Infer return value from promise in Typescript

Question

I have a very simple function that takes two arguments - node object and async function that somehow processes given node and returns processed node which can be anything.

I would like to make it generic and infer type from async function. This is my try but TS is complaining and I don't really understand why.

// ensure that callback is of a correct type
type NodeCallback<H> = H extends (node: Node) => Promise<infer R> ? (node: Node) => Promise<R> : never

// retrieve return value
type NodeCallbackReturnValue<H> = H extends (node: Node) => Promise<infer R> ? R : never


const myAsyncFunction = <_, C>(node: Node, cb: NodeCallback<C>) => {
  return cb(node)
}


myAsyncFunction(document, (node: Node) => Promise.resolve(node.nodeType))

Playground available here

Answer 1

Alternative solution without explicit generic

type NodeCallback<T> = (node: Node) => Promise<T>

const myAsyncFunction = <R,>(node: Node, cb: NodeCallback<R>) => {
  return cb(node)
}

const result = myAsyncFunction(
  document,
  (node: Node) => Promise.resolve(node.nodeType)
) // Promise<number>

I don't think we need conditional types in NodeCallback type at all, since TS is able to infer the type

Playground

Here you can find more information about working with callbacks

Answer 2

That's because the implicitly declared generic type is unknown in myAsyncFunction(...), and it can't inferred to other type

You can solve it by following:

// change
type NodeCallback<H> = H extends (node: Node) => Promise<infer R> ? (node: Node) => Promise<R> : never
// into
type NodeCallback<H> = H extends (node: Node) => Promise<infer R> ? H : never

Or

const myAsyncFunction = <C,>(node: Node, cb: NodeCallback<C>) => {
  return cb(node)
}

type T = (node: Node) => Promise<number>
myAsyncFunction<T>(document, (node: Node) => Promise.resolve(node.nodeType))

Playground