Async components for lazy loading in Vue - Mini Project: Build & Apply

Async Components for Lazy Loading in Vue

📖 Scenario: You are building a Vue app that shows different sections on demand. To make the app faster, you want to load some parts only when needed.

🎯 Goal: Create a Vue app that uses an async component to lazy load a ProfileCard component only when it is shown.

📋 What You'll Learn

Create a Vue app with a showProfile boolean state

Create an async component called ProfileCard using defineAsyncComponent

Add a button to toggle showProfile

Render ProfileCard only when showProfile is true

💡 Why This Matters

🌍 Real World

Lazy loading components helps speed up web apps by loading parts only when needed, improving user experience.

💼 Career

Understanding async components is important for building efficient Vue apps and is a common skill required in frontend developer roles.

1

Set up Vue app with showProfile state

Create a Vue component with a showProfile state variable initialized to false using ref. Use script setup syntax.

Vue

// Create a ref called showProfile and set it to false
// Your code here

Hint

Use import { ref } from 'vue' and then const showProfile = ref(false).

2

Create async component ProfileCard

Import defineAsyncComponent from 'vue' and create an async component called ProfileCard that loads from './ProfileCard.vue'.

Vue

import { ref, defineAsyncComponent } from 'vue'
const showProfile = ref(false)
// Create async component ProfileCard loading './ProfileCard.vue'
// Your code here

Hint

Use defineAsyncComponent(() => import('./ProfileCard.vue')) to create ProfileCard.

3

Add button to toggle showProfile

Add a button element with a click event that toggles showProfile.value between true and false. The button text should be 'Toggle Profile'.

Vue

import { ref, defineAsyncComponent } from 'vue'
const showProfile = ref(false)
const ProfileCard = defineAsyncComponent(() => import('./ProfileCard.vue'))



  
  

Hint

Use @click="showProfile = !showProfile" on the button to toggle the boolean.

4

Render ProfileCard only when showProfile is true

Inside the template, add a <ProfileCard /> component that only shows when showProfile is true using v-if.

Vue

import { ref, defineAsyncComponent } from 'vue'
const showProfile = ref(false)
const ProfileCard = defineAsyncComponent(() => import('./ProfileCard.vue'))



  Toggle Profile
  
  

Hint

Use <ProfileCard v-if="showProfile" /> to show the component only when true.

Practice

1. What is the main purpose of using defineAsyncComponent in Vue?

easy

A. To load a component only when it is needed, improving app performance

B. To define a component that updates automatically on data change

C. To create a component that runs only on the server side

D. To make a component that never re-renders after first load

Solution

1. Step 1: Understand what lazy loading means

   Lazy loading means loading something only when it is needed, not before.

2. Step 2: Connect lazy loading to defineAsyncComponent

   This function helps Vue load components only when they are required, saving resources.

3. Final Answer:

   To load a component only when it is needed, improving app performance -> Option A

4. Quick Check:

   Lazy loading = load on demand [OK]

Hint: Async components load only when needed, boosting speed [OK]

Common Mistakes:

• Thinking it updates components automatically
• Confusing with server-side rendering
• Believing it prevents re-renders forever

2. Which of the following is the correct syntax to create an async component in Vue 3?

easy

A. const AsyncComp = async () => import('./MyComp.vue')

B. const AsyncComp = defineAsyncComponent(() => import('./MyComp.vue'))

C. const AsyncComp = defineComponent(async () => import('./MyComp.vue'))

D. const AsyncComp = createAsyncComponent('./MyComp.vue')

Solution

1. Step 1: Recall the Vue 3 async component syntax

   Vue 3 uses defineAsyncComponent with a function returning an import promise.

2. Step 2: Match the syntax with options

   const AsyncComp = defineAsyncComponent(() => import('./MyComp.vue')) matches the correct pattern: defineAsyncComponent(() => import('...')).

3. Final Answer:

   const AsyncComp = defineAsyncComponent(() => import('./MyComp.vue')) -> Option B

4. Quick Check:

   Correct async syntax = defineAsyncComponent + import [OK]

Hint: Use defineAsyncComponent with arrow function returning import [OK]

Common Mistakes:

• Using async arrow function without defineAsyncComponent
• Using defineComponent instead of defineAsyncComponent
• Calling a non-existent createAsyncComponent function

3. Given this Vue code snippet, what will be rendered initially?

const AsyncComp = defineAsyncComponent(() => new Promise(resolve => {
  setTimeout(() => resolve({ template: '
Loaded!
' }), 2000)
}))

export default {
  components: { AsyncComp },
  template: `
    
    
Loading...
  `
}

medium

A. Loading... (fallback content) for 2 seconds, then Loaded!

B. Loaded! immediately without delay

C. Nothing renders until the component loads

D. An error because Suspense cannot be used with async components

Solution

1. Step 1: Understand Suspense fallback behavior

   Suspense shows fallback content while async component loads.

2. Step 2: Analyze the async component delay

   The component resolves after 2 seconds, so fallback shows first.

3. Final Answer:

   Loading... (fallback content) for 2 seconds, then Loaded! -> Option A

4. Quick Check:

   Suspense fallback shows during async load [OK]

Hint: Suspense fallback shows until async component resolves [OK]

Common Mistakes:

• Expecting immediate component render
• Thinking nothing shows during loading
• Believing Suspense cannot wrap async components

4. Identify the error in this async component usage:

import { defineAsyncComponent } from 'vue'

const AsyncComp = defineAsyncComponent(() => import('./MyComp.vue'))

export default {
  components: { AsyncComp },
  template: `
`
}

medium

A. Async component must be registered globally, not locally

B. The template uses instead of self-closing

C. Missing Suspense wrapper to handle loading state

D. The import path './MyComp.vue' is invalid syntax

Solution

1. Step 1: Check async component usage rules

   Async components should be wrapped in <Suspense> to show fallback during loading.

2. Step 2: Analyze the given code

   The code lacks a <Suspense> wrapper, so no loading state is handled.

3. Final Answer:

   Missing Suspense wrapper to handle loading state -> Option C

4. Quick Check:

   Async components need Suspense for loading fallback [OK]

Hint: Always wrap async components in Suspense for loading UI [OK]

Common Mistakes:

• Thinking self-closing tags are mandatory
• Believing async components must be global
• Assuming import path syntax is wrong

5. You want to lazy load two components, CompA and CompB, but only show CompB after CompA finishes loading. How can you achieve this using Vue async components and Suspense?

hard

A. Wrap CompA in Suspense, and inside its default slot, include CompB as a normal component

B. Use defineAsyncComponent for both, wrap both in a single Suspense, and use v-if to show CompB only after CompA loads

C. Load both components with defineAsyncComponent and render them side by side without Suspense

D. Wrap CompA in a Suspense with fallback, then in CompA's mounted hook, render CompB inside another Suspense

Solution

1. Step 1: Understand the loading order requirement

   You want CompB to appear only after CompA finishes loading.

2. Step 2: Use sequential triggering with lifecycle

   Wrap CompA in Suspense; in CompA's mounted hook, dynamically render CompB (async) inside its own Suspense for true sequential loading.

3. Step 3: Why other options fail

   Wrapping both in a single Suspense loads them in parallel (shows both after slowest finishes); loading side-by-side without Suspense skips fallback and loads parallel; including normal CompB in CompA's default slot loads it immediately upon CompA resolution.

4. Final Answer:

   Wrap CompA in a Suspense with fallback, then in CompA's mounted hook, render CompB inside another Suspense -> Option D

5. Quick Check:

   Sequential async = trigger next in mounted [OK]

Hint: Trigger next async component in mounted hook for sequential loading [OK]

Common Mistakes:

• Using single Suspense for parallel loading
• Skipping Suspense for async components
• Treating second component as normal (non-async)