Effective scope for cleanup in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of cleanup in Vue's lifecycle?

Cleanup helps remove side effects like timers, event listeners, or subscriptions when a component is no longer needed. This prevents memory leaks and keeps the app efficient.

beginner

Where should you place cleanup code in Vue 3 Composition API?

Use the onUnmounted lifecycle hook inside the setup() function to run cleanup code when the component is destroyed.

intermediate

Why is it important to keep cleanup code scoped inside setup()?

Keeping cleanup code inside setup() ensures it only affects the current component instance and avoids accidental interference with other parts of the app.

beginner

How do you clean up a timer in Vue 3 Composition API?

Create the timer inside setup(), store its ID, and clear it inside onUnmounted(() => clearTimeout(timerId)) to stop it when the component unmounts.

beginner

What happens if you forget to clean up side effects in Vue components?

Side effects like timers or event listeners keep running, causing memory leaks and unexpected behavior, which can slow down or break your app over time.

In Vue 3 Composition API, which lifecycle hook is used for cleanup?

A onCreated

B onUnmounted

C onUpdated

D onMounted

The onUnmounted hook runs when the component is removed, making it the right place for cleanup.

Where should cleanup code be placed to keep it effective and scoped?

A Inside setup() function

B Outside any function

C In the template section

D In a global script

Placing cleanup inside setup() keeps it tied to the component's lifecycle and scope.

What is a common side effect that needs cleanup in Vue components?

A Template markup

B Static HTML content

C CSS styles

D Timers like setTimeout or setInterval

Timers continue running unless cleared, so they need cleanup.

What problem can happen if cleanup is not done properly?

A Memory leaks and slow app performance

B Faster app loading

C Better user experience

D Automatic bug fixes

Not cleaning up causes memory leaks, which slow down the app.

Which of these is NOT a good practice for cleanup in Vue?

A Using onUnmounted inside setup()

B Clearing timers when component unmounts

C Leaving event listeners active after component removal

D Removing subscriptions on unmount

Leaving event listeners active causes memory leaks and bugs.

Explain how to properly scope cleanup code in a Vue 3 component using the Composition API.

Think about where side effects start and where they should end.

Describe why cleanup is important in Vue components and what can happen if it is neglected.

Consider what happens when components disappear but their effects keep running.

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