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Comment by pcwalton | Hacker News Reader

TypeScript isn't sound, so you can't rely on the types for optimization. The shape/hidden class optimization that JS engines already have is probably about as good as you can do.

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The last soundness issue I'm aware of is the bivariant function issue which they resolved in 2.6

You can still do something like this:

    let x: number = "some string" as any;

I don't get it, I though the whole purpose of typescript is to prevent things like that?

That's sort of tautological. "Any" and type assertions are designed as the escape hatch of the type system. If you're trying to develop type safe typescript you obviously won't use those. The followup question is for typed code (excluding usage of any or type assertions) does typescript have an unsound type system? Afaik the answer is no.

In the real world you are forced to use "any" when the type definitions for a third-party library are not perfect.

This actually happens a lot with types from DefinitelyTyped that are not maintained by the original author of the library.

That would be the equivalent of static_cast in c++, or casting via (object) in C#. Both of those are considered type safe but can still be abused also.

You could still infer sound areas such as certain structs and patterns for using them. They could introduce sound mode or patterns for areas as well.

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