Function onEffectCleanup

packages/reactivity/src/effect.ts:558–567

(fn: () => void, failSilently = false)

 * an active effect.
 */
export function onEffectCleanup(fn: () => void, failSilently = false): void {
  if (activeSub instanceof ReactiveEffect) {
    activeSub.cleanup = fn
  } else if (__DEV__ && !failSilently) {
    warn(
      `onEffectCleanup() was called when there was no active effect` +
        ` to associate with.`,
    )
  }
}

function cleanupEffect(e: ReactiveEffect) {
  const { cleanup } = e