Composable with lifecycle hooks in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the composable's onMounted hook runs?

Consider this Vue 3 composable using onMounted:

import { ref, onMounted } from 'vue'

export function useTimer() {
  const count = ref(0)
  onMounted(() => {
    count.value = 5
  })
  return { count }
}

If a component uses useTimer(), what will count.value be immediately after the component mounts?

A 5

B 0

C undefined

D An error occurs because <code>onMounted</code> is used outside setup

💡 Hint

Remember that onMounted runs after the component is mounted, so the value changes then.

The onMounted hook runs after the component is mounted, setting count.value to 5. So after mounting, count.value is 5.

❓ state_output

intermediate

What is the value of message after the composable's onUnmounted hook runs?

Given this composable:

import { ref, onUnmounted } from 'vue'

export function useMessage() {
  const message = ref('Hello')
  onUnmounted(() => {
    message.value = 'Goodbye'
  })
  return { message }
}

If a component uses useMessage() and then unmounts, what will message.value be?

A undefined

B 'Hello'

C An error because you cannot change state in onUnmounted

D 'Goodbye'

💡 Hint

Think about when onUnmounted runs and if state can be changed then.

The onUnmounted hook runs just before the component is removed. It changes message.value to 'Goodbye'. So after unmounting, message.value is 'Goodbye'.

📝 Syntax

advanced

Which option correctly uses onBeforeUnmount inside a composable?

Choose the option that correctly registers a cleanup function with onBeforeUnmount in a Vue 3 composable.

A

import { onBeforeUnmount } from 'vue'

export function useCleanup() {
  onBeforeUnmount = () => {
    console.log('Cleaning up')
  }
}

B

import { onBeforeUnmount } from 'vue'

export function useCleanup() {
  onBeforeUnmount(() => {
    console.log('Cleaning up')
  })
}

C

import { onBeforeUnmount } from 'vue'

export function useCleanup() {
  onBeforeUnmount(() => {
    console.log('Cleaning up')
  })()
}

D

import { onBeforeUnmount } from 'vue'

export function useCleanup() {
  onBeforeUnmount(() => {
    console.log('Cleaning up')
  }
}

💡 Hint

Remember onBeforeUnmount is a function you call with a callback.

Option B correctly calls onBeforeUnmount with a callback function. Option B tries to assign to onBeforeUnmount, which is invalid. Option B calls the result of onBeforeUnmount, which is not a function. Option B has a syntax error (missing closing parenthesis).

🔧 Debug

advanced

Why does this composable's onMounted hook not run?

Look at this composable:

import { ref, onMounted } from 'vue'

export function useData() {
  const data = ref(null)
  onMounted(() => {
    data.value = 'Loaded'
  })
  return { data }
}

But when used in a component, data.value stays null. What is the likely cause?

A The <code>onMounted</code> hook runs too early before the component mounts

B The <code>ref</code> is not reactive, so changes don't update

C The composable is called outside the component's setup function, so lifecycle hooks don't run

D The composable must return a function to trigger <code>onMounted</code>

💡 Hint

Lifecycle hooks only work inside component setup or other lifecycle hooks.

If the composable is called outside the component's setup function, Vue does not run lifecycle hooks like onMounted. So data.value stays null.

🧠 Conceptual

expert

What is the main benefit of using lifecycle hooks inside composables?

Why do Vue developers use lifecycle hooks like onMounted inside composables instead of only inside components?

A To reuse lifecycle-dependent logic across multiple components easily

B To force components to mount earlier than usual

C To prevent components from unmounting automatically

D To replace the component's own lifecycle hooks completely

💡 Hint

Think about how composables help organize code.

Using lifecycle hooks inside composables lets developers share code that depends on component lifecycle events, making code reusable and cleaner.

Practice

1. What is the main purpose of using lifecycle hooks inside a Vue composable?

easy

A. To run code automatically when a component starts or stops

B. To define the component's template structure

C. To style the component with CSS

D. To create new Vue components

Solution

1. Step 1: Understand lifecycle hooks role

   Lifecycle hooks let you run code at specific times in a component's life, like when it starts or stops.

2. Step 2: Connect lifecycle hooks with composables

   Using lifecycle hooks inside composables allows shared logic to run automatically when components use the composable.

3. Final Answer:

   To run code automatically when a component starts or stops -> Option A

4. Quick Check:

   Lifecycle hooks run code at component start/stop = A [OK]

Hint: Lifecycle hooks run code on component start or cleanup [OK]

Common Mistakes:

• Thinking lifecycle hooks define templates
• Confusing styling with lifecycle hooks
• Believing composables create components

2. Which of the following is the correct way to import and use the onMounted lifecycle hook inside a Vue 3 composable?

easy

A. import { onMounted } from 'vue'; onMounted = () => { /* code */ };

B. import onMounted from 'vue'; onMounted(() => { /* code */ });

C. import { onMounted } from 'vue'; onMounted(() => { /* code */ });

D. import { onMounted } from 'vue'; onMounted.function(() => { /* code */ });

Solution

1. Step 1: Check import syntax

   Vue lifecycle hooks like onMounted are named exports, so use curly braces in import.

2. Step 2: Check usage syntax

   onMounted is a function that takes a callback, so call it with a function argument.

3. Final Answer:

   import { onMounted } from 'vue'; onMounted(() => { /* code */ }); -> Option C

4. Quick Check:

   Correct import and call syntax = A [OK]

Hint: Use curly braces to import lifecycle hooks from 'vue' [OK]

Common Mistakes:

• Importing without curly braces
• Assigning onMounted to a function instead of calling it
• Using incorrect method like onMounted.function

3. Given this composable code, what will be logged when a component using it is mounted?

import { ref, onMounted } from 'vue';
export function useLogger() {
  const message = ref('Hello');
  onMounted(() => {
    console.log(message.value);
  });
  return { message };
}

medium

A. Hello

B. undefined

C. null

D. An error is thrown

Solution

1. Step 1: Understand ref initialization

   The message ref is initialized with the string 'Hello'.

2. Step 2: Check onMounted callback

   When the component mounts, the callback logs message.value, which is 'Hello'.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Ref value logged on mount = B [OK]

Hint: Ref values accessed with .value inside lifecycle hooks [OK]

Common Mistakes:

• Logging ref without .value
• Expecting undefined before mount
• Thinking onMounted runs before ref initialization

4. Identify the error in this composable using lifecycle hooks:

import { onMounted, onUnmounted } from 'vue';
export function useTimer() {
  let timer = null;
  onMounted(() => {
    timer = setInterval(() => console.log('tick'), 1000);
  });
  onUnmounted(() => {
    clearInterval(timer);
  });
  return {};
}

medium

A. timer should be a ref to persist between hooks

B. onUnmounted should be onBeforeUnmount

C. setInterval callback syntax is wrong

D. No error, code works correctly

Solution

1. Step 1: Check variable scope and usage

   Variable timer is declared in composable scope and updated in onMounted, accessible in onUnmounted.

2. Step 2: Verify lifecycle hooks usage

   onMounted sets interval, onUnmounted clears it correctly to avoid memory leaks.

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   Proper timer setup and cleanup = D [OK]

Hint: Variables in composable scope persist across lifecycle hooks [OK]

Common Mistakes:

• Thinking timer must be a ref
• Confusing onUnmounted with onBeforeUnmount
• Misunderstanding setInterval callback syntax

5. You want to create a composable that fetches data when a component mounts and cancels the fetch if the component unmounts before completion. Which lifecycle hooks and pattern should you use inside the composable?

hard

A. Use onUnmounted to start fetch and onMounted to abort fetch

B. Use onMounted to start fetch and onUnmounted to abort fetch with an AbortController

C. Use onBeforeMount to start fetch and onMounted to abort fetch

D. Use onMounted to start fetch and no cleanup is needed

Solution

1. Step 1: Determine when to start fetch

   Fetching data should start when the component is fully mounted, so use onMounted.

2. Step 2: Handle cleanup on unmount

   To cancel fetch if component unmounts early, use onUnmounted to abort the fetch using an AbortController.

3. Final Answer:

   Use onMounted to start fetch and onUnmounted to abort fetch with an AbortController -> Option B

4. Quick Check:

   Fetch start on mount, abort on unmount = C [OK]

Hint: Start fetch on mount, abort on unmount with AbortController [OK]

Common Mistakes:

• Starting fetch too early or too late
• Not aborting fetch on unmount causing leaks
• Using wrong lifecycle hooks for cleanup