Defining components in Vue - Performance & Optimization

Performance: Defining components

MEDIUM IMPACT

This affects initial page load speed and rendering performance by controlling how Vue processes and renders UI parts.

Creating reusable UI parts in Vue

Vue

<script setup>
import MyButton from './MyButton.vue'
</script>

<template>
  <MyButton />
  <MyButton />
  <MyButton />
</template>

Importing components from separate files allows Vue to cache and reuse component definitions, reducing setup cost.

📈 Performance GainSingle component definition reused, reducing JS execution and improving LCP.

Creating reusable UI parts in Vue

Vue

<script setup>
const MyButton = {
  template: `<button>Click me</button>`
}
</script>

<template>
  <MyButton />
  <MyButton />
  <MyButton />
</template>

Defining components inline without proper separation can cause Vue to recreate component definitions on each render, increasing overhead.

📉 Performance CostTriggers multiple component initializations and increases JS execution time on load.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Inline component definitions recreated each render	High (many nodes recreated)	Multiple reflows per render	High paint cost due to frequent DOM updates	[X] Bad
Imported single component reused multiple times	Low (nodes reused)	Single reflow per update	Lower paint cost due to efficient updates	[OK] Good

Rendering Pipeline

Vue compiles component templates into render functions, which create virtual DOM nodes. These nodes are then patched to the real DOM. Defining components efficiently reduces the work in creating and patching nodes.

→JS Execution

→Virtual DOM Creation

→DOM Patching

⚠️ BottleneckJS Execution and Virtual DOM Creation when many components are defined inefficiently

Core Web Vital Affected

LCP

This affects initial page load speed and rendering performance by controlling how Vue processes and renders UI parts.

Optimization Tips

1Define Vue components once and import them to reuse.

2Avoid inline component definitions inside setup scripts to reduce JS overhead.

3Reuse component instances to minimize DOM node creation and reflows.

Performance Quiz - 3 Questions

What is a performance downside of defining Vue components inline inside the setup script repeatedly?

A It reduces the number of DOM nodes created, improving performance.

B It causes Vue to recreate component definitions on each render, increasing JS execution time.

C It automatically caches components, reducing memory usage.

D It delays the initial page load by lazy loading components.

DevTools: Performance

How to check: Record a performance profile while loading the page and interacting with components. Look for long scripting times and many component initializations.

What to look for: High JS execution time and repeated component setup indicate inefficient component definitions.

Practice

1. What is the main purpose of defining components in Vue?

easy

A. To handle server-side rendering only

B. To create reusable pieces of UI that can be used throughout the app

C. To connect the app to a database

D. To write CSS styles for the app

Solution

1. Step 1: Understand what components do in Vue

   Components are small, reusable parts of the user interface that can be used multiple times in an app.

2. Step 2: Identify the correct purpose

   Among the options, only creating reusable UI pieces matches the role of components.

3. Final Answer:

   To create reusable pieces of UI that can be used throughout the app -> Option B

4. Quick Check:

   Components = reusable UI blocks [OK]

Hint: Components build reusable UI parts in Vue apps [OK]

Common Mistakes:

• Confusing components with styling or backend logic
• Thinking components handle database connections
• Believing components only work on the server side

2. Which of the following is the correct way to start a Vue component using the Composition API with <script setup>?

easy

A.

{{ message }}

B.

{{ message }}

C.
{{ message }}

D.

{{ message }}

Solution

1. Step 1: Recognize the <script setup> syntax

   In Vue 3, <script setup> is a special syntax that allows direct use of variables without export default.

2. Step 2: Check which option uses <script setup> correctly

   <script setup> const message = 'Hello' </script> <template> <p>{{ message }}</p> </template> declares a variable inside <script setup> and uses it in the template, which is correct.

3. Final Answer:

   <script setup> const message = 'Hello' </script> <template> <p>{{ message }}</p> </template> -> Option A

4. Quick Check:

   <script setup> uses direct variable declaration [OK]

Hint: Use

Common Mistakes:

• Using export default inside
• Declaring variables outside
• Placing <script> after <template> incorrectly

3. Given this Vue component code, what will be displayed when rendered?

<script setup>
import { ref } from 'vue'
const count = ref(0)
function increment() {
  count.value++
}
</script>

<template>
  <button @click="increment">Clicked {{ count }} times</button>
</template>

medium

A. A button showing 'Clicked 0 times' initially, increasing count on each click

B. A button showing 'Clicked 1 times' initially, increasing count on each click

C. A button showing 'Clicked count times' as a literal string

D. An error because count is not reactive

Solution

1. Step 1: Understand ref and reactivity

   The variable count is a reactive reference starting at 0. Using count.value++ updates it reactively.

2. Step 2: Check the template interpolation

   The template uses {{ count }}, which Vue unwraps automatically to show the current value of count.

3. Final Answer:

   A button showing 'Clicked 0 times' initially, increasing count on each click -> Option A

4. Quick Check:

   ref count starts at 0 and updates on click [OK]

Hint: ref variables start at initial value and update reactively [OK]

Common Mistakes:

• Thinking count starts at 1 instead of 0
• Expecting literal 'count' string instead of value
• Assuming count is not reactive without .value

4. Identify the error in this Vue component code:

<script setup>
const props = defineProps({
  title: String
})

function showTitle() {
  console.log(props.title.toUpperCase())
}

showTitle()
</script>

<template>
  <h1>{{ title }}</h1>
</template>

medium

A. Missing import of defineProps

B. Calling showTitle() before props are available

C. Using title in template instead of props.title

D. Using toUpperCase() on a possibly undefined prop

Solution

1. Step 1: Analyze prop definition and usage

   defineProps({ title: String }) declares title without required: true or default value, so props.title can be undefined.

2. Step 2: Locate the error in JS code

   In showTitle(), props.title.toUpperCase() assumes title is a string, but calling toUpperCase() on undefined causes a runtime error.

3. Final Answer:

   Using toUpperCase() on a possibly undefined prop -> Option D

4. Quick Check:

   Validate props before string operations [OK]

Hint: Guard undefined props before methods like toUpperCase [OK]

Common Mistakes:

• Applying string methods to props that may be undefined
• Forgetting to import defineProps (though auto-imported in some setups)
• Assuming props are reactive variables directly

5. You want to create a Vue component that accepts a user prop object with name and age, and displays "Name: [name], Age: [age]". Which is the best way to define and use this prop in a <script setup> component?

hard

A.

Name: {{ user.name }}, Age: {{ user.age }}

B.

Name: {{ props.name }}, Age: {{ props.age }}

C.

Name: {{ props.user.name }}, Age: {{ props.user.age }}

D.

Name: {{ user.name }}, Age: {{ user.age }}

Solution

1. Step 1: Define the prop as an object with expected properties

   The prop user is an object containing name and age, so define it as an Object type and mark required if needed.

2. Step 2: Access the nested properties correctly in the template

   Use props.user.name and props.user.age to display the values inside the template.

3. Final Answer:

   Define user as an object prop and access nested properties via props.user.name and props.user.age -> Option C

4. Quick Check:

   Object prop with nested access = <script setup> const props = defineProps({ user: { type: Object, required: true } }) </script> <template> <p>Name: {{ props.user.name }}, Age: {{ props.user.age }}</p> </template> [OK]

Hint: Define object prop fully and access nested fields via props [OK]

Common Mistakes:

• Defining nested fields as separate props instead of one object
• Destructuring props incorrectly in
• Using defineProps with wrong syntax for objects