$ cat then.js
const effects = [], effect = x => effects.push(x);
Promise.resolve({
    then() {
        effect(1);
    }
});
(async function() {
    return {
        then() {
            effect(2);
        }
    };
})();
(async function() {
    await ({
        then: function() {
            effect(3);
        }
    });
})();
(async function() {
    await ({
        get then() {
            effect(4);
            return () => effect(5);
        }
    });
})();
(async function() {
    await ({
        then() {
            effect(6);
        }
    });
})();
(async function() {
    await await ({
        then(resolve) {
            resolve({
                then() {
                    effect(7);
                }
            });
        }
    });
})();
async function action(x) {
    return x;
}
action({}); // no side effects - may be dropped
var promise = action(8);
action({
    then(success, fail) {
        success(effect(9));
    }
});
promise.then(x => effect(x));
action({
    then(resolve) {
        resolve({
            then() {
                console.log(effects.join(" "));
            }
        });
    }
});

$ cat then.js | node
4 1 2 3 5 6 9 8 7

$ cat then.js | rollup --silent | node
$ 

$ cat then.js | rollup --silent
const effects = [], effect = x => effects.push(x);
async function action(x) {
    return x;
}
var promise = action(8);
promise.then(x => effect(x));