Directive hooks (mounted, updated, unmounted) in Vue - Mini Project: Build & Apply

Using Directive Hooks in Vue 3

📖 Scenario: You are building a Vue 3 app where you want to create a custom directive that changes the background color of an element when it is added to the page, updates the color when the bound value changes, and cleans up when the element is removed.

🎯 Goal: Create a custom Vue directive called v-highlight that uses the mounted, updated, and unmounted hooks to manage the background color of an element.

📋 What You'll Learn

Create a Vue 3 app with a custom directive named highlight

Use the mounted hook to set the element's background color to the directive's value

Use the updated hook to update the background color when the value changes

Use the unmounted hook to reset the background color when the element is removed

💡 Why This Matters

🌍 Real World

Custom directives are useful when you want to add reusable behavior to HTML elements in Vue apps, like animations, event handling, or style changes.

💼 Career

Understanding directive hooks helps you build maintainable Vue components and extend Vue's functionality, a valuable skill for frontend developer roles.

1

Create the Vue app and initial directive setup

Create a Vue 3 app using createApp and define a custom directive called highlight with an empty object as its definition.

Vue

import { createApp } from 'vue'

const app = createApp({})

app.directive('highlight', {
  // Your code here
})

app.mount('#app')

Hint

Start by importing createApp from Vue and create an app instance. Then add a directive named highlight with an empty object.

2

Add the mounted hook to set background color

Inside the highlight directive, add the mounted hook that sets the element's background color to the directive's value using el.style.backgroundColor = binding.value.

Vue

import { createApp } from 'vue'

const app = createApp({})

app.directive('highlight', {
  mounted(el, binding) {
    // Your code here
  }
})

app.mount('#app')

Hint

The mounted hook runs when the element is added to the DOM. Use binding.value to get the color value.

3

Add the updated hook to change background color on value change

Add the updated hook inside the highlight directive that updates the element's background color to the new binding.value when the directive's value changes.

Vue

import { createApp } from 'vue'

const app = createApp({})

app.directive('highlight', {
  mounted(el, binding) {
    el.style.backgroundColor = binding.value
  },
  updated(el, binding) {
    // Your code here
  }
})

app.mount('#app')

Hint

The updated hook runs when the directive's value changes. Update the background color similarly to mounted.

4

Add the unmounted hook to reset background color

Add the unmounted hook inside the highlight directive that resets the element's background color to an empty string '' when the element is removed from the DOM.

Vue

import { createApp } from 'vue'

const app = createApp({})

app.directive('highlight', {
  mounted(el, binding) {
    el.style.backgroundColor = binding.value
  },
  updated(el, binding) {
    el.style.backgroundColor = binding.value
  },
  unmounted(el) {
    // Your code here
  }
})

app.mount('#app')

Hint

The unmounted hook runs when the element is removed. Reset the background color to remove the highlight.

Practice

1. What does the mounted hook in a Vue directive do?

easy

A. Runs every time the component updates

B. Runs once when the directive is attached to the element

C. Runs when the directive is removed from the element

D. Runs only when the element is clicked

Solution

1. Step 1: Understand the purpose of mounted hook

   The mounted hook runs once when the directive is first attached to the element in the DOM.

2. Step 2: Compare with other hooks

   The updated hook runs on data or element changes, and unmounted runs when the directive is removed, so they do not match the description of mounted.

3. Final Answer:

   Runs once when the directive is attached to the element -> Option B

4. Quick Check:

   mounted = runs once on attach [OK]

Hint: Remember: mounted runs once when directive starts [OK]

Common Mistakes:

• Confusing mounted with updated hook
• Thinking mounted runs multiple times
• Mixing unmounted with mounted

2. Which of the following is the correct syntax to define the unmounted hook in a Vue directive?

easy

A. onUnmounted(el, binding) { /* code */ }

B. unmount(el, binding) { /* code */ }

C. destroyed(el, binding) { /* code */ }

D. unmounted(el, binding) { /* code */ }

Solution

1. Step 1: Recall the correct hook name

   The correct hook name for cleanup when a directive is removed is unmounted.

2. Step 2: Check syntax correctness

   The hook is defined as a function with parameters el and binding, so unmounted(el, binding) { /* code */ } is correct.

3. Final Answer:

   unmounted(el, binding) { /* code */ } -> Option D

4. Quick Check:

   Correct hook name is unmounted [OK]

Hint: Use exact hook names: unmounted, not unmount or destroyed [OK]

Common Mistakes:

• Using 'unmount' instead of 'unmounted'
• Confusing with lifecycle hooks like 'destroyed'
• Adding 'on' prefix incorrectly

3. Given this directive code, what will be logged when the bound element's text changes?

const myDirective = {
  mounted(el) {
    console.log('Mounted')
  },
  updated(el) {
    console.log('Updated')
  },
  unmounted(el) {
    console.log('Unmounted')
  }
}

medium

A. 'Mounted' then 'Updated' will log on text change

B. No logs will appear

C. 'Unmounted' will log on text change

D. Only 'Mounted' will log once

Solution

1. Step 1: Understand when each hook runs

   mounted runs once when directive attaches, updated runs on element or data changes, unmounted runs when directive is removed.

2. Step 2: Analyze the scenario

   When the element's text changes, the updated hook runs, so both Mounted (once) and Updated (on change) will log.

3. Final Answer:

   'Mounted' then 'Updated' will log on text change -> Option A

4. Quick Check:

   mounted once, updated on change [OK]

Hint: Updated logs on changes, mounted only once [OK]

Common Mistakes:

• Thinking updated runs only once
• Expecting unmounted to run on text change
• Ignoring mounted logs

4. Identify the error in this directive definition:

const directive = {
  mounted(el) {
    console.log('Mounted')
  },
  updated() {
    console.log('Updated')
  },
  unmounted(el) {
    console.log('Unmounted')
  }
}

medium

A. The mounted hook should not have parameters

B. The unmounted hook should be named 'destroyed'

C. The updated hook is missing the 'el' parameter

D. There is no error in this code

Solution

1. Step 1: Check parameters for each hook

   Directive hooks receive the element as the first parameter. The updated hook is missing the el parameter.

2. Step 2: Verify other hooks

   mounted and unmounted correctly have el parameter. The unmounted hook is correctly named.

3. Final Answer:

   The updated hook is missing the 'el' parameter -> Option C

4. Quick Check:

   All hooks need 'el' parameter [OK]

Hint: All directive hooks need 'el' parameter [OK]

Common Mistakes:

• Omitting parameters in hooks
• Confusing lifecycle hook names
• Assuming no parameters needed

5. You want to create a directive that adds a click event listener when attached and removes it when the directive is removed. Which hooks should you use to add and remove the listener?

hard

A. Add listener in mounted, remove in unmounted

B. Add listener in updated, remove in mounted

C. Add listener in unmounted, remove in updated

D. Add listener in updated, remove in updated

Solution

1. Step 1: Determine when to add event listener

   The listener should be added once when the directive attaches, so use mounted.

2. Step 2: Determine when to remove event listener

   Cleanup should happen when the directive is removed, so use unmounted to remove the listener.

3. Final Answer:

   Add listener in mounted, remove in unmounted -> Option A

4. Quick Check:

   mounted = add, unmounted = cleanup [OK]

Hint: Add in mounted, clean up in unmounted [OK]

Common Mistakes:

• Adding/removing listeners in updated hook
• Removing listener before adding it
• Using unmounted to add listener