Discriminated union about typebox

[sinclairzx81]

@thibauds Hi,

Unfortunately, this is a known limitation with TS enums. The problem is that TypeScript treats enum values as opaque, meaning they're invisible to the type system. Because they're invisible to the type system, they are also invisible to TypeBox (which is why you're seeing any inferred).

There's been a few attempts to try an get constant values from TS enums in the past, unfortunately each attempt has lead to type scalability issues (and very slow inference performance for large enums). Because of this, using enum discriminators is generally not recommended (for now)

This said, you can get around this issue by not using enums, and instead using string literal values. These are observable to TypeBox, and generally are a better way to represent as discriminator properties (as they're directly encodable in Json Schema and don't require auxiliary knowledge of the enum data structure)

Workbench Link Here

type Moving = {
  id: string
  disc: 'user' | 'poney'
}
type Inert = {
  disc: 'rock'
}
export type Item = {
  name: string
} & (Moving | Inert)

As a general rule of thumb, Enum are usually more useful for representing options, and less so for discriminator properties. For discriminators, always preference literal values if possible.

Hope this helps
S

[thibauds]

Thank you for this detailed answer 🙏