[hajile]

> It's explicitly allowed by the language.

This is like arguing that a null exception is fine because it's allowed by the language. If you get `undefined` when you expect another type, most future interaction are guaranteed to have JS throw because of the JS equivalent of a null pointer exception. They are technically different because a dynamic language runtime can prevent a total crash, but the effect on your web app is going to be essentially the same.

    [1,2,3][4].toFixed(2)

> It's a failure of any type system that fails to account for this specified behavior in the language.

Haskell has the ability to handle the error.

How do you recommend a compiler to detect out-of-bounds at compile time? It can certainly do this for our trivial example, but that example will also be immediately evident the first time you run the code too, so it's probably not worth the effort. What about the infinite number of more subtle variants?

[recursive]

> How do you recommend a compiler to detect out-of-bounds at compile time?

I wouldn't make the recommendation that they do at all. Full soundness is not my thing. But... if Flow wanted to do it, it would have to change the type of indexing into `(Element[])[number]` with a read from `Element` to `Element | undefined`.