Vue Academy · Lesson

onUpdated and onBeforeUpdate

Reacting to reactive data changes, avoiding infinite update loops in onUpdated.

onUpdated and onBeforeUpdate is a free Vue Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Vue Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Hooks Around Updates

When reactive data changes, Vue re-renders the component. Two hooks bracket this: onBeforeUpdate fires before the DOM is patched and onUpdated fires after.

onUpdated: After the Patch

onUpdated runs once the DOM has been updated to reflect new reactive state. At this point the rendered DOM matches your data.

<script setup>
import { onUpdated } from 'vue'
onUpdated(() => {
  console.log('DOM is now up to date')
})
</script>

Reading Updated DOM

Because the patch is complete, onUpdated is the right place to read new element sizes or positions that depend on the latest render.

<script setup>
import { ref, onUpdated } from 'vue'
const list = ref(null)
onUpdated(() => {
  console.log(list.value.scrollHeight)
})
</script>

The Infinite Loop Trap

Never mutate reactive state inside onUpdated. Doing so triggers another render, which fires onUpdated again, which mutates again — an infinite loop that freezes the app.

<script setup>
import { ref, onUpdated } from 'vue'
const count = ref(0)
onUpdated(() => {
  count.value++ // BAD: infinite update loop
})
</script>

Use watch Instead

If you want to react to a specific value changing, use watch, not onUpdated. A watch targets one source and avoids the loop risk, while onUpdated fires for every render.

watch(count, (newVal) => {
  console.log('count became', newVal)
})

onBeforeUpdate: Before the Patch

onBeforeUpdate runs after a reactive change but before Vue patches the DOM. The DOM still shows the old values here.

<script setup>
import { onBeforeUpdate } from 'vue'
onBeforeUpdate(() => {
  console.log('DOM still has old values')
})
</script>

Use Case: Pre-Update Snapshot

Because onBeforeUpdate sees the old DOM, it is perfect for capturing a snapshot before the change — for example, recording the current scroll position to restore it after the update.

<script setup>
import { ref, onBeforeUpdate, onUpdated } from 'vue'
const el = ref(null)
let savedScroll = 0
onBeforeUpdate(() => { savedScroll = el.value.scrollTop })
onUpdated(() => { el.value.scrollTop = savedScroll })
</script>

They Fire on Every Render

Both hooks run on each update cycle, which can be frequent. Keep their callbacks lightweight to avoid performance problems.

Not for Data Side Effects

These hooks are about the DOM, not your data. For data-driven reactions prefer computed (for derived values) or watch (for side effects on specific sources).

Order Recap

On a reactive change the sequence is: state changes, onBeforeUpdate runs (old DOM), Vue patches the DOM, then onUpdated runs (new DOM).

Choosing Between Them

onBeforeUpdate: capture old DOM state, like scroll or selection.
onUpdated: read the new DOM after patch — but never mutate reactive state here.

Quick Check

Test your update hooks knowledge.

Recap

Update hooks summary:

onUpdated runs after the DOM patch — read new sizes here.
Never mutate reactive state inside it (infinite loop); use watch instead.
onBeforeUpdate runs before the patch with old DOM — great for snapshots.
Both fire on every render, so keep them light.

Frequently asked questions

Is the “onUpdated and onBeforeUpdate” lesson free?

Yes — the full text of “onUpdated and onBeforeUpdate” is free to read here on the web, and the Vue Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Vue Academy course, upgrade to CoddyKit PRO.

What will I learn in “onUpdated and onBeforeUpdate”?

Reacting to reactive data changes, avoiding infinite update loops in onUpdated. You practise Vue Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Vue Academy?

No prior experience is required. Vue Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “onUpdated and onBeforeUpdate” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Vue Academy lesson?

Yes. Every Vue Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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