Event loop trong Node.js hoạt động như thế nào?

Node.js event loop xử lý async ops theo phases: timers→poll→check — giữa mỗi phase flush toàn bộ microtask queue (nextTick → Promises); thứ tự: process.nextTick(C), Promise.then(B), setTimeout(A, 0) → chạy C, B, A.

Event loop là vòng lặp liên tục xử lý callbacks theo 6 phases:

1. Timers: thực thi callbacks của setTimeout/setInterval đã đến hạn;
2. Pending callbacks: I/O errors từ vòng trước;
3. Idle/prepare: internal use;
4. Poll: fetch I/O events mới — nếu queue empty và không có timers pending, block tại đây chờ I/O;
5. Check: setImmediate callbacks;
6. Close callbacks: socket.on('close')

Giữa MỖI phase, Node.js xử lý toàn bộ microtask queue: Promise callbacks (then/catch) + queueMicrotask() + process.nextTick() (nextTick ưu tiên hơn Promise).

Starvation: nếu microtask queue không bao giờ empty (Promise resolve tạo Promise mới), event loop không bao giờ qua phase tiếp theo.

Ví dụ thực tế execution order: setTimeout(A, 0) → Promise.resolve().then(B) → process.nextTick(C) — thứ tự chạy: C, B, A.

Node.js event loop processes async ops through phases: timers→poll→check — microtask queue (nextTick → Promises) is flushed between every phase; execution order: process.nextTick(C), Promise.then(B), setTimeout(A, 0) → runs C, B, A.

The event loop is a continuously running cycle that processes callbacks across 6 phases:

1. Timers: executes callbacks from setTimeout/setInterval that have elapsed;
2. Pending callbacks: I/O errors from the previous cycle;
3. Idle/prepare: internal use;
4. Poll: fetches new I/O events — if the queue is empty and no timers are pending, blocks here waiting for I/O;
5. Check: setImmediate callbacks;
6. Close callbacks: socket.on('close')

Between EACH phase, Node.js processes the entire microtask queue: Promise callbacks (then/catch) + queueMicrotask() + process.nextTick() (nextTick has higher priority than Promises).

Starvation: if the microtask queue never empties (a resolved Promise creating new Promises), the event loop never advances to the next phase.

Practical execution order example: setTimeout(A, 0) → Promise.resolve().then(B) → process.nextTick(C) — execution order: C, B, A.