Why Vue performance matters - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a reactive variable in Vue.

Vue

const count = [1](0);

Drag options to blanks, or click blank then click option'

Awatch

Breactive

Cref

Dcomputed

Attempts:

3 left

💡 Hint

Common Mistakes

Using reactive instead of ref for a simple number.

Using computed when you want a reactive variable.

In Vue, ref creates a reactive variable that tracks changes.

2fill in blank

medium

Complete the code to watch changes on a reactive variable.

Vue

watch([1], (newVal, oldVal) => { console.log(newVal); });

Drag options to blanks, or click blank then click option'

Acount

BcountReactive

CcountRef

Dcount.value

Attempts:

3 left

💡 Hint

Common Mistakes

Watching count.value instead of count.

Using a variable name that does not exist.

You watch the reactive variable itself, not its value property.

3fill in blank

hard

Fix the error in the Vue template binding.

Vue

<template>
  <div>{{ [1] }}</div>
</template>

Drag options to blanks, or click blank then click option'

AcountReactive

Bcount.value

CcountRef

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using count.value inside the template interpolation.

Using a variable name that is not defined.

In Vue templates, refs are unwrapped automatically, so use the variable name directly.

4fill in blank

hard

Fill both blanks to create a computed property that doubles a count.

Vue

const doubleCount = computed(() => [1] * [2]);

Drag options to blanks, or click blank then click option'

Acount.value

Bcount

C2

DcountRef

Attempts:

3 left

💡 Hint

Common Mistakes

Using count instead of count.value inside computed.

Using a variable name that is not reactive.

Computed properties use the inner value of refs, so use count.value multiplied by 2.

5fill in blank

hard

Fill all three blanks to create a reactive object and update its property.

Vue

const state = [1]({ count: 0 });

function increment() {
  state.[2] += [3];
}

Drag options to blanks, or click blank then click option'

Areactive

Bcount

C1

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using ref instead of reactive for an object.

Trying to increment a property that does not exist.

Use reactive to create a reactive object, then update its count property by 1.

Practice

1. Why is Vue performance important for web applications?

easy

A. It keeps the app smooth and users happy.

B. It makes the app look colorful.

C. It increases the size of the app files.

D. It removes all animations from the app.

Solution

1. Step 1: Understand user experience impact

   Good performance means the app runs smoothly without delays, which keeps users happy.

2. Step 2: Identify the role of performance

   Performance does not affect colors or file size directly but affects how fast and responsive the app feels.

3. Final Answer:

   It keeps the app smooth and users happy. -> Option A

4. Quick Check:

   Vue performance = smooth app and happy users [OK]

Hint: Performance means smooth and happy users, not looks [OK]

Common Mistakes:

• Confusing performance with visual design
• Thinking performance increases app size
• Assuming performance removes animations

2. Which Vue feature helps improve performance by updating only what changes?

easy

A. Using global variables

B. Direct DOM manipulation

C. Reactive data binding

D. Inline CSS styles

Solution

1. Step 1: Identify Vue's reactive system

   Vue uses reactive data binding to track changes and update only affected parts of the page.

2. Step 2: Compare other options

   Direct DOM manipulation is manual and less efficient; global variables and inline styles do not improve update efficiency.

3. Final Answer:

   Reactive data binding -> Option C

4. Quick Check:

   Reactive data = efficient updates [OK]

Hint: Reactive data updates only changed parts [OK]

Common Mistakes:

• Choosing direct DOM manipulation instead of reactive binding
• Confusing global variables with performance features
• Thinking inline styles affect update efficiency

3. What will be the output behavior of this Vue component when the button is clicked?

<template>
  <div>
    <p>Count: {{ count }}</p>
    <button @click="increment">Add</button>
  </div>
</template>

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

medium

A. The count number stays at 0 and does not change.

B. The count number decreases by 1 each click.

C. The app crashes with an error on click.

D. The count number increases by 1 each click and updates on screen.

Solution

1. Step 1: Understand reactive ref usage

   The count is a reactive reference starting at 0, updated by increment function increasing count.value.

2. Step 2: Check template binding and event

   The template shows count and button click calls increment, so count increases and updates on screen.

3. Final Answer:

   The count number increases by 1 each click and updates on screen. -> Option D

4. Quick Check:

   Reactive ref updates count display [OK]

Hint: ref value++ updates reactive display [OK]

Common Mistakes:

• Forgetting to use .value with ref
• Assuming count does not update without manual DOM change
• Thinking count decreases instead of increases

4. Identify the error in this Vue code that affects performance:

<template>
  <div>
    <p>Message: {{ message }}</p>
    <button @click="updateMessage">Change</button>
  </div>
</template>

<script setup>
import { reactive } from 'vue'
const state = reactive({ message: 'Hello' })
function updateMessage() {
  state.message = 'Hi'
  state = { message: 'Hey' }
}
</script>

medium

A. Using reactive instead of ref is wrong here.

B. Reassigning reactive object 'state' causes an error.

C. The template syntax is invalid.

D. The button click event is missing.

Solution

1. Step 1: Check reactive object usage

   Reactive objects cannot be reassigned; only their properties can be changed.

2. Step 2: Identify the error line

   The line 'state = { message: 'Hey' }' tries to reassign the reactive object, causing an error and hurting performance.

3. Final Answer:

   Reassigning reactive object 'state' causes an error. -> Option B

4. Quick Check:

   Reactive object reassignment = error [OK]

Hint: Do not reassign reactive objects, only update properties [OK]

Common Mistakes:

• Trying to reassign reactive objects
• Confusing reactive with ref usage
• Ignoring error messages in console

5. You want to optimize a Vue app that has a large list rendering slowly. Which approach best improves performance?

hard

A. Use v-for with a unique :key and virtual scrolling.

B. Render all list items at once without keys.

C. Use inline styles for each list item.

D. Avoid using computed properties for list filtering.

Solution

1. Step 1: Understand list rendering performance

   Rendering large lists can be slow; Vue recommends using unique keys with v-for to track items efficiently.

2. Step 2: Apply virtual scrolling

   Virtual scrolling renders only visible items, greatly improving performance on large lists.

3. Final Answer:

   Use v-for with a unique :key and virtual scrolling. -> Option A

4. Quick Check:

   Keys + virtual scroll = faster list rendering [OK]

Hint: Use keys and virtual scroll for big lists [OK]

Common Mistakes:

• Rendering all items without keys
• Ignoring virtual scrolling techniques
• Avoiding computed properties unnecessarily