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It's not perfect, but TypeScript goes a long way toward making frontend feel as safe as backend. You still have the potential for weird bugs when interacting with external JavaScript, but your internal code is pretty safe. Also, TypeScript's type system is impressively flexible, and I often find myself missing features (like unions) when working with other languages.

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> missing features (like unions) when working with other languages.

Which languages are missing unions? Off the top of my head, they exist in C/C++ and mypy python.

Typescript allows you to have literal type discriminators in their unions, which combined with control flow based type narrowing of unions effectively gives you sum types. That is what he is talking about, because most mainstream langauges don't have sum types and you need to go to strongly typed functional langauges (haskell, scala, etc...) or modern strongly typed langauges such as Rust to get access to such features.

Yes, this. Sum types in most languages are approximated by something like kotlin's sealed classes. Sometimes you don't want to declare a whole new class hierarchy, you just want to say "this function can either return this or this".

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