[lacampbell]

Plus, JS/TS already have proper union types without a need for a construct like Either.

I disagree - what if on success it returns a string representing some data, and on failure it returns a string detailing the error? A `string | string` return type is not very useful, while an `Either<string, string>` is IMO.

[boubiyeah]

But that's the simplest union type.

You could have:

type Result = {type: 'ok', value: string} | {type: 'err', value: string}