Creating custom directives in Vue - Try It Yourself

Creating Custom Directives in Vue

📖 Scenario: You are building a Vue 3 app where you want to highlight text elements when the user hovers over them. Instead of adding the same style repeatedly, you will create a custom directive to handle the highlight effect.

🎯 Goal: Build a Vue 3 component that uses a custom directive called v-highlight to change the background color of text when hovered.

📋 What You'll Learn

Create a Vue 3 component with a list of text items

Define a custom directive named highlight

Make the directive change background color on mouse enter and revert on mouse leave

Apply the v-highlight directive to each text item

💡 Why This Matters

🌍 Real World

Custom directives help you reuse common DOM behavior like animations, focus management, or styling across many components without repeating code.

💼 Career

Understanding custom directives is valuable for Vue developers to create clean, maintainable, and reusable UI behaviors in professional projects.

1

Set up the Vue component with text items

Create a Vue 3 component using the Options API. Inside the component object, define a data() function that returns { items: ["Apple", "Banana", "Cherry"] }. In the template, use <li> elements inside a <ul> to display each item using v-for with item and index as variables.

Vue

export default {
  data() {
    // Create items with ["Apple", "Banana", "Cherry"]
    // Your code here
  }
}



  
    
    
  

Hint

Remember to define data() that returns { items: ["Apple", "Banana", "Cherry"] }. Use v-for with (item, index) in items to loop.

2

Define the custom directive

Before the export default, create a constant called highlight that is an object with two methods: beforeMount and unmounted. In beforeMount, add event listeners to the element parameter el for mouseenter and mouseleave. On mouseenter, set el.style.backgroundColor = 'yellow'. On mouseleave, reset el.style.backgroundColor = ''. Leave unmounted empty for now.

Vue

// Define highlight directive object with beforeMount and unmounted
// Your code here
export default {
  data() {
    return {
      items: ["Apple", "Banana", "Cherry"]
    }
  }
}



  
    {{ item }}
  

Hint

Use el.addEventListener inside beforeMount to add mouse events that change el.style.backgroundColor.

3

Register the directive in the component

Add a directives property to the component object (export default). Set directives to an object with a key highlight and value highlight (shorthand).

Vue

const highlight = {
  beforeMount(el) {
    el.addEventListener('mouseenter', () => {
      el.style.backgroundColor = 'yellow'
    })
    el.addEventListener('mouseleave', () => {
      el.style.backgroundColor = ''
    })
  },
  unmounted() {
    // No cleanup needed for this example
  }
}

export default {
  data() {
    return {
      items: ["Apple", "Banana", "Cherry"]
    }
  }
  // Add directives: { highlight }
  // Your code here
}



  
    {{ item }}
  

Hint

Add directives: { highlight } to the component options.

4

Apply the custom directive in the template

In the <li> element inside the v-for, add the directive v-highlight to apply the highlight effect on hover.

Vue

const highlight = {
  beforeMount(el) {
    el.addEventListener('mouseenter', () => {
      el.style.backgroundColor = 'yellow'
    })
    el.addEventListener('mouseleave', () => {
      el.style.backgroundColor = ''
    })
  },
  unmounted() {
    // No cleanup needed for this example
  }
}

export default {
  data() {
    return {
      items: ["Apple", "Banana", "Cherry"]
    }
  },
  directives: {
    highlight
  }
}



  
    >{{ item }}
  

Hint

Add v-highlight inside the <li> tag to apply the directive.

Practice

1. What is the main purpose of creating a custom directive in Vue?

easy

A. To add reusable behavior to HTML elements

B. To define new components

C. To manage Vuex state

D. To handle routing between pages

Solution

1. Step 1: Understand what directives do in Vue

   Directives are special markers on elements that tell Vue to do something with that element.

2. Step 2: Identify the role of custom directives

   Custom directives let you add reusable behavior to elements, like focusing an input or changing styles dynamically.

3. Final Answer:

   To add reusable behavior to HTML elements -> Option A

4. Quick Check:

   Custom directives = reusable element behavior [OK]

Hint: Custom directives add behavior to elements, not components [OK]

Common Mistakes:

• Confusing directives with components
• Thinking directives manage app state
• Assuming directives handle routing

2. Which of the following is the correct syntax to register a global custom directive named v-focus in Vue 3?

easy

A. Vue.directive('v-focus', function(el) { el.focus() })

B. app.directive('focus', { mounted(el) { el.focus() } })

C. app.directive('v-focus', { bind(el) { el.focus() } })

D. Vue.createDirective('focus', { mounted(el) { el.focus() } })

Solution

1. Step 1: Recall Vue 3 global directive registration

   In Vue 3, you use the app instance's directive method with the directive name without the 'v-' prefix.

2. Step 2: Check the lifecycle hook and syntax

   The correct hook for when the element is inserted is 'mounted', and the function receives the element as argument.

3. Final Answer:

   app.directive('focus', { mounted(el) { el.focus() } }) -> Option B

4. Quick Check:

   Vue 3 global directive = app.directive('name', { mounted }) [OK]

Hint: Use app.directive('name', { mounted }) without 'v-' prefix [OK]

Common Mistakes:

• Including 'v-' prefix in directive name
• Using Vue.directive instead of app.directive in Vue 3
• Using deprecated hooks like 'bind'

3. Given this custom directive registered globally in Vue 3:

app.directive('color', {
  mounted(el, binding) {
    el.style.color = binding.value
  },
  updated(el, binding) {
    el.style.color = binding.value
  }
})

What will be the text color of the paragraph after the component updates if used as <p v-color="'red'">Hello</p> and later the binding value changes to 'blue'?

medium

A. The text color will be black (default)

B. The text color will remain red

C. The text color will change to blue

D. There will be a runtime error

Solution

1. Step 1: Understand directive lifecycle hooks

   The 'mounted' hook sets the color initially, and the 'updated' hook runs when the binding value changes.

2. Step 2: Trace the color change on update

   When the binding value changes to 'blue', the 'updated' hook sets the element's color style to 'blue'.

3. Final Answer:

   The text color will change to blue -> Option C

4. Quick Check:

   updated hook changes color to new binding value [OK]

Hint: updated hook runs on binding change to update element [OK]

Common Mistakes:

• Assuming color stays as initial value
• Ignoring the updated lifecycle hook
• Thinking directive causes errors on update

4. Consider this custom directive code snippet in Vue 3:

app.directive('tooltip', {
  mounted(el, binding) {
    el.setAttribute('title', binding.value)
  },
  updated(el, binding) {
    el.setAttribute('title', binding.oldValue)
  }
})

What is the problem with this directive when the binding value changes?

medium

A. The tooltip title does not update to the new value

B. The directive causes a syntax error

C. The title attribute is removed on update

D. The directive works correctly with no issues

Solution

1. Step 1: Analyze the updated hook usage

   The updated hook sets the title attribute to binding.oldValue, which is the previous value, not the new one.

2. Step 2: Understand the effect on the tooltip

   Because it sets the old value on update, the tooltip does not reflect the new binding value after change.

3. Final Answer:

   The tooltip title does not update to the new value -> Option A

4. Quick Check:

   updated hook must use binding.value, not binding.oldValue [OK]

Hint: Use binding.value in updated hook, not binding.oldValue [OK]

Common Mistakes:

• Using binding.oldValue instead of binding.value on update
• Assuming no update needed for title
• Confusing syntax error with logic error

5. You want to create a custom directive v-auto-focus that automatically focuses an input element only when it is inserted into the DOM and not on updates. Which is the best way to implement this directive in Vue 3?

hard

A. Use the created hook to call el.focus()

B. Use the updated hook to call el.focus() every time the binding changes

C. Use both mounted and updated hooks to call el.focus()

D. Use the mounted hook to call el.focus() and leave updated empty

Solution

1. Step 1: Identify when to focus the element

   The input should be focused only once when inserted, so the 'mounted' hook is the right place.

2. Step 2: Avoid focusing on updates

   Calling focus in 'updated' would focus repeatedly, which is not desired.

3. Step 3: Understand Vue 3 lifecycle hooks

   There is no 'created' hook for directives; 'mounted' is the earliest for DOM access.

4. Final Answer:

   Use the mounted hook to call el.focus() and leave updated empty -> Option D

5. Quick Check:

   Focus on mounted only, not updated [OK]

Hint: Focus in mounted hook only to avoid repeated focusing [OK]

Common Mistakes:

• Focusing in updated hook causing repeated focus
• Using non-existent created hook for directives
• Calling focus multiple times unnecessarily