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What is the call stack? What is the difference between the microtask queue and the macrotask (task) queue? How does the event loop decide what to run next? Why does a long synchronous function block UI updates and event handling? 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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JavaScript › Event Loop & Concurrency

Why does Promise.resolve().then(cb) run before setTimeout(cb, 0), even with a 0ms delay?

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Answer

setTimeout(fn, 0) doesn't run immediately — it's still scheduled as a macrotask, which the browser also clamps to a minimum delay (often ~4ms after nesting). Promise callbacks are microtasks, and ALL pending microtasks are drained before the event loop even considers the next macrotask.
setTimeout(() => console.log('macro'), 0);
Promise.resolve().then(() => console.log('micro'));
// logs: 'micro' then 'macro', every time
Real-world example A subtle bug where code assumed setTimeout(fn, 0) would run 'immediately after' the current synchronous code, before any promises.

Common follow-ups: Is there any way to schedule work that runs before all microtasks?

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