Timers

setTimeout

setTimeout(fn, delay) schedules fn to run as a macrotask after at least delay milliseconds. The delay is a minimum — not a guarantee. If the call stack is busy when the timer fires, the callback waits until the stack is clear.

var id = setTimeout(function() {
  console.log('Runs after at least 100 ms');
}, 100);

// Cancel before it fires:
clearTimeout(id);

setInterval

setInterval(fn, delay) repeatedly schedules fn every delay ms. Use clearInterval to stop it.

var count = 0;
var id = setInterval(function() {
  count++;
  console.log('tick', count);
  if (count >= 3) clearInterval(id);
}, 500);

setTimeout with delay 0

setTimeout(fn, 0) does not mean “run immediately.” It queues fn as a macrotask. Any pending microtasks (Promise callbacks, queueMicrotask) will run first.

setTimeout(function() { console.log('Timer'); }, 0);
Promise.resolve().then(function() { console.log('Promise'); });
// Output: Promise, then Timer

queueMicrotask

To schedule a callback before the next macrotask, use queueMicrotask:

queueMicrotask(function() {
  console.log('This runs in the microtask queue, before any timer');
});

Runnable demo

This demo shows that setTimeout(fn, 0) still runs after a resolved Promise .then, because the timer callback is a macrotask and microtasks drain first.

JavaScript

setTimeout(function() {
  console.log('Timer ran second');
}, 0);

Promise.resolve().then(function() {
  console.log('Promise ran first');
});

Output is always: Promise ran first, then Timer ran second.