CODEWITHSUNDEEP
typescripttypescript-typings
zedryas
zedryas

Reputation: 974

Howto extract only async function from type/interface

Given a utility ExtractKeysOfType that allows to retrieve only key of a type/interface that match a target and the Pick type i can easily extract function from a given type/interface.
What I want to accomplish is to extract only the async function (and not all signatures) from the base type.

Any clue on how to achieve that ?

type ExtractKeysOfType<T, Target> = {
  [K in keyof T]: T[K] extends Target ? K : never
}[keyof T]

type Type = {
  prop: boolean
  fn (): boolean
  fn (): Promise<boolean>
}

type OnlyFunc = Pick<
  Type,
  ExtractKeysOfType<Type, (...as: any) => any>
> // {fn (): {boolean; Promise<boolean>}

type OnlyAsync = ??? // Someway to build type such as OnlyAsync: {fn (): Promise<boolean>}

Upvotes: 2

Views: 673

Answers (1)

spender
spender

Reputation: 120450

Drawing on this answer as inspiration, we can define an OnlyAsync type using mapped and conditional types as follows:

// finds keys of Promise returning function members
type PromiseFuncKeys<T> = {
    [K in keyof T]: T[K] extends ((...args: any[]) => Promise<any>) ? K : never;
}[keyof T]
// so we can Pick them
type OnlyAsync<T> = Pick<T, PromiseFuncKeys<T>>;

Here's another answer that should shed some light on what's going on here.

Upvotes: 2

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