Keep-alive for expensive components in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a component is wrapped with in Vue 3?

Consider a Vue 3 app where a component is wrapped inside a tag. What is the main effect on the component's lifecycle?

Vue

<template>
  <keep-alive>
    <ExpensiveComponent v-if="show" />
  </keep-alive>
</template>

<script setup>
import { ref } from 'vue'
import ExpensiveComponent from './ExpensiveComponent.vue'
const show = ref(true)
</script>

A The component is destroyed immediately when toggled off and recreated fresh when toggled on.

B The component is cached and its state is preserved when toggled off and on again.

C The component's template is cached but its data and state are reset every time.

D The component is never rendered because <keep-alive> disables rendering.

💡 Hint

Think about what does to component instances in Vue.

Wrapping a component with caches the component instance. When the component is toggled off (unmounted), Vue keeps it in memory. When toggled back on, Vue reuses the cached instance, preserving its state and avoiding re-creation.

❓ lifecycle

intermediate

Which lifecycle hooks are called when a component is toggled off and then on again?

In Vue 3, if a component wrapped in is hidden (v-if false) and then shown again, which lifecycle hooks run?

Vue

<script setup>
import { onMounted, onUnmounted, onActivated, onDeactivated } from 'vue'

onMounted(() => console.log('mounted'))
onUnmounted(() => console.log('unmounted'))
onActivated(() => console.log('activated'))
onDeactivated(() => console.log('deactivated'))
</script>

A mounted, unmounted, activated

B mounted, unmounted, mounted

C mounted, deactivated, activated, unmounted

D mounted, deactivated, activated

💡 Hint

Remember that caches the component instead of destroying it.

When a component is hidden, it triggers the deactivated hook instead of unmounted. When shown again, it triggers activated. The mounted hook runs only once when first created.

📝 Syntax

advanced

Which option correctly uses with dynamic components and includes an include prop?

Choose the correct syntax to cache only components named 'ExpensiveComponent' and 'HeavyWidget' using with dynamic components.

Vue

<template>
  <keep-alive :include="['ExpensiveComponent', 'HeavyWidget']">
    <component :is="currentComponent" />
  </keep-alive>
</template>

A

<keep-alive :include="['ExpensiveComponent', 'HeavyWidget']">
  <component :is="currentComponent" />
</keep-alive>

B

<keep-alive include="['ExpensiveComponent', 'HeavyWidget']">
  <component :is="currentComponent" />
</keep-alive>

C

<keep-alive include={ExpensiveComponent, HeavyWidget}>
  <component :is="currentComponent" />
</keep-alive>

D

<keep-alive include="ExpensiveComponent,HeavyWidget">
  <component :is="currentComponent" />
</keep-alive>

💡 Hint

Check how to bind props dynamically in Vue templates.

The include prop expects a string or an array. To pass an array, use the binding syntax :include with an array value. Option A correctly uses :include with an array.

🔧 Debug

advanced

Why does the component inside lose its state after toggling?

A developer wraps a component with but notices the component's data resets every time it is toggled off and on. What is the most likely cause?

Vue

<template>
  <keep-alive>
    <ExpensiveComponent v-if="show" :key="uniqueKey" />
  </keep-alive>
</template>

<script setup>
import { ref } from 'vue'
const show = ref(true)
const uniqueKey = ref(1)
</script>

A The component is missing the <keep-alive> include prop to enable caching.

B The <keep-alive> tag does not cache components with v-if directives.

C The :key changes on toggle, forcing Vue to recreate the component instead of reusing it.

D The component's data is reset because it uses reactive refs inside setup.

💡 Hint

Think about how Vue uses keys to identify components.

When the :key changes, Vue treats the component as a new instance and recreates it, ignoring the cached version. This causes the state to reset despite .

🧠 Conceptual

expert

How does improve performance in Vue apps with expensive components?

Explain the main reason why using around expensive components can improve app performance.

A <keep-alive> caches component instances to avoid costly re-creation and re-rendering when toggled.

B <keep-alive> delays rendering of components until they are visible on screen.

C <keep-alive> compresses component data to reduce memory usage during runtime.

D <keep-alive> automatically splits component code into smaller chunks for faster loading.

💡 Hint

Think about what happens when a component is removed and added back without caching.

keeps component instances alive in memory, so Vue does not destroy and recreate them. This saves time and resources, especially for components that take a long time to mount or fetch data.