• Call stack — runs synchronous code, one frame at a time
• Web APIs / Node APIs — handle timers, I/O and network calls in the background
• Microtask queue — Promise callbacks (.then, async/await) — runs before macrotasks
• Macrotask (callback) queue — setTimeout, setInterval, I/O callbacks
• The event loop keeps pulling from the microtask queue, then the macrotask queue, whenever the call stack is empty

console.log('1');
setTimeout(() => console.log('2'), 0);
Promise.resolve().then(() => console.log('3'));
console.log('4');

// Output: 1, 4, 3, 2
// Microtasks (Promises) always run before macrotasks (setTimeout)