I'm trying to infer return types based on input types into a function. Specifically, I have a mini-communication homemade protocol via MessagePorts between iframes that sends requests and receives responses. I would like to infer the response type based on the request type that was sent.
Here is a fake, non-working example:
enum RequestKind {
Foo = 'foo',
Bar = 'bar',
}
// Discriminated union of request types
type Request = (
{ kind: RequestKind.Foo, fooParam: number } |
{ kind: RequestKind.Bar, barParam: string }
)
// Requests' associated response types.
type Response = {
[RequestKind.Foo]: boolean
[RequestKind.Bar]: { result: string, error: string }
}
// This has a syntax error:
// - Type 'T["kind"]' cannot be used to index type 'Response'.
function send<T extends Request>(req: T): Promise<Response[T['kind']]> {
// send request and return response
}
Is there a way to infer a separate return/Response type from input/Request type?
Answer
You have duplicated Request and Response types with lib.dom.d.ts. If you enclose your code into a block, it works ok:
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