CODEWITHSUNDEEP
typescriptmapped-types
robkuz
robkuz

Reputation: 9934

Extend Discriminated Unions

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}

Upvotes: 3

Views: 1487

Answers (1)

Sean Vieira
Sean Vieira

Reputation: 160015

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;
})

Upvotes: 5

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