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0 pointsSebastianKra3mo ago0 comments

Have you engaged with the advanced features? TS is the only mainstream language that lets you express pretty much everything in the typesystem, to the point where I frequently have to hold myself back.

Having used Swift and Kotlin in the past, their typesystems don’t come close to the flexibility with which you can wrangle data in TS. Many concepts that I would consider fundamental are either not present or require complicated syntax eg.: product types A & B, sum types A | B, { …spreading }.

And where Swift needs custom syntax such as `guard let` and `case let` to narrow types, Typescript is able to analyze normal control flow.

For practical reasons, there are a ton of ugly workarounds, but I’d rather live with those than go back to the rigid “Java++” languages.

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afiori3mo ago

> product types A & B, sum types A | B

those are intersection types and union types and are much more unique and rare in type systems eg all typed functional languages have product/sum types but typescript is the only mainstream language I know to have unions and intersection.

satvikpendem3mo ago

The problem is the type system is not strong, where structural typing can cause a lot of issues, such as adding a property to an object for example and the typechecker will not care. This does not happen in more strongly, non-structural typed languages.

SebastianKraOP3mo ago

Yeah that's one of the practical workarounds I mentioned. It mostly comes up while spreading (which is why spreads should usually go first) and when persisting (which is why your ORM should have a runtime schema).

But it's a double edged sword:

- You can brand types with additional information. For example, React Query does this to keep track of which type is stored under which cache-key. This is where most strongly typed languages just give up.

- You can progressively extend/narrow types without having to worry about inheritance. Invariants can be statically checked by encoding them in a type: eg IssuedInvoice = Invoice & { … }. A function that needs only a subset of a domain-object can specify this rather than requiring the email-address to verify a phone number.

- You can even emulate nominal types using keyed unions.

The first two alone have saved me from more errors than structural typing introduced.

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