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Definitely to get the most ergonomic programming experience, while also having a sound type system, you'd need to change some of the semantics of the language.

A prime example is that if you index into an array of type `T[]`, JS semantics mean the value you get back could be undefined as well as a `T`. So to describe existing JS semantics in a sound type system, the type would have to be `T | undefined`, which would be a big pain. Alternatively you could make the type `T` and have that be sound, but only if you make the runtime semantics be that an out-of-bounds access throws instead of returning undefined.