How to return type Promise<void> or Promise<any>?

Question

In TypeScript How can I return a promise of either type Promise<void> or Promise<any>

I'ved tried

• Promise<void | any>
• Promise<void> | Promise<any>

The resulting error is for Promise<void | any>:

error TS2794: Expected 1 arguments, but got 0. Did you forget to include 'void' in your type argument to 'Promise'?

I ran into this error when upgrading from Angular 11 -> 12. This TypeScript version now throws this error when Promise type isn't provided.

See example type error in below...

type SpecialPromise = (number: number) => Promise<void>;

const returnsSomething: SpecialPromise = (number: number) => {
  return new Promise((resolve) => setTimeout(() => resolve(number), 2000));
};
const returnsVoid: SpecialPromise = (number: number) => {
  return new Promise((resolve) => setTimeout(() => resolve(), 2000));
};
Promise.all([
  returnsSomething(Math.random()),
  returnsVoid(Math.random())
]).then((numbers) => console.log(numbers));

The problem is when I create a new promise it by default expects a type. void (nothing) is not a type.

new <T>(executor: (resolve: (value: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): Promise<T>;

A simple fix is to specify the type when creating the promise.

new Promise<void>

But still I get the error if I decide to use:

new Promise<void | any>

Answer 1

One way to solve this is to resolve a promise with undefined like Promise.resolve(undefined)