden by runtime profiling data in later stages), but:
- That there is a benefit at all is complete speculation at this stage.
- Type checking and inference takes time, and time is precious for baseline execution. Browsers want to start executing scripts as soon as possible.
- It would have to be worth the extra complexity and implementation effort.
- see the first paragraph.