process.nextTick

TL;DR

process.nextTick() adds a callback to the “next tick queue”. This queue is processed immediately after the current operation finishes, before the Event Loop moves on to any other phases, and even before Promises (Microtasks) are resolved.

Mental Model

How It Works

Callbacks passed to process.nextTick are resolved before the Event Loop takes its next step. It is the absolute highest priority asynchronous queue in Node.js.

Example

console.log('1. Start');

setTimeout(() => console.log('4. setTimeout'), 0);

Promise.resolve().then(() => console.log('3. Promise'));

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

console.log('Start finished');

/* Output:
1. Start
Start finished
2. nextTick
3. Promise
4. setTimeout
*/

Common Interview Questions

Can process.nextTick starve the Event Loop?

Yes! If you recursively call process.nextTick(), Node.js will continually process the next tick queue and will never reach the Event Loop phases (like processing I/O or timers).

function starve() {
    process.nextTick(starve); // Blocks the event loop forever
}

When should you use process.nextTick?

1. To allow users to initialize event handlers (like .on('event')) before an event is emitted.
2. To ensure a callback is always executed asynchronously, even if the data is available synchronously, keeping API behavior consistent.