core-js/tests/unit-global/es.promise.with-resolvers.js

const { getPrototypeOf } = Object;

QUnit.test('Promise.withResolvers', assert => {
  const { withResolvers } = Promise;
  assert.isFunction(withResolvers);
  assert.arity(withResolvers, 0);
  assert.name(withResolvers, 'withResolvers');
  assert.nonEnumerable(Promise, 'withResolvers');
  assert.looksNative(withResolvers);

  const d1 = Promise.withResolvers();
  assert.same(getPrototypeOf(d1), Object.prototype, 'proto is Object.prototype');
  assert.true(d1.promise instanceof Promise, 'promise is promise');
  assert.isFunction(d1.resolve, 'resolve is function');
  assert.isFunction(d1.reject, 'reject is function');

  const promise = {};
  const resolve = () => { /* empty */ };
  let reject = () => { /* empty */ };

  function P(exec) {
    exec(resolve, reject);
    return promise;
  }

  const d2 = withResolvers.call(P);
  assert.same(d2.promise, promise, 'promise is promise #2');
  assert.same(d2.resolve, resolve, 'resolve is resolve #2');
  assert.same(d2.reject, reject, 'reject is reject #2');

  reject = {};

  assert.throws(() => withResolvers.call(P), TypeError, 'broken resolver');
  assert.throws(() => withResolvers.call({}), TypeError, 'broken constructor #1');
  assert.throws(() => withResolvers.call(null), TypeError, 'broken constructor #2');
});

QUnit.test('Promise.withResolvers, resolve', assert => {
  const d = Promise.withResolvers();
  d.resolve(42);
  return d.promise.then(it => {
    assert.same(it, 42, 'resolved as expected');
  }, () => {
    assert.avoid();
  });
});

QUnit.test('Promise.withResolvers, reject', assert => {
  const d = Promise.withResolvers();
  d.reject(42);
  return d.promise.then(() => {
    assert.avoid();
  }, error => {
    assert.same(error, 42, 'rejected as expected');
  });
});