Lifecycle and watchers

onMount {
  const res = await fetch("/api/posts")
  posts = await res.json()
  loading = false
}

onDestroy {
  clearInterval(timer)
}

watch query {
  results = search(value)          -- `value` is the new value, `previous` the old one
}

These work at page level and inside components. onMount runs once after the page or component is in the DOM. watch runs whenever the expression's value changes (arrays and objects are compared by content).