setTimeout vs setImmediate vs process.nextTick in Node.js

setTimeout vs setImmediate vs process.nextTick in Node.js

setTimeout, setImmediate, and process.nextTick schedule callbacks at different times in the event loop. Here is the difference and execution order.

setTimeout

Schedules a callback in the timers phase, after a delay. setTimeout(fn, 0) runs on the next tick's timers phase, after the current stack and all microtasks complete.

setImmediate

Schedules a callback in the check phase, right after the poll phase. setImmediate runs after I/O events are processed in the current tick, before the next timers phase.

process.nextTick

Schedules a callback to run after the current operation, before the next phase of the event loop and before promise microtasks. It is the most urgent of the three.

Execution Order

In general: process.nextTick runs first (before promises and before the next event loop phase), then promises, then the event loop phases (timers, poll, check). So nextTick, then promises, then setImmediate, then setTimeout on the next tick.

When to Use Each

Use setImmediate to run code after the current I/O events. Use setTimeout for a delayed callback. Use process.nextTick rarely, since it can starve the event loop by running before I/O. Most code uses setImmediate or async/await.

The Common Confusion

People confuse setImmediate and nextTick. setImmediate runs in the check phase after I/O. nextTick runs before the next phase, even before I/O. nextTick is more urgent but can starve the loop if overused.

The Takeaway

setTimeout runs in timers phase after a delay. setImmediate runs in check phase after I/O. process.nextTick runs before the next phase, even before promises. Order: nextTick, promises, then event loop phases. Use setImmediate for most deferred work.