Creating custom directives in Vue

Introduction

Custom directives let you add special behavior to HTML elements easily. They help you reuse code for common tasks.

You want to automatically focus an input box when a page loads.

You need to change an element's style based on some condition.

You want to add or remove event listeners on elements in a reusable way.

You want to create a tooltip that shows when hovering over elements.

You want to detect clicks outside an element to close a dropdown.

Syntax

Vue

app.directive('directive-name', {
  mounted(el, binding) {
    // code to run when element is added to DOM
  },
  updated(el, binding) {
    // code to run when component updates
  },
  unmounted(el) {
    // cleanup code when element is removed
  }
})

The el is the HTML element the directive is attached to.

The binding contains the value passed to the directive.

Examples

This directive automatically focuses an input element when it appears.

Vue

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

This directive changes the text color of an element based on the passed value.

Vue

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

Sample Program

This Vue component uses two custom directives. The v-focus directive sets focus on the input when the page loads. The v-color directive changes the paragraph text color based on the textColor variable. Clicking the button toggles the color between blue and green.

Vue

<template>
  <input v-focus placeholder="Focus on load" />
  <p v-color="textColor">This text changes color.</p>
  <button @click="changeColor">Change Color</button>
</template>

<script setup>
import { ref } from 'vue'

const textColor = ref('blue')

function changeColor() {
  textColor.value = textColor.value === 'blue' ? 'green' : 'blue'
}
</script>

<script>
export default {
  directives: {
    focus: {
      mounted(el) {
        el.focus()
      }
    },
    color: {
      mounted(el, binding) {
        el.style.color = binding.value
      },
      updated(el, binding) {
        el.style.color = binding.value
      }
    }
  }
}
</script>

Important Notes

Always clean up event listeners or timers in the unmounted hook to avoid memory leaks.

Use binding.value to get the value passed to the directive.

Directives are great for DOM-related tasks that don't fit well in components.

Summary

Custom directives add reusable behavior to HTML elements.

Use lifecycle hooks like mounted and updated to control behavior.

Directives help keep your templates clean and your code organized.

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