Union of generics in array

Issue

I am trying to get the union type of generics in an array but can only retrieve what the generic is extended from not the actual implementation.

type Params = Record<string, number | string | null | undefined> | undefined;
type Route<T extends Params = undefined> = {
  params: T
}
type Stack = {
  routes: Route<Params>[];
}

const route1: Route<{ detailUrl: string }> = { ... };
const route2: Route<{ head: string }> = { ... };
const route3: Route = { ... };

const routeRegistry: Stack = {
  routes: [route1, route2, route3]
};

type UnionOfParams = ExtractGeneric<typeof routeRegistry['routes'][number]>;
// expected: { detailUrl: string } | { head: string } | undefined
// received: Params

I know const assertion is useful to limit widening of the inference of the types, but using it in this case didn't produce any result.

I couldn't find anything related to this particular problem. Is there a way to accomplish what I ask?

Solution

You can't have inference and an annotation for a variable. This can only be done with a function. A function can have a generic type parameter that both constrains a parameter and offers a vehicle for inference:

function createRouteRegistry<T extends Stack>(p: T): T {
  return p
}

const routeRegistry = createRouteRegistry({
  routes: [route1, route2, route3]
})

type ExtractGeneric<T> = T extends Route<infer P> ? P : never
type UnionOfParams = ExtractGeneric<typeof routeRegistry['routes'][number]>;

Playground Link

Answered By - Titian Cernicova-Dragomir