typescriptgenericstypesarrow-functionstypescript-generics

Typescript: generic functional type/interface for arrow function

Ok, I have the following scenario:

type IHashFn = (arg: string) => number;
const hash: IHashFn = (arg) => {
  return 42;
}

So far, so good. Now I want the function to be generic.

const hash: <T> (arg: T) => number = (arg) => {
  return 42;
}

That works. But this doesn't:

type IHashFn<T> = (arg: T) => number;
const hash: <T> IHashFn<T> = (arg) => {
  return 42;
}

I found no way to calm down the TS compiler. Using interfaces instead of type aliases doesn't work either.

Note: I dont't want hash to be an implementation for IHashFn<string> but generic as well.

Is there a way to declare generic function types or interfaces in TypeScript at all?

Solution

  • You aren't really doing anything with your generic parameter.

    But it sounds like you want a generic function, not a generic type alias. So leave it on the function, and off of the type alias.

    type IHashFn = <T>(arg: T) => number;

const hash: IHashFn = (arg) => {
  return 42;
}