Effective scope for cleanup in Vue - Mini Project: Build & Apply

Effective scope for cleanup in Vue 3

📋 What You'll Learn

💡 Why This Matters

🌍 Real World

Tracking window size is common in responsive web design to adjust layouts or load different components based on screen size.

💼 Career

Understanding lifecycle hooks and cleanup in Vue is essential for building efficient, bug-free applications that manage resources well.

1

Setup reactive window width

Create a Vue 3 component with setup() function. Inside it, create a ref called width and set it to window.innerWidth.

Vue

// Create a ref called width and set it to window.innerWidth
// Your code here



  Window width: {{ width }}

Hint

Use ref from Vue to create a reactive variable.

2

Add window resize event listener

Inside the setup() function, import and use onMounted. Add a window resize event listener that updates width.value to window.innerWidth when the window resizes.

Vue

import { ref, onMounted } from 'vue'

const width = ref(window.innerWidth)

// Add onMounted to add window resize event listener
// Your code here



  Window width: {{ width }}

Hint

Use onMounted to add the event listener only after the component is ready.

3

Add cleanup with onUnmounted

Import onUnmounted and use it to remove the window resize event listener when the component unmounts. Store the event handler function in a variable called updateWidth so you can remove it properly.

Vue

import { ref, onMounted, onUnmounted } from 'vue'

const width = ref(window.innerWidth)

// Define updateWidth function
// Use onMounted to add event listener with updateWidth
// Use onUnmounted to remove event listener with updateWidth
// Your code here



  Window width: {{ width }}

Hint

Store the event handler in a variable so you can remove it later with removeEventListener.

4

Complete component with reactive display

Ensure the template displays the reactive width value inside a <div>. The component should update the displayed width as the window resizes and clean up the event listener on unmount.

Vue

import { ref, onMounted, onUnmounted } from 'vue'

const width = ref(window.innerWidth)

const updateWidth = () => {
  width.value = window.innerWidth
}

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

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




  

Hint

The template should show the current window width reactively.

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