What is process.nextTick and how does it differ from setImmediate?

nextTick is not a timer and not a promise. It runs immediately after the current operation completes, before Node continues with anything else. The nextTick queue is processed before the microtask queue, and both are drained between event loop phases.

setImmediate(() => console.log("immediate"));
setTimeout(() => console.log("timeout"), 0);
Promise.resolve().then(() => console.log("promise"));
process.nextTick(() => console.log("nextTick"));

console.log("sync");

Why it exists

• Let an API finish its current operation before invoking a user callback
• Emit events or errors after the caller has had a chance to attach handlers
• Defer work until the current stack unwinds without waiting for a timer
• Guarantee ordering ahead of promise continuations when that is truly required

The danger is starvation. If a nextTick callback schedules another nextTick, the queue never empties and the event loop never reaches I/O. The same is true of a promise chain that keeps resolving itself. Use nextTick sparingly and never in a recursive pattern without a bound.