onMount

Runs once when the component initializes. Signal reads inside it are not tracked, so it never re-runs.

import { onMount } from '@barefootjs/client'

onMount(fn: () => void): void

onMount(() => {
  const onHashChange = () => setFilter(window.location.hash === '#/active' ? 'active' : 'all')
  window.addEventListener('hashchange', onHashChange)
  onCleanup(() => window.removeEventListener('hashchange', onHashChange))
})

#Runs during hydration

onMount(fn) is createEffect(() => untrack(fn)), so onCleanup works inside it and, like every effect, it runs synchronously during hydration. A signal written here changes the server-rendered DOM as soon as the component hydrates; if the value is knowable up front, seed the signal with it instead. See Effects run during hydration.