It's not just the ADTs but the exhaustive pattern matching that help make sure you've covered all of the cases in your ADT. You'll see JavaScripters use objects as a poor man's ADT where they'll use the key name as a the constructor or a { tag: key, ... }, but that has caveats (aside from feeling less first-class): 1) JavaScript can't do exhaustive pattern matching so you'll always need to handle null cases, 2) checks must all happen at runtime which means you'll end up having to throw/catch exceptions and nothing will hold your hand to say that you've missed a case. TypeScripters can handle 1 & 2 for safety, but the ergonomics are bad and verbose so you'll see a lot of folks skip it. Similar languages will have pattern matching, safety, but lack the lightweight/dense ergonomics. When you dive into a language in the ML family (Haskell, OCaml, Standard ML, et. al.), the ergonomics make you want to use them, and they are idiomatic to the ecosystem and you'll want to use them for just about everything--either the ones in the Preludes/stdlib like Maybe, Either, List, etc. or by building them yourself.
An example in the wild that demonstrates this `fp-ts`'s explanation of how to do ADTs in TypeScript where you can see the comparison in PureScript is a one-liner (both data and polymorphic data): https://github.com/gcanti/fp-ts/blob/master/docs/guides/pure...
That Typescript ADT example is heavily outdated (or explicitly obtuse)
class Bar { tag="bar" as const; constructor(public value:string){} }
class Baz { tag="baz" as const; constructor(public value:boolean){} }
type Foo = Bar|Baz;
That's true if pattern matching specifically is what you want, in practice we don't constructs ADT's for the sake of it and rolls with interfaces that also meshes well with data imported from other sources (ie JSON).
Also while pattern matching isn't an explicit thing in TS you get most benefits from compiler checked accesses that can recognize type-tests in your code and derive types and do property access checks.
An example in the wild that demonstrates this `fp-ts`'s explanation of how to do ADTs in TypeScript where you can see the comparison in PureScript is a one-liner (both data and polymorphic data): https://github.com/gcanti/fp-ts/blob/master/docs/guides/pure...