All 10 Event Loop & Concurrency questions
More in Event Loop & Concurrency
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? 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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JavaScript › Event Loop & Concurrency

Is JavaScript single-threaded, and what does that mean in practice?

Beginner

Answer

Yes, JavaScript runs on a single main thread, executing one operation at a time. It achieves the appearance of doing multiple things at once through asynchronous callbacks, Promises, and the event loop — not through true parallel execution on that thread.
console.log('1');
setTimeout(() => console.log('2'), 0);
console.log('3');
// logs: 1, 3, 2
Real-world example Understanding why a long synchronous loop freezes the entire page, including scrolling and clicks.

Common follow-ups: How do Web Workers get around the single-threaded limitation?

Web Workers & Multithreading
All 10 Event Loop & Concurrency questions