How to distinguish different functions signature with conditional type checks?-CodePudding

I'd like to be distinguish the following function types in conditional type checks:

type SyncFn = () => void;
type AsyncFn = (data: number) => Promise<void>;
type SyncFnWithArg = (data: number) => void;

So I can then use the

I also noticed that if I modify AsyncFn and remove its argument then I have more problems since the type definition for onlySyncFn is now incorrect since now it's "f1" | "f2" instead of only being "f1" as it is in the first TS Playground above.

Second TS Playground

I guess that's related with how function overloading in typescript is done, but I don't really know, so that's why I'm reaching out for help.... maybe it's not related, but are we able to do such function type distinction in TS?

CodePudding user response：

The problem can be addressed by changing the KeyOfType type as follows:

Check both directions of the type relationship (A extends B and B extends A):
Wrap the types used in the conditional type clause in tuples ([A] extends [B]).

type KeyOfType<T, V> = keyof {
  [P in keyof T as [T[P]] extends [V]
    ? [V] extends [T[P]]
      ? P
      : never
    : never
  ]: any
}

Find a playground example here, and an interesting discussion here about various ways to test for type equality (each with their own caveats).