What is the output of nested Promise microtasks?

Direct answer

Output: 1, 5, 2, 3, 4 — sync first; microtasks drain fully (including nested .then) before macrotasks; each Promise.then queues one microtask.

promise-microtasks.mjs

console.log("1");

Promise.resolve().then(() => console.log("2"));

Promise.resolve().then(() => {
  console.log("3");
  Promise.resolve().then(() => console.log("4"));
});

setTimeout(() => console.log("timeout"), 0);

console.log("5");

Output

1
5
2
3
4
timeout

The microtask queue drains completely before the timers phase — nested Promise in the second .then schedules 4 before timeout runs.

Sync — 1, 5.
First microtask batch — 2 from first Promise; 3 from second.
Nested microtask — 4 queued during 3's callback, runs before leaving microtask checkpoint.
Macrotask — timeout last.