javascript - Constructor of a custom promise class is called twice (extending standard Promise)

Question

I'm playing with Promise Extensions for JavaScript (prex) and I want to extend the standard Promise class with cancellation support using prex.CancellationToken, complete code here.

Unexpectedly, I'm seeing the constructor of my custom class CancellablePromise being called twice. To simplify things, I've now stripped down all the cancellation logic and left just a bare minimum required to repro the issue:

class CancellablePromise extends Promise {
  constructor(executor) {
    console.log("CancellablePromise::constructor");
    super(executor);
  }
}

function delayWithCancellation(timeoutMs, token) {
  // TODO: we've stripped all cancellation logic for now
  console.log("delayWithCancellation");
  return new CancellablePromise(resolve => {
    setTimeout(resolve, timeoutMs);
  }, token);
}

async function main() {
  await delayWithCancellation(2000, null);
  console.log("successfully delayed.");
}

main().catch(e => console.log(e));

Answer 1

First Update:

I first thought .catch( callback) after 'main' would return a new, pending promise of the extended Promise class, but this is incorrect - calling an async function returns a Promise promise.

Cutting the code down further, to only produce a pending promise:

class CancellablePromise extends Promise {
  constructor(executor) {
    console.log("CancellablePromise::constructor");
    super(executor);
  }
}

async function test() {
   await new CancellablePromise( ()=>null);
}
test();