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Is JavaScript single-threaded, and what does that mean in practice? What is the call stack? How does the event loop decide what to run next? Why does a long synchronous function block UI updates and event handling? Why does Promise.resolve().then(cb) run before setTimeout(cb, 0), even with a 0ms delay? How would you break a large synchronous task into chunks to keep the UI responsive? What is 'starvation' of the macrotask queue, and how can recursive microtasks cause it? How does async/await map onto the microtask queue under the hood? What's the relationship between requestAnimationFrame and the event loop's rendering step?
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What is the difference between the microtask queue and the macrotask (task) queue?

Intermediate

Answer

Promise callbacks (.then, async/await continuations) go into the microtask queue, which is fully drained after every single macrotask completes. setTimeout, setInterval, and I/O callbacks go into the macrotask queue, processed one at a time between event loop ticks.
console.log('1');
setTimeout(() => console.log('2 (macrotask)'), 0);
Promise.resolve().then(() => console.log('3 (microtask)'));
console.log('4');
// logs: 1, 4, 3, 2
Real-world example Understanding why a Promise callback always runs before a setTimeout(fn, 0), even though both are 'async'.

Common follow-ups: What happens if a microtask queues another microtask — does it get processed in the same cycle?

Promises & async/await
All 10 Event Loop & Concurrency questions