Consider the following code:

function main() {
    console.log('START');
    test('A');
    test('B');
    console.log('END');
}

function waitForSomething(label) {
    return {
        then(onFulfilled) {
            console.log(`FULFILL: ${label}`);
            onFullfilled();
        }
    }
}

async function test(label) {
    console.log(`BEFORE: ${label}`);
    await waitForSomething(label);
    console.log(`AFTER: ${label}`);
}

It gives the following output:

START
BEFORE: A
BEFORE: B
END
FULFILL: A
FULFILL: B
AFTER: A
AFTER: B

I would like AFTER A to trigger immediately after FULFILL A, in a synchronous way. The idea is that I’m writting a library with a global state that I want to stay consistent accross a chain of asynchronous calls written by the user. Because multiple chains may be running at the same time, I need to run some code before a call chain “resumes”. I understand it may seem like a weird or bad idea at first glance, but this is a separate question. Is there a way to do that?

I’ve thought of returning an object with something like an unwrap method that would trigger the code, like:

(await waitForSomething(label)).unwrap();

But this forces the user to use unwrap() on all relevant calls, which is tedious. Moreover they may forget to do it and run into unexpected issues without understanding why.