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I think the discussion about TypeScript, and particularly type guards, really misses the point. Structural typing and manual type narrowing is basically what yo

I think the discussion about TypeScript, and particularly type guards, really misses the point. Structural typing and manual type narrowing is basically what you’d do anywhere you don’t know some data’s type in the first place—for instance when you get some JSON from an external service. It just happens that that’s fairly common in JS/TS. It’s also a natural consequence of TS explicitly choosing not to expose its type system to the runtime; a choice which confuses many, but makes sense given the potentially unpredictable performance impact that could have. But there are plenty of good tools for declaring validation and getting types “for free”, allowing you to choose explicitly where you want to incur that runtime cost—typically at API/network boundaries. This is not much different from interface/protocol/contract based typing, which often appears in a goofy way in Java: class Foo implements IFoo. But when you work in an environment where interface typing is commonplace and done well, it really helps guide good modular design.

otterley 6y ago
I really like the fact that you have to declare when a class implements an interface. I consider it a valuable feature in Java. It’s a tremendous boon for documentation because it makes finding compatible classes much easier. Documentation indexers make this a snap, and it makes it easy for IDEs to autocomplete for you. I have spent more time than I ought to have trying to find conforming types in TypeScript and Go code through trial and error. Fortunately I get feedback pretty quickly in my editor, but it is still a frustrating exercise. I want to build mechanisms, not solve jigsaw puzzles.
- eyelidlessness 6y ago
  > I really like the fact that you have to declare when a class implements an interface. I consider it a valuable feature in Java. It’s a tremendous boon for documentation because it makes finding compatible classes much easier. Documentation indexers make this a snap, and it makes it easy for IDEs to autocomplete for you. In some contexts I miss this in TS, in others I find it unnecessarily limiting. Probably an even split. Though I find myself wanting nominal types more for primitives (sure email is a string, but not every string is; or is this number really an unsigned int?).

otterley 6y ago

I really like the fact that you have to declare when a class implements an interface. I consider it a valuable feature in Java. It’s a tremendous boon for documentation because it makes finding compatible classes much easier. Documentation indexers make this a snap, and it makes it easy for IDEs to autocomplete for you. I have spent more time than I ought to have trying to find conforming types in TypeScript and Go code through trial and error. Fortunately I get feedback pretty quickly in my editor, but it is still a frustrating exercise. I want to build mechanisms, not solve jigsaw puzzles.

eyelidlessness 6y ago
> I really like the fact that you have to declare when a class implements an interface. I consider it a valuable feature in Java. It’s a tremendous boon for documentation because it makes finding compatible classes much easier. Documentation indexers make this a snap, and it makes it easy for IDEs to autocomplete for you. In some contexts I miss this in TS, in others I find it unnecessarily limiting. Probably an even split. Though I find myself wanting nominal types more for primitives (sure email is a string, but not every string is; or is this number really an unsigned int?).