Lifecycle

Preferred names:

Hook	When
onBeforeMount	After the instance exists, before the first render()
onMount	After the first DOM commit
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.

Clear timers and subscriptions in onDestroy. The composition demo does this for its interval.