When are chained callbacks in promises resolved?

Promise.then's "onFulfilledHandler" is called asynchronously. Meaning the callback will enter the queue for immediate execution after current stack fully unraveled. Calling 1000 then's takes a few milliseconds:

function f(x) {
    if (x > 0) {
        return Promise.resolve(--x).then(f);
    }
    console.timeEnd('f');
}

console.time('f');
f(1000);