Lifecycle

Preferred names:

Hook	When
onBeforeMount	After the instance exists, before the first render()
onMount	After the first DOM commit (the whole tree is inserted; refs have run)
onUpdate	After every later patch
onDestroy	Before the instance is removed

2.x aliases still work: _beforeMount, _didMount, _didUpdate. There was no unmount hook before; use onDestroy.

onMount may call setState. That schedules one extra patch after mount — it does not re-run onMount. Nested children run onMount before their parent. Refs run before onMount, so a ref callback can stash an element that onMount then focuses.

isConnected is true in onMount only when the root container is in the document. Detached roots still mount; hooks still run.

onBeforeMount also runs during renderToString. Put browser-only work in onMount. SSR never runs refs or onMount.

onDestroy, ref teardown, and observe unsubscribe functions always run to completion even if one of them throws. The first error is rethrown after the rest of the cleanup.

Clear timers and subscriptions in onDestroy. The composition demo does this for its interval. unmountRoot(container) is safe to call more than once.