Node.js Timers setTimeout setInterval

What is Timers setTimeout setInterval in Node.js?

setTimeout schedules a callback once, setInterval repeats it, and setImmediate runs on the next event loop iteration. All three return handles you must clear to avoid keeping the process alive.

Timers setTimeout setInterval example

const t = setTimeout(() => console.log('later'), 1000);
clearTimeout(t);

const i = setInterval(poll, 5000);
clearInterval(i);

// promise-based timers
import { setTimeout as sleep } from 'node:timers/promises';
await sleep(1000);
console.log('one second later');

Key points to remember

• The delay is a minimum, not a guarantee — a blocked event loop delays it further.
• An uncleared interval keeps the Node process running forever.
• unref() lets a timer exist without holding the process open.
• setInterval can overlap if the callback takes longer than the interval — prefer a self-scheduling setTimeout for polling.

Common mistakes with Timers setTimeout setInterval

• A polling setInterval whose work takes longer than the interval, stacking overlapping runs.
• Forgetting clearInterval in a test, so the test runner never exits.