Aren't these two forms isomorphic:

    type MyUnion = { type: "foo"; foo: string } | { type: "bar"; bar: string };

vs

    type MyUnion = Foo of { foo: string } | Bar of { bar: string };

You still need some runtime encoding of which branch of the union your data is; otherwise, your code could not pick a branch at runtime.

There's a slight overhead to the TypeScript version (which uses strings instead of an int to represent the branch) but it allows discriminated unions to work without having to introduce them as a new data type into JavaScript. And if you really wanted to, you could use a literal int as the `type` field instead.

Isn’t it the same in TypeScript? You don’t need an explicit type field.

It depends on what you're trying to achieve. If there are sufficient structural differences, you're fine (`"foo" in myThing` can discrimate) but if two types in your union have the same structure, TS doesn't give you a way to tell them apart. (This relates back to branded types.)

A good example would be `type Money = Dollars | Euros` where both types in the union alias `number`. You need a tag. In other languages, you don't.

True, although I think that's just missing branded types, not discriminated unions.