Lifecycle hooks let you run code at specific moments in a component's life: when it's created, mounted to the DOM, updated, or destroyed. In the Composition API, you register them as functions inside <script setup>.

The main hooks

import {
  onBeforeMount,
  onMounted,
  onBeforeUpdate,
  onUpdated,
  onBeforeUnmount,
  onUnmounted
} from 'vue'

onBeforeMount(() => {
  // DOM not available yet
})

onMounted(() => {
  // DOM is ready, safe to access template refs, start timers, fetch data
})

onBeforeUpdate(() => {
  // reactive state changed, DOM not yet re-rendered
})

onUpdated(() => {
  // DOM re-rendered with new state
})

onBeforeUnmount(() => {
  // component still functional, clean up before removal
})

onUnmounted(() => {
  // component removed from DOM, all watchers stopped
})

Lifecycle flow

setup()
  │
  ├── onBeforeMount
  ├── onMounted          ← DOM ready
  │
  │   (reactive state changes)
  ├── onBeforeUpdate
  ├── onUpdated          ← DOM re-rendered
  │
  │   (component removed)
  ├── onBeforeUnmount
  └── onUnmounted        ← fully cleaned up

Which hook for what

Task	Hook
Fetch initial data	onMounted
Access template refs	onMounted
Start a timer or listener	onMounted (clean up in onUnmounted)
React to DOM changes after update	onUpdated
Clean up timers, listeners, subscriptions	onUnmounted

Common pattern: setup + cleanup

onMounted(() => {
  window.addEventListener('resize', handleResize)
})

onUnmounted(() => {
  window.removeEventListener('resize', handleResize)
})

Options API equivalent

If you see older code using Options API, the mapping is direct:

Composition API	Options API
onBeforeMount	beforeMount
onMounted	mounted
onBeforeUpdate	beforeUpdate
onUpdated	updated
onBeforeUnmount	beforeUnmount
onUnmounted	unmounted

There is no onCreated or onBeforeCreate in the Composition API. Code that would go there runs directly in setup() (or at the top level of <script setup>), since setup itself runs at creation time.

See also: What is the Composition API and how does it differ from the Options API? · Can you use await directly in script setup?

References

• onMounted - Vue.js docs
• Lifecycle Hooks guide - Vue.js docs
• Composition API: Lifecycle Hooks - Vue.js docs