onMounted and onUnmounted in Vue - Performance & Optimization

Performance: onMounted and onUnmounted

MEDIUM IMPACT

These lifecycle hooks affect the timing of DOM interactions and resource cleanup, impacting rendering responsiveness and memory usage.

Managing event listeners in a Vue component

Vue

import { onMounted, onUnmounted } from 'vue';
onMounted(() => {
  window.addEventListener('resize', onResize);
});
onUnmounted(() => {
  window.removeEventListener('resize', onResize);
});

Removes event listener when component unmounts, freeing memory and avoiding unnecessary event handling.

📈 Performance GainReduces memory leaks and improves interaction responsiveness by cleaning up listeners.

Managing event listeners in a Vue component

Vue

import { onMounted } from 'vue';
onMounted(() => {
  window.addEventListener('resize', onResize);
});
// No cleanup on unmount

Event listener stays active after component unmount, causing memory leaks and unnecessary event handling.

📉 Performance CostIncreases memory usage and causes unnecessary event processing, leading to slower input responsiveness.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Add event listener in onMounted only	1 addEventListener	0 reflows	0 paint	[X] Bad
Add in onMounted and remove in onUnmounted	1 addEventListener + 1 removeEventListener	0 reflows	0 paint	[OK] Good

Rendering Pipeline

onMounted runs after the component's DOM is inserted, allowing safe DOM reads and writes without blocking initial paint. onUnmounted runs before the component is removed, enabling cleanup to prevent memory leaks and extra work.

→DOM Insertion

→Layout

→Paint

→Memory Management

⚠️ BottleneckMemory Management due to leftover event listeners or timers if cleanup is missing

Core Web Vital Affected

INP

These lifecycle hooks affect the timing of DOM interactions and resource cleanup, impacting rendering responsiveness and memory usage.

Optimization Tips

1Always add event listeners or timers in onMounted.

2Always remove event listeners or timers in onUnmounted.

3Cleaning up prevents memory leaks and improves input responsiveness.

Performance Quiz - 3 Questions

What is the main performance risk of adding event listeners only in onMounted without removing them in onUnmounted?

A Memory leaks and slower input responsiveness

B Slower initial page load

C Increased CSS paint cost

D Delayed DOM insertion

DevTools: Performance

How to check: Record a session while mounting and unmounting the component. Look for event listeners and memory usage in the summary and flame chart.

What to look for: Check if event listeners persist after unmount and if memory usage drops after cleanup, indicating good performance.

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