setImmediate

TL;DR

setImmediate() schedules a callback to execute in the Check phase of the Event Loop, which runs immediately after the Poll phase (I/O callbacks) completes.

Mental Model

How It Works

Despite the name, setImmediate does not run immediately. It yields to the event loop, allowing I/O operations to process, and then runs during the Check phase.

It was designed specifically to execute a script once the current Poll phase completes.

Example

const fs = require('fs');

// Outside an I/O cycle, the order of setTimeout and setImmediate is non-deterministic 
// due to process performance.
setTimeout(() => console.log('Timeout'), 0);
setImmediate(() => console.log('Immediate'));

// INSIDE an I/O cycle, setImmediate ALWAYS runs first
fs.readFile(__filename, () => {
  setTimeout(() => console.log('Timeout Inside IO'), 0);
  setImmediate(() => console.log('Immediate Inside IO'));
});

/* Output for I/O:
Immediate Inside IO
Timeout Inside IO
*/

Common Interview Questions

What is the difference between setTimeout(fn, 0) and setImmediate()?

• setTimeout(0) runs in the Timers phase.
• setImmediate() runs in the Check phase.
  If called within an I/O callback (like reading a file), setImmediate is guaranteed to execute before setTimeout(0), because the Check phase immediately follows the Poll phase.

setImmediate vs process.nextTick?

• process.nextTick fires before the event loop continues, pausing everything.
• setImmediate fires on the next iteration of the loop (Check phase), allowing other I/O to be processed first.