What is the output order when synchronous code, Promise callbacks, and setTimeout(0) are mixed?

Direct answer

Synchronous logs run first, then queued Promise microtasks, then the timer task in a later event-loop turn.

event-loop-order.js

console.log("A");
setTimeout(() => console.log("B"), 0);
Promise.resolve().then(() => console.log("C"));
console.log("D");

Output

A
D
C
B

For console.log('A'); setTimeout(() => console.log('B'), 0); Promise.resolve().then(() => console.log('C')); console.log('D');, the common browser/host ordering is A, D, C, B.

A prints during the current synchronous execution.
The timer callback is scheduled for a future task.
Promise.then() schedules a Promise reaction job that the host processes as a microtask.
D prints before the current stack finishes.
At the microtask checkpoint, C runs before the event loop takes the timer task that prints B.

A stronger interview follow-up is to add another Promise.then() inside the first microtask. That new microtask is normally processed in the same checkpoint before the event loop advances to the timer task.