CODEWITHSUNDEEP
typescripttypescript-generics
ThomasReggi
ThomasReggi

Reputation: 59345

How do I infer nested argument type?

I'm using typescript generics and I'd like to infer the type of fn aka P, however it's not working as I'd expect.

Playground

Here's the code:

type Callback = (...args: any[]) => any 

interface Route<
  T extends Callback
> {
  fn: T
}

function route <
  P extends Callback,
  R extends Route<P>
> (pathname: string, handler: R) {
  return handler.fn
}

const x = route('/hi', {fn: (name: string) => `hi ${name}`})
//    ^?

I'd expect x to return the type (name: string) => string, but instead it's returning Callback.

Upvotes: 0

Views: 39

Answers (1)

ThomasReggi
ThomasReggi

Reputation: 59345

I just realized I can remove R and call Route<P> and it works. I suppose this makes sense.

function route <
  P extends Callback,
> (pathname: string, handler: Route<P>) {
  return handler.fn
}

The rational is the thing you don't know should be the thing extended. The thing you "know" shouldn't.

Upvotes: 2

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