Lifecycle hooks in Composition API in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the onMounted hook runs?

Consider this Vue 3 component using Composition API:

import { ref, onMounted } from 'vue';

export default {
  setup() {
    const message = ref('');
    onMounted(() => {
      message.value = 'Hello from mounted!';
    });
    return { message };
  }
}

What will the component display after it is mounted?

A The component initially shows an empty string, then updates to 'Hello from mounted!' after mounting.

B The component shows 'Hello from mounted!' immediately before mounting.

C The component never updates the message and stays empty.

D The component throws an error because <code>onMounted</code> cannot access <code>message</code>.

💡 Hint

Remember that onMounted runs after the component is inserted into the DOM.

The onMounted hook runs after the component is mounted. It updates the reactive message value, causing the component to re-render and show the new message.

❓ state_output

intermediate

What is the value of count after onBeforeUpdate runs?

Look at this Vue 3 setup function:

import { ref, onBeforeUpdate } from 'vue';

export default {
  setup() {
    const count = ref(0);
    onBeforeUpdate(() => {
      count.value += 1;
    });
    function increment() {
      count.value += 1;
    }
    return { count, increment };
  }
}

If the user clicks a button that calls increment once, what will be the value of count after the onBeforeUpdate hook runs?

A 1

B 0

C 2

D 3

💡 Hint

The onBeforeUpdate hook runs before the DOM updates but after reactive state changes.

When increment is called, count increases from 0 to 1. Then onBeforeUpdate runs and adds 1 more, making count equal to 2.

📝 Syntax

advanced

Which option correctly uses onUnmounted in Composition API?

Select the code snippet that correctly registers a cleanup function to run when the component unmounts.

A onUnmounted(() => { console.log('Component unmounted'); });

B onUnmounted { console.log('Component unmounted'); }

C onUnmount(() => { console.log('Component unmounted'); });

D onUnmounted(() => console.log('Component unmounted'))

💡 Hint

Check the exact function name and syntax for arrow functions.

The correct hook name is onUnmounted and it requires a function argument. Option A uses correct syntax with an arrow function and parentheses.

🔧 Debug

advanced

Why does this onUpdated hook cause an infinite loop?

Examine this setup function:

import { ref, onUpdated } from 'vue';

export default {
  setup() {
    const count = ref(0);
    onUpdated(() => {
      count.value++;
    });
    return { count };
  }
}

What causes the infinite update loop?

A The component lacks a template, so updates never finish.

B The <code>count</code> ref is not initialized properly.

C The <code>onUpdated</code> hook is missing a cleanup function.

D The <code>onUpdated</code> hook changes reactive state, triggering another update endlessly.

💡 Hint

Think about what happens when reactive state changes inside update hooks.

Modifying reactive state inside onUpdated triggers another update, causing an endless loop of updates.

🧠 Conceptual

expert

Which lifecycle hook runs only once during the component's lifetime?

In Vue 3 Composition API, which lifecycle hook is guaranteed to run exactly once per component instance?

A onUnmounted

B onMounted

C onBeforeUpdate

D onUpdated

💡 Hint

Think about hooks that run multiple times versus those that run once.

onMounted runs once after the component is inserted into the DOM. Other hooks like onUpdated and onBeforeUpdate can run many times. onUnmounted runs once but only when the component is removed.