Async components for lazy loading in Vue - Step-by-Step Execution

Concept Flow - Async components for lazy loading

Start Vue app

↓

Render parent component

↓

Encounter async component

↓

Load component code asynchronously

↓

Show loading state (optional)

↓

Component code loaded

↓

Render async component

↓

User interacts with component

↓

App continues

Vue app starts and renders a parent component. When it meets an async component, it loads it in the background, optionally shows a loading message, then renders the component once loaded.

Execution Sample

Vue

import { defineAsyncComponent } from 'vue';

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

export default {
  components: { AsyncComp },
  template: '<AsyncComp />'
}

Defines an async component that loads MyComp.vue lazily and uses it in the template.

Execution Table

Step	Action	Component State	Output/Render
1	Start Vue app	No components loaded	Blank or root app shell
2	Render parent component	Parent ready	Parent template rendered
3	Encounter AsyncComp	AsyncComp not loaded	Show loading placeholder (if any)
4	Begin loading MyComp.vue asynchronously	Loading in progress	Loading indicator visible
5	MyComp.vue loaded	AsyncComp ready	AsyncComp rendered with MyComp content
6	User interacts with AsyncComp	AsyncComp active	Component responds to user
7	App continues running	All components loaded	Full UI interactive

💡 Async component finishes loading and renders, no more loading steps.

Variable Tracker

Variable	Start	After Step 3	After Step 5	Final
AsyncComp state	not loaded	loading	loaded	loaded

Key Moments - 3 Insights

Why do we see a loading placeholder before the component appears?

Because the async component code is still being fetched (see execution_table step 4), Vue shows a loading state to keep the UI responsive.

What happens if the async component fails to load?

Vue can show an error component or fallback UI if loading fails, but in this trace, loading succeeds at step 5.

Is the async component code included in the main bundle initially?

No, it is loaded separately on demand (step 4), which helps reduce initial app size.

Visual Quiz - 3 Questions

Look at the execution table, what is the AsyncComp state at step 4?

A loading

B loaded

C not loaded

D error

💡 Hint

Check the 'Component State' column at step 4 in the execution_table.

At which step does the async component finish loading and render?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for when 'AsyncComp rendered' appears in the 'Output/Render' column.

If we remove the loading placeholder, what changes in the execution table?

A Step 5 would be skipped

B Step 3 output would be blank instead of loading placeholder

C AsyncComp would load faster

D Parent component would not render

💡 Hint

Focus on the 'Output/Render' column at step 3 in the execution_table.

Concept Snapshot

Async components in Vue let you load parts of your app only when needed.
Use defineAsyncComponent with a dynamic import.
Vue shows a loading state while fetching the code.
Once loaded, Vue renders the component normally.
This improves app speed by reducing initial load size.

Full Transcript

This visual trace shows how Vue handles async components for lazy loading. The app starts and renders the parent component. When it encounters an async component, Vue starts loading its code in the background. During loading, a placeholder or loading message can appear to keep the UI responsive. Once the component code finishes loading, Vue renders the async component normally. The variable tracker shows the async component state changing from not loaded to loading to loaded. Key moments clarify why loading placeholders appear and how lazy loading reduces initial bundle size. The quiz tests understanding of component states and rendering steps. This helps beginners see how Vue loads components only when needed, improving app performance.