What is the difference between process.nextTick and setImmediate?

Direct answer

process.nextTick runs before the next event loop phase continues — highest priority, can starve I/O; setImmediate runs in the check phase after poll, designed to defer work until after current I/O callbacks finish.

Both schedule callbacks asynchronously, but they sit in different queues with different priority. nextTick is not part of libuv phases — Node drains the entire nextTick queue between every phase transition.

	process.nextTick	setImmediate
Queue	nextTick queue (Node-specific)	check phase of event loop
Priority	Higher — before Promises and next phase	After poll phase
Use for	Defer error propagation, run after sync assign	Split long sync work across turns
Risk	Recursive nextTick starves I/O	Safer for yielding; still use workers for CPU

nexttick-vs-immediate.js

console.log("start");

setImmediate(() => console.log("setImmediate"));

process.nextTick(() => console.log("nextTick 1"));

process.nextTick(() => console.log("nextTick 2"));

Promise.resolve().then(() => console.log("promise"));

console.log("end");

Output

start
end
nextTick 1
nextTick 2
promise
setImmediate

nextTick use case — emit 'error' on next tick so all 'error' listeners attach first (EventEmitter pattern).
setImmediate use case — break up sync CPU without blocking poll as aggressively as nextTick chains.
queueMicrotask — standard microtask API; runs after nextTick, before macrotasks.