Directive hooks (mounted, updated, unmounted) in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Vue directive's mounted hook runs?

Consider a custom directive in Vue 3 using the mounted hook. What is the main purpose of this hook?

A It runs when the directive is removed from the element, to clean up resources.

B It runs every time the component updates, to refresh the directive's behavior.

C It runs once when the directive is first attached to the element, allowing setup like event listeners.

D It runs before the element is inserted into the DOM.

💡 Hint

Think about when you want to set up something once when the element appears.

The mounted hook in Vue directives runs once when the directive is first bound to the element and inserted into the DOM. It's used to set up things like event listeners or initial styles.

❓ component_behavior

intermediate

When does the updated hook of a Vue directive run?

In Vue 3, what triggers the updated hook of a custom directive?

A Every time the component's reactive data changes and the element is re-rendered.

B Only when the directive is unbound from the element.

C When the directive is first bound to the element.

D Before the element is inserted into the DOM.

💡 Hint

Think about when the element's content or attributes might change after initial render.

The updated hook runs after the containing component has updated and the element has been re-rendered. It lets you respond to changes in the element or its data.

❓ lifecycle

advanced

What is the correct order of Vue directive hooks when an element is inserted, updated, and then removed?

Arrange the Vue directive hooks in the order they are called during the element's lifecycle: insertion, update, removal.

A 2,1,3

B 1,2,3

C 3,1,2

D 1,3,2

💡 Hint

Think about the lifecycle from element creation to removal.

The mounted hook runs first when the element is inserted. Then updated runs on reactive updates. Finally, unmounted runs when the element is removed.

🔧 Debug

advanced

Why does this Vue directive's unmounted hook not run?

Given this directive code, the unmounted hook never triggers. What is the likely cause?

export default {
  mounted(el) { console.log('mounted'); },
  updated(el) { console.log('updated'); },
  // unmounted hook missing
}

A Because the directive must use <code>beforeUnmount</code> instead.

B Because the <code>updated</code> hook prevents <code>unmounted</code> from running.

C Because the element is never removed from the DOM.

D Because the <code>unmounted</code> hook is not defined in the directive object.

💡 Hint

Check if the hook function exists in the directive.

If the unmounted hook is not defined, Vue has no code to run when the element is removed. Defining it is necessary for cleanup.

📝 Syntax

expert

Which Vue directive hook signature is correct for accessing the element and binding value?

Select the correct syntax for a Vue 3 directive hook that accesses the element and the binding value.

A mounted(el, binding) { console.log(binding.value); }

B mounted(binding, el) { console.log(binding.value); }

C mounted(el) { console.log(el.binding.value); }

D mounted(el, binding, vnode) { console.log(vnode.value); }

💡 Hint

Remember the order of parameters in directive hooks.

The correct order is el first, then binding. The binding object contains the value.

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