Node.jsTrong I/O callback context, setImmediate() LUÔN chạy trước setTimeout(fn, 0) — vì check phase đến trước timers phase sau poll; bên ngoài I/O context thứ tự không đảm bảo. setTimeout(fn, 0) thực ra là setTimeout(fn, 1) minimum — chạy trong timers phase nếu timer đã expired. setImmediate() chạy trong check phase (ngay sau poll phase).
- Ngoài I/O context (ví dụ main script): thứ tự giữa hai cái không đảm bảo — phụ thuộc vào system clock resolution và thời gian setup event loop.
- Trong I/O callback context: setImmediate() LUÔN chạy trước setTimeout(fn, 0) — vì I/O callback chạy ở poll phase, sau poll là check phase (setImmediate), sau đó mới quay lại timers phase.
Ví dụ: fs.readFile(file, () => { setImmediate(() => console.log('immediate')); setTimeout(() => console.log('timeout'), 0); }) — luôn in 'immediate' trước 'timeout'.
Inside an I/O callback, setImmediate() ALWAYS fires before setTimeout(fn, 0) — check phase comes before timers phase after poll; outside I/O context order is not guaranteed. setTimeout(fn, 0) is actually setTimeout(fn, 1) minimum — runs in the timers phase when the timer has expired. setImmediate() runs in the check phase (immediately after the poll phase).
- Outside an I/O context (e.g., in the main script): the order between the two is not guaranteed — depends on system clock resolution and event loop setup time.
- Inside an I/O callback context: setImmediate() ALWAYS runs before setTimeout(fn, 0) — because I/O callbacks run in the poll phase, and after poll comes the check phase (setImmediate), then timers.
- Example:
fs.readFile(file, () => { setImmediate(() => console.log('immediate')); setTimeout(() => console.log('timeout'), 0); })— always prints 'immediate' before 'timeout'. - Practical implication: when you want code to run after the current I/O operation but before any timers, use setImmediate.