What AsyncGenerator TypeScript type yields a Promise?

Question

I'm trying to assign a return type to the function below:

async function *sleepyNumbers() {  // what TypeScript type is this?
  let n = 0;
  while (true) {
    yield new Promise(resolve => resolve(n++));
    await new Promise(resolve => setTimeout(resolve, 500));
  }
}

(async () => {
  for await (const i of sleepyNumbers())
    console.log(i);
})();

The generator is yielding a Promise that resolves to a number. Setting the type to Promise<number> fails with this error message:

TS2739: Type 'AsyncGenerator' is missing the following properties from type 'Promise': then, catch, [Symbol.toStringTag], finally

Iterable resulted in a similar error.

I can set the type to AsyncGenerator but that's not specific enough. What is the proper TypeScript syntax for the return type of this function?

Answer 1

It will be AsyncGenerator<number, never, void>:

number - next result
never returns
void - next doesn't get any parameter

You'll also need to explicitly type a promise resolve:

yield new Promise<number>(resolve => resolve(n++));

All together:

async function *sleepyNumbers(): AsyncGenerator<number, never, void> {
    let n = 0;
    while (true) {
        yield new Promise<number>(resolve => resolve(n++));
        await new Promise(resolve => setTimeout(resolve, 500));
    }
}

(async () => {
    for await (const i of sleepyNumbers())
        console.log(i);
})();