onCleanup

onCleanup(fn) registers fn on the current owner. It runs when that owner is disposed or, for an effect or memo, before it re-runs. Cleanups run newest first and untracked; one that throws does not stop the others. Called with no owner, onCleanup warns NO_OWNER_CLEANUP and the function never runs.

Use it in a component body for resources the component holds (timers, listeners, subscriptions):

import { onCleanup } from "@shadow-js/core";

export function Clock() {
  const timer = setInterval(() => {}, 1000);
  onCleanup(() => clearInterval(timer));
  return <span>tick</span>;
}