different promise execution on browsers, what happened?

问题

const b = () => {
  return new Promise(resolve => {
    resolve();
    Promise.resolve()
      .then(() => {
        console.log(1);
      })
      .then(() => console.log(3));
  });
};

const a = async () => {
  await b();
  console.log(2);
};

a();

Different behavior on safari , chrome(firefox), any standard described about this ?

回答1:

You have two completely independent promise chains here:

Promise.resolve()
  .then(() => {
    console.log(1);
  })
  .then(() => console.log(3));


(async () => {
  await new Promise(resolve => {
    resolve();
  });
  console.log(2);
}());

There is no guaranteed ordering other that 3 happens after 1. The rest is affected by how promise callbacks are queued exactly, and there was a change in the spec of await (omitting one unnecessary thenable resolution procedure) that is probably not yet implemented in the Safari engine.

来源：https://stackoverflow.com/questions/61234256/different-promise-execution-on-browsers-what-happened