onMounted and onUnmounted in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the onMounted lifecycle hook do in Vue 3?

It runs a function right after the component is added to the page (DOM). Think of it like saying "Hey, I am ready now!"

beginner

When is the onUnmounted hook called in a Vue component?

It runs a function just before the component is removed from the page. It's like cleaning up your desk before leaving.

beginner

How do you import <code>onMounted</code> and <code>onUnmounted</code> in Vue 3 Composition API?

You import them from 'vue' like this: <br><code>import { onMounted, onUnmounted } from 'vue'</code>

intermediate

Why is onUnmounted useful in Vue components?

It helps you stop timers, remove event listeners, or clean resources so your app stays fast and tidy.

intermediate

Show a simple example of using onMounted and onUnmounted in a Vue component.

Example:<br><pre>import { ref, onMounted, onUnmounted } from 'vue'

export default {
  setup() {
    const count = ref(0)
    let timer

    onMounted(() => {
      timer = setInterval(() => {
        count.value++
      }, 1000)
    })

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

    return { count }
  }
}</pre>

What is the main purpose of onMounted in Vue 3?

A Run code after the component appears on the page

B Run code before the component is created

C Run code when the component is destroyed

D Run code before the component appears on the page

onMounted runs after the component is added to the page (DOM), so option A is correct.

Which hook should you use to clean up timers or event listeners in Vue 3?

A onUnmounted

B onMounted

C onCreated

D onUpdated

onUnmounted is used to clean up before the component is removed, so A is correct.

How do you import onMounted in a Vue 3 component?

A import { onMounted } from 'vue-router'

B import { onMounted } from 'vue'

C import onMounted from 'vue'

D import onMounted from 'vue-router'

You import onMounted from 'vue' using curly braces, so C is correct.

When does onUnmounted run in the component lifecycle?

A Right after the component is mounted

B Before the component is mounted

C Before the component is removed from the DOM

D After the component updates

onUnmounted runs just before the component is removed from the DOM, so B is correct.

Which of these is a good use case for onMounted?

A Stopping a timer when the component disappears

B Removing event listeners

C Resetting component data before removal

D Starting a timer when the component appears

onMounted is perfect for starting things like timers when the component appears, so D is correct.

Explain in your own words what onMounted and onUnmounted hooks do in Vue 3.

Think about when you start and stop something when entering or leaving a room.

Describe a practical example where you would use both onMounted and onUnmounted in a Vue component.

Imagine turning on a light when you enter a room and turning it off when you leave.

Practice

1. What is the main purpose of the onMounted lifecycle hook in Vue?

easy

A. To run code when the component is destroyed

B. To run code before the component is created

C. To update the component's data reactively

D. To run code right after the component appears on the screen

Solution

1. Step 1: Understand the lifecycle timing of onMounted

   The onMounted hook runs after the component is inserted into the DOM and visible to the user.

2. Step 2: Compare with other lifecycle hooks

   onUnmounted runs before removal, and creation hooks run earlier. Updating data is reactive and not tied to onMounted.

3. Final Answer:

   To run code right after the component appears on the screen -> Option D

4. Quick Check:

   onMounted = runs after component appears [OK]

Hint: onMounted runs code after component shows up [OK]

Common Mistakes:

• Confusing onMounted with onUnmounted
• Thinking onMounted runs before creation
• Assuming onMounted updates data reactively

2. Which of the following is the correct syntax to use onUnmounted in a Vue 3 component with Composition API?

easy

A. onUnmounted(() => { console.log('Component removed') })

B. onUnmounted = () => { console.log('Component removed') }

C. onUnmounted { console.log('Component removed') }

D. onUnmounted(function) { console.log('Component removed') }

Solution

1. Step 1: Recall the correct function call syntax

   onUnmounted is a function that takes a callback function as an argument, so it must be called with parentheses and a function inside.

2. Step 2: Check each option's syntax

   onUnmounted(() => { console.log('Component removed') }) correctly calls onUnmounted with an arrow function. Options B, C, and D have invalid syntax or missing parentheses.

3. Final Answer:

   onUnmounted(() => { console.log('Component removed') }) -> Option A

4. Quick Check:

   onUnmounted(callback) syntax correct [OK]

Hint: Use onUnmounted as a function with a callback [OK]

Common Mistakes:

• Missing parentheses when calling onUnmounted
• Assigning onUnmounted instead of calling it
• Using incorrect braces or keywords

3. Consider this Vue 3 component code snippet:

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

export default {
  setup() {
    const count = ref(0)
    let timer

    onMounted(() => {
      timer = setInterval(() => {
        count.value++
      }, 1000)
    })

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

    return { count }
  }
}

What will happen to count when the component is removed from the page?

medium

A. count will keep increasing even after component removal

B. count will stop increasing because the timer is cleared on unmount

C. count will reset to zero automatically

D. An error will occur because timer is undefined

Solution

1. Step 1: Understand the timer setup in onMounted

   The code starts a timer that increments count every second when the component mounts.

2. Step 2: Understand the cleanup in onUnmounted

   The timer is cleared with clearInterval(timer) when the component unmounts, stopping the increments.

3. Final Answer:

   count will stop increasing because the timer is cleared on unmount -> Option B

4. Quick Check:

   onUnmounted clears timer stopping count increment [OK]

Hint: onUnmounted cleans timers to stop background tasks [OK]

Common Mistakes:

• Thinking count resets automatically on unmount
• Assuming timer keeps running after unmount
• Confusing onMounted and onUnmounted roles

4. Identify the error in this Vue 3 setup function:

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

export default {
  setup() {
    const message = ref('Hello')

    onMounted(() => {
      console.log(message.value)
    })

    onUnmounted(() => {
      console.log('Goodbye')
    })

    onMounted(() => {
      console.log('Another mount')
    })

    return { message }
  }
}

medium

A. No error; multiple onMounted hooks are allowed and run in order

B. Multiple onMounted hooks are not allowed in the same setup

C. onUnmounted must be called before onMounted

D. message ref must be declared inside onMounted

Solution

1. Step 1: Check Vue 3 Composition API rules for lifecycle hooks

   Vue 3 allows multiple onMounted hooks inside the same setup function; they run in the order declared.

2. Step 2: Verify other options

   There is no requirement for onUnmounted to be before onMounted, and message can be declared outside hooks.

3. Final Answer:

   No error; multiple onMounted hooks are allowed and run in order -> Option A

4. Quick Check:

   Multiple onMounted hooks allowed [OK]

Hint: Multiple onMounted hooks run fine in setup [OK]

Common Mistakes:

• Thinking only one onMounted hook is allowed
• Believing lifecycle hooks must be in specific order
• Assuming refs must be declared inside hooks

5. You want to create a Vue 3 component that starts a WebSocket connection when it appears and closes it when it disappears. Which is the best way to use onMounted and onUnmounted to achieve this?

import { onMounted, onUnmounted } from 'vue'

export default {
  setup() {
    let socket

    // What should go here?

    return {}
  }
}

hard

A. Use onUnmounted to open the socket and onMounted to close it

B. Open the socket outside lifecycle hooks and close it inside onUnmounted

C. Use onMounted to open the socket and onUnmounted to close it

D. Open and close the socket both inside onMounted

Solution

1. Step 1: Understand lifecycle timing for resource management

   Resources like WebSocket should be opened when the component appears (onMounted) and cleaned up before it disappears (onUnmounted).

2. Step 2: Match options with correct lifecycle usage

   Use onMounted to open the socket and onUnmounted to close it correctly uses onMounted to open and onUnmounted to close the socket. Other options misuse lifecycle hooks or open/close at wrong times.

3. Final Answer:

   Use onMounted to open the socket and onUnmounted to close it -> Option C

4. Quick Check:

   Open onMounted, close onUnmounted for resources [OK]

Hint: Open resource onMounted, close onUnmounted [OK]

Common Mistakes:

• Opening resource outside lifecycle hooks
• Closing resource in onMounted instead of onUnmounted
• Reversing open and close lifecycle hooks