Extend Discriminated Unions

Question

is it possible to do transform this DU using mappend/conditional types

type MyDU =
| {kind: 'foo'}
| {kind: 'bar'}

type Transformed = DUTransformer<MyDU>

such that we get the folllowing result

type Transformed =
| {kind: 'foo', foo: boolean}
| {kind: 'bar', bar: boolean}

Answer 1

Yes, because TypeScript will distribute mapped types over a union :

type MyDU =
| {kind: 'foo'}
| {kind: 'bar'}

type Kinded<T extends string> = { kind: T }

type DUTransformer<T> = T extends Kinded<infer K> ? T & {[K1 in K]: boolean} : never

type Transformed = DUTransformer<MyDU>

The type of Transformed is:

type Transformed = ({
    kind: 'foo';
} & {
    foo: boolean;
}) | ({
    kind: 'bar';
} & {
    bar: boolean;
})