Missing default rejection tracking and console.log() polyfill to the Hermes CLI

[wcandillon]

Bug Description

The following code returns 10 on v8 but doesn't return anything using the hermes binary:

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

The Expected Behavior

To return 10;

[tmikov]

I am afraid I can't reproduce this. I just tried it and it works as expected. See the attached screenshot:

[tmikov]

Your example uses console.log(), which isn't provided by the Hermes CLI. An exception is thrown in the primise.

[wcandillon]

$ cat oo.js
function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();
$ ./bin/hermes foo.js
$  echo $?
0

no output nor exception. How do I reconcile the difference of behaviour?
Using print() does yield the "expected" result.

[tmikov]

@wcandillon there is no real difference in behavior. The spec does not specify how unhandled Promise rejection should be handled by the host environment. It is up to the integrator of the JS engine. Browsers and NodeJS provide their own mechanisms. I suspect React Native does too, or it should.

The Hermes CLI tool is a very minimalistic integration of the engine, it only provides print() and a couple of more things. It doesn't configure a rejection hook.

If you need this behavior, you can configure the rejection hook yourself from JS like this:

HermesInternal.enablePromiseRejectionTracker({
  allRejections: true,
  onUnhandled: function(id, error) {
    print("Unhandled Promise rejection:", error.stack);
  },
});

I think there is a good point to be made that the Hermes CLI tool should set this by default and it should also probably provide a very simple polyfill for console.log(). We will look into it.