Effective scope for cleanup in Vue

Introduction

Cleaning up resources like timers or event listeners helps keep your app fast and bug-free. It stops things from running when they are no longer needed.

When you start a timer or interval inside a component

When you add event listeners to the window or document

When you subscribe to external data or services

When you want to avoid memory leaks in your app

When you want to reset or clear resources before a component is removed

Syntax

Vue

<script setup>
import { onUnmounted } from 'vue'

const timer = setInterval(() => {
  console.log('Tick')
}, 1000)

onUnmounted(() => {
  clearInterval(timer)
})
</script>

onUnmounted runs cleanup code when the component is removed from the page.

Always pair setup code (like timers) with cleanup code to avoid leftover effects.

Examples

This example adds a click listener and removes it when the component unmounts.

Vue

<script setup>
import { onUnmounted } from 'vue'

const listener = () => console.log('Clicked')
window.addEventListener('click', listener)

onUnmounted(() => {
  window.removeEventListener('click', listener)
})
</script>

This example updates a counter every second and stops the timer when the component is gone.

Vue

<script setup>
import { ref, onUnmounted } from 'vue'

const count = ref(0)
const interval = setInterval(() => {
  count.value++
}, 1000)

onUnmounted(() => {
  clearInterval(interval)
})
</script>

Sample Program

This component shows seconds passed. It starts a timer when created and clears it when removed to stop counting.

Vue

<template>
  <div>
    <p>Seconds passed: {{ seconds }}</p>
  </div>
</template>

<script setup>
import { ref, onUnmounted } from 'vue'

const seconds = ref(0)

const timer = setInterval(() => {
  seconds.value++
}, 1000)

onUnmounted(() => {
  clearInterval(timer)
  console.log('Timer cleared on unmount')
})
</script>

Important Notes

Always use onUnmounted to clean up side effects in Vue components.

Forgetting to clean up can cause bugs or slow down your app over time.

Cleanup functions run automatically when the component is removed from the page.

Summary

Use onUnmounted to run cleanup code in Vue components.

Cleanup stops timers, listeners, or subscriptions to keep your app healthy.

Always match setup code with cleanup to avoid memory leaks.

Practice

1. In Vue, which lifecycle hook is best to use for cleaning up timers or event listeners when a component is removed?

easy

A. onUpdated

B. onMounted

C. onUnmounted

D. onCreated

Solution

1. Step 1: Understand Vue lifecycle hooks

   onMounted runs when the component appears, onUnmounted runs when it disappears.

2. Step 2: Identify cleanup timing

   Cleanup like stopping timers or removing listeners should happen when the component is removed, so onUnmounted is the right hook.

3. Final Answer:

   onUnmounted -> Option C

4. Quick Check:

   Cleanup runs on unmount = C [OK]

Hint: Cleanup always goes in onUnmounted hook [OK]

Common Mistakes:

• Using onMounted for cleanup
• Confusing onUpdated with cleanup
• Trying to cleanup in onCreated

2. Which of the following is the correct syntax to register a cleanup function in Vue's setup() using the onUnmounted hook?

easy

A. onUnmounted(() => { clearInterval(timer) })

B. onUnmounted = () => { clearInterval(timer) }

C. onUnmounted.clear(() => { clearInterval(timer) })

D. onUnmounted.clearInterval(timer)

Solution

1. Step 1: Recall Vue hook syntax

   Vue lifecycle hooks like onUnmounted are functions that take a callback to run on that event.

2. Step 2: Match correct syntax

   The correct way is calling onUnmounted with a function: onUnmounted(() => { ... }). Other options misuse assignment or methods.

3. Final Answer:

   onUnmounted(() => { clearInterval(timer) }) -> Option A

4. Quick Check:

   Hook usage = function call with callback [OK]

Hint: Hooks are called with a function, not assigned [OK]

Common Mistakes:

• Assigning instead of calling onUnmounted
• Using nonexistent methods on onUnmounted
• Missing arrow function wrapper

3. What will be the console output when this Vue component unmounts?

setup() {
  const timer = setInterval(() => console.log('tick'), 1000)
  onUnmounted(() => clearInterval(timer))
}

medium

A. Logs 'tick' only once when component unmounts

B. Logs 'tick' every second until component unmounts, then stops

C. No logs appear at all

D. Throws an error on unmount

Solution

1. Step 1: Understand setInterval behavior

   setInterval logs 'tick' every 1000ms continuously until cleared.

2. Step 2: Analyze cleanup with onUnmounted

   When the component unmounts, clearInterval(timer) stops the interval, so logging stops after unmount.

3. Final Answer:

   Logs 'tick' every second until component unmounts, then stops -> Option B

4. Quick Check:

   Interval runs until cleared on unmount [OK]

Hint: Intervals run until cleared in onUnmounted [OK]

Common Mistakes:

• Thinking logs stop immediately without unmount
• Expecting logs only once on unmount
• Assuming error on clearInterval

4. Identify the problem in this Vue setup function and how to fix it:

setup() {
  const timer = setInterval(() => console.log('tick'), 1000)
  onMounted(() => clearInterval(timer))
}

medium

A. Cleanup runs too early; should use onUnmounted instead

B. Timer is never created; setInterval is missing

C. clearInterval is called without timer variable

D. No problem; code works correctly

Solution

1. Step 1: Check when cleanup runs

   Using onMounted to clear interval means it stops immediately after mounting, so no ticks happen.

2. Step 2: Correct cleanup timing

   Cleanup should happen when component unmounts, so onUnmounted is the correct hook to clear the timer.

3. Final Answer:

   Cleanup runs too early; should use onUnmounted instead -> Option A

4. Quick Check:

   Cleanup must be on unmount, not mount [OK]

Hint: Cleanup belongs in onUnmounted, not onMounted [OK]

Common Mistakes:

• Clearing timer on mount instead of unmount
• Assuming onMounted is for cleanup
• Ignoring lifecycle hook purpose

5. You have a Vue component that subscribes to a WebSocket in setup(). Which pattern correctly ensures the subscription is cleaned up only when the component unmounts?

setup() {
  const socket = new WebSocket('wss://example.com')
  socket.onmessage = (event) => console.log(event.data)
  // What cleanup code and where?
}

hard

A. Call socket.close() immediately after creating socket

B. No cleanup needed for WebSocket in Vue

C. Use onMounted(() => socket.close()) to close socket on mount

D. Use onUnmounted(() => socket.close()) inside setup

Solution

1. Step 1: Understand WebSocket lifecycle

   WebSocket stays open until closed; closing it too early stops messages.

2. Step 2: Place cleanup in onUnmounted

   To avoid leaks, close the socket when component unmounts using onUnmounted(() => socket.close()).

3. Final Answer:

   Use onUnmounted(() => socket.close()) inside setup -> Option D

4. Quick Check:

   Cleanup subscriptions on unmount with onUnmounted [OK]

Hint: Close subscriptions in onUnmounted hook [OK]

Common Mistakes:

• Closing socket immediately after creation
• Closing socket on mount instead of unmount
• Skipping cleanup causing memory leaks