onBeforeMount and onBeforeUnmount in Vue - Mini Project: Build & Apply

Using onBeforeMount and onBeforeUnmount in Vue 3

📖 Scenario: You are building a simple Vue 3 component that shows a message when it is about to appear on the page and another message when it is about to be removed. This helps you understand how Vue manages component lifecycle events.

🎯 Goal: Create a Vue 3 component using the Composition API that logs messages using onBeforeMount and onBeforeUnmount lifecycle hooks.

📋 What You'll Learn

Use the onBeforeMount lifecycle hook to log 'Component is about to mount' before the component appears.

Use the onBeforeUnmount lifecycle hook to log 'Component is about to unmount' before the component disappears.

Use the <script setup> syntax with Composition API.

Render a simple message inside a <div> element.

💡 Why This Matters

🌍 Real World

Lifecycle hooks like onBeforeMount and onBeforeUnmount help manage setup and cleanup tasks in Vue components, such as starting or stopping timers, fetching data, or cleaning event listeners.

💼 Career

Understanding Vue lifecycle hooks is essential for frontend developers working with Vue.js to build efficient, maintainable, and bug-free user interfaces.

1

Create the Vue component skeleton

Write a Vue 3 Single File Component using <script setup>. Import ref from 'vue' to create a reactive message string set to 'Hello Vue!'. Render the message inside a <template> with a <div>.

Vue

  {{ message }}



// Your code here

Hint

Use ref to create a reactive message variable inside <script setup>.

2

Add onBeforeMount lifecycle hook

Import onBeforeMount from 'vue' and use it to log 'Component is about to mount' before the component appears.

Vue

  {{ message }}



import { ref, onBeforeMount } from 'vue'

const message = ref('Hello Vue!')

// Use onBeforeMount here

Hint

Use onBeforeMount(() => { ... }) to run code before the component mounts.

3

Add onBeforeUnmount lifecycle hook

Import onBeforeUnmount from 'vue' and use it to log 'Component is about to unmount' before the component is removed.

Vue

  {{ message }}



import { ref, onBeforeMount, onBeforeUnmount } from 'vue'

const message = ref('Hello Vue!')

onBeforeMount(() => {
  console.log('Component is about to mount')
})

// Use onBeforeUnmount here

Hint

Use onBeforeUnmount(() => { ... }) to run code before the component unmounts.

4

Complete the component with proper imports and template

Ensure the component uses <template> with a <div> showing {{ message }}, and the <script setup> imports ref, onBeforeMount, and onBeforeUnmount from 'vue'. The lifecycle hooks should log the correct messages.

Vue

  {{ message }}



// Final check: imports and lifecycle hooks

Hint

Make sure all imports and lifecycle hooks are present and the template shows the message.

Practice

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

easy

A. It runs when the component is updated.

B. It runs after the component is removed from the screen.

C. It runs only once when the app starts.

D. It runs right before the component appears on the screen.

Solution

1. Step 1: Understand the timing of onBeforeMount

   The onBeforeMount hook runs just before the component is rendered and shown on the screen.

2. Step 2: Compare with other lifecycle hooks

   Unlike hooks that run after removal or updates, onBeforeMount is specifically before the first render.

3. Final Answer:

   It runs right before the component appears on the screen. -> Option D

4. Quick Check:

   onBeforeMount = before component appears [OK]

Hint: onBeforeMount = just before showing component [OK]

Common Mistakes:

• Confusing onBeforeMount with onBeforeUnmount
• Thinking it runs after component removal
• Mixing it with update hooks

2. Which of the following is the correct way to use onBeforeUnmount inside the setup() function in Vue 3?

easy

A. onBeforeUnmount(function) { console.log('Unmount') }

B. onBeforeUnmount = () => { console.log('Component will unmount') }

C. onBeforeUnmount(() => { console.log('Component will unmount') })

D. setup(() => { onBeforeUnmount(console.log('Unmount')) })

Solution

1. Step 1: Recall correct syntax for lifecycle hooks in setup()

   Lifecycle hooks like onBeforeUnmount are called as functions with a callback inside setup().

2. Step 2: Identify correct arrow function usage

   onBeforeUnmount(() => { console.log('Component will unmount') }) correctly passes an arrow function to onBeforeUnmount to run code before unmount.

3. Final Answer:

   onBeforeUnmount(() => { console.log('Component will unmount') }) -> Option C

4. Quick Check:

   Use onBeforeUnmount(callback) inside setup() [OK]

Hint: Use onBeforeUnmount(() => {...}) inside setup() [OK]

Common Mistakes:

• Assigning onBeforeUnmount instead of calling it
• Calling onBeforeUnmount with wrong syntax
• Trying to use it outside setup()

3. Consider this Vue 3 component code snippet inside setup():

import { ref, onBeforeMount, onBeforeUnmount } from 'vue'

export default {
  setup() {
    const count = ref(0)
    onBeforeMount(() => { count.value = 5 })
    onBeforeUnmount(() => { count.value = 0 })
    return { count }
  }
}

What will be the value of count just before the component is removed from the screen?

medium

A. 0

B. 5

C. undefined

D. It causes an error

Solution

1. Step 1: Understand onBeforeMount effect

   When the component is about to mount, count.value is set to 5.

2. Step 2: Understand onBeforeUnmount effect

   Right before the component unmounts, count.value is reset to 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   onBeforeUnmount resets count to 0 [OK]

Hint: onBeforeUnmount runs last, resetting count to 0 [OK]

Common Mistakes:

• Thinking count stays 5 after unmount
• Confusing onBeforeMount and onBeforeUnmount effects
• Assuming unmount hook does not change reactive data

4. What is wrong with this Vue 3 code inside setup()?

import { onBeforeUnmount } from 'vue'

export default {
  setup() {
    onBeforeUnmount = () => {
      console.log('Cleaning up')
    }
  }
}

medium

A. Missing import of onBeforeMount.

B. onBeforeUnmount is assigned instead of called with a callback.

C. Arrow function syntax is incorrect.

D. setup() must return a value.

Solution

1. Step 1: Check how onBeforeUnmount is used

   The code assigns a new function to onBeforeUnmount instead of calling it with a callback.

2. Step 2: Correct usage of onBeforeUnmount

   It should be called like onBeforeUnmount(() => { ... }) to register the cleanup function.

3. Final Answer:

   onBeforeUnmount is assigned instead of called with a callback. -> Option B

4. Quick Check:

   Use onBeforeUnmount(callback), not assignment [OK]

Hint: Call onBeforeUnmount(callback), don't assign it [OK]

Common Mistakes:

• Assigning lifecycle hook instead of calling it
• Forgetting parentheses for function call
• Confusing import errors with usage errors

5. You want to track how many times a component is about to mount and unmount using onBeforeMount and onBeforeUnmount. Which code correctly updates two counters mountCount and unmountCount inside setup()?

hard

A. const mountCount = ref(0); const unmountCount = ref(0);
onBeforeMount(() => { mountCount.value++ });
onBeforeUnmount(() => { unmountCount.value++ });
return { mountCount, unmountCount }

B. let mountCount = 0; let unmountCount = 0;
onBeforeMount(() => { mountCount++ });
onBeforeUnmount(() => { unmountCount++ });
return { mountCount, unmountCount }

C. const mountCount = 0; const unmountCount = 0;
onBeforeMount(() => { mountCount += 1 });
onBeforeUnmount(() => { unmountCount += 1 });
return { mountCount, unmountCount }

D. const mountCount = ref(0); const unmountCount = ref(0);
onBeforeMount(() => mountCount++);
onBeforeUnmount(() => unmountCount++);
return { mountCount, unmountCount }

Solution

1. Step 1: Use reactive refs for counters

   To track changes reactively, counters must be ref(0), not plain numbers.

2. Step 2: Increment ref values inside lifecycle hooks

   Inside onBeforeMount and onBeforeUnmount, increment with mountCount.value++ and unmountCount.value++.

3. Step 3: Return refs for template usage

   Return the refs so the template can react to changes.

4. Final Answer:

   const mountCount = ref(0); const unmountCount = ref(0); onBeforeMount(() => { mountCount.value++ }); onBeforeUnmount(() => { unmountCount.value++ }); return { mountCount, unmountCount } -> Option A

5. Quick Check:

   Use ref(0) and increment .value inside hooks [OK]

Hint: Use ref(0) and increment .value inside hooks [OK]

Common Mistakes:

• Using plain numbers instead of refs
• Incrementing refs without .value
• Not returning counters for template