alt Hacker News1. Porting typescript is made easier by the enormous test suite they can copy for verification.
2. They're doing this for workers, so with a systems-level language, there's a much higher potential for CPU and memory savings (an order of magnitude on this first pass, apparently). Driving their compute costs down is very revenue-efficient at this scale.
For workers, you only need to strip the types. There's no need to do the expensive checking.