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12. The Event Loop

Call stack, microtasks, and macrotasks.

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JavaScript uses a single-threaded event loop. When the call stack is clear, all pending microtasks (Promises, queueMicrotask) execute before the next macrotask (setTimeout, I/O).

js
setTimeout(() => console.log("macro"));
Promise.resolve().then(() => console.log("micro"));
// micro logs before macro

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  1. 1. Which queue executes first when the call stack clears?

  2. 2. What blocks the browser UI thread?

  3. 3. Is JavaScript execution multi-threaded by default?

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