For new code, I think most JS devs have already switched to using the class syntax. Also FP in JS is alive and well if you want to avoid dealing with prototype chains altogether.

I think we can all agree the Date API sucks, but life can still be good if you just give in and include a date library. Also, there's some light at the end of the tunnel with Temporal proposal coming along [1].

Object.keys returning string[] is purely a TypeScript design decision, coming from how TypeScript chooses to model object subtyping [2].

  type Foo = {
    a: string
  }
  
  const keysOfFoo = (obj: Foo) => Object.keys(obj)
  
  const foo = {a: '', b: 5}
  keysOfFoo(foo)

The fact that this passes type checks is a conscious TS design decision that comes with both advantages and disadvantages. It wouldn't have to be this way; Exact types [3] could potentially be used to describe that if the type is exactly Foo, it's safe to assume Object.keys(foo) is (keyof Foo)[].

For first class tuples (and records), there's a stage 2 EcmaScript proposal coming along [4]. There's of course also the readonly [] type in TypeScript if you only need the safety of not accidentally pushing to an "immutable" array. First class language support could have nice additional features though, like strict equality.

Anyways, if these are the things you don't like about JS/TS, I don't think AssemblyScript will be the answer for you, as the goals of that language seem to be entirely different from "fixing" old JS cruft. Apart from the i32, i64 stuff I guess.

[1] https://github.com/tc39/proposal-temporal

[2] https://github.com/Microsoft/TypeScript/pull/12253#issuecomm...

[3] https://github.com/microsoft/TypeScript/issues/12936

[4] https://github.com/tc39/proposal-record-tuple