Type Void[] Array Not Assignable to Type Void

Question

I expect type void[] to be compatible with type void. Specifically, when using Promise.all.

class Foo {
  delete(): Promise<void> {
    // do delete
    return Promise.resolve();
  }
  deleteMany(list: Foo[]): Promise<void> {
    return Promise.all(list.map((x) => x.delete()));
  }

typescript error:

'Type 'Promise<void[]>' is not assignable to type 'Promise<void>'. Type 'void[]' is not assignable to type 'void'.'

I can solve this two ways that I know of:

1. Mark deleteMany as returning Promise<void[]>.

2. Promise chain to Promise.all, to return a resolved promise. e.g.

   return Promise.all(list.map((x) => x.delete())).then(() => Promise.resolve());
   

The second one is worse, since that bit of code executes in JavaScript, but the first one is confusing to developers. Does Typescript have poor support for Promise.all or is it omitted from their documentation? Anyone find a better solution?

Answer 1

The 1 (Mark deleteMany as returning Promise<void[]>) is absolutely ok at least for me.

However, you can use async/await if you really want to return Promise<void>:

class Foo {
  delete(): Promise<void> {
    // do delete
    return Promise.resolve();
  }
  async deleteMany(list: Foo[]): Promise<void> {
    await Promise.all(list.map((x) => x.delete()));
  }

The function will behave exactly the same way.