SyncFunction type

[alcuadrado]

Hi Crhis!

I'm wondering if it's possible to define a type that forbids async functions. I'll explain why I'd like something like that, and what I tried so far.

Suppose I have a function like this one:

function receivesACallback(c:() => any) {
  c();
  doSomethingElse();
}

Passing an async function to receivesACallback can be problematic, as it doesn't await it.

My best try so far is this:

type SyncFunction<T> = () => Exclude<T, Promise<any>>;

function notAsyncCallback<T>(callback: SyncFunction<T>) {
  callback();
  doSomethingElse();
}

notAsyncCallback(() => {});
notAsyncCallback(() => 123);

notAsyncCallback(async () => {}); // Type error
notAsyncCallback(async () => 123); // Type error

I don't like having to define a useless type param though.

Do you find this idea interesting? And do you know how to do it without the type param?

[krzkaczor]

Oh, that's pretty useful, floating promises are cancer. I think SyncFunction type would be a pretty cool addition to ts-essentials 👍

I don't see a way to do this without type parameter right now. Probably Negated types that are cooking in TS backlog as we speak would solve this: microsoft/TypeScript#29317

[alcuadrado]

I don't think this can be achieved without the type parameter either.

The not proposal is very interesting! It even has an example related to this:

declare function ignore<T extends not (object & Promise<any>)>(value: T): void;
declare function readFileAsync(): Promise<string>;
declare function readFileSync(): string;
ignore(readFileSync());     // OK
ignore(readFileAsync());    // Should error

[krzkaczor]

@all-contributors please add @alcuadrado for ideas

[allcontributors]

@krzkaczor

I've put up a pull request to add @alcuadrado! 🎉

[Beraliv]

Is it possible to do with ESLint rule? https://typescript-eslint.io/rules/no-floating-promises/