v-once for static content in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to render static content only once using Vue's directive.

Vue

<div [1]>
  <p>This content will not update.</p>
</div>

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

Av-if

Bv-once

Cv-for

Dv-bind

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if instead of v-once causes conditional rendering, not static rendering.

Using v-for is for lists, not static content.

Using v-bind is for binding attributes, not controlling rendering.

The v-once directive tells Vue to render the element and its children only once, skipping future updates.

2fill in blank

medium

Complete the code to apply v-once to a component's root element.

Vue

<template>
  <section [1]>
    <h1>Welcome</h1>
  </section>
</template>

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

Av-once

Bv-show

Cv-model

Dv-if

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if will conditionally render but not prevent updates.

v-show toggles visibility but still updates the DOM.

v-model is for two-way binding, not rendering control.

Applying v-once on the root element ensures the whole section renders only once.

3fill in blank

hard

Fix the error in the code to correctly use v-once on a list item.

Vue

<ul>
  <li v-for="item in items" [1]>{{ item.name }}</li>
</ul>

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

Av-if

Bv-bind

Cv-once

Dv-model

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if inside v-for can cause unexpected behavior.

v-bind is for binding attributes, not controlling rendering.

v-model is unrelated to rendering control.

Using v-once on each list item renders them once and skips updates, which is useful for static lists.

4fill in blank

hard

Fill both blanks to create a static list where each item renders only once.

Vue

<ul>
  <li v-for="item in items" :key="BLANK_1" {{BLANK_2}}>{{ item.text }}</li>
</ul>

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

Aitem.id

Bv-once

Citem.name

Dv-if

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if instead of v-once will not prevent re-rendering.

Using item.name as key may not be unique.

Omitting the key causes Vue to warn about list rendering.

Using :key="item.id" helps Vue track list items, and v-once ensures each item renders only once.

5fill in blank

hard

Fill all three blanks to optimize static content rendering with v-once and unique keys.

Vue

<template>
  <div>
    <article v-for="post in posts" :key="BLANK_1" [2]>
      <h2>{{ post.title }}</h2>
      <p>{{ post.content }}</p>
    </article>
    <footer>{{BLANK_3}}</footer>
  </div>
</template>

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

Apost.id

Bv-once

Cnew Date().getFullYear()

Dv-if

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if instead of v-once for static rendering.

Using a non-unique key causes rendering issues.

Not using JavaScript expression for dynamic footer content.

Use post.id as a unique key, v-once to render articles once, and display the current year in the footer.

Practice

1. What does the v-once directive do in a Vue template?

easy

A. Makes the element reactive to data changes

B. Binds an event listener that triggers only once

C. Renders the element or component only once and skips future updates

D. Delays rendering until a condition is true

Solution

1. Step 1: Understand the purpose of v-once

   v-once tells Vue to render the element or component only one time and then never update it again, even if reactive data changes.

2. Step 2: Compare with other directives

   Unlike event listeners or conditional rendering, v-once is about skipping future updates to improve performance for static content.

3. Final Answer:

   Renders the element or component only once and skips future updates -> Option C

4. Quick Check:

   v-once = Render once [OK]

Hint: Remember: v-once means render once only [OK]

Common Mistakes:

• Confusing v-once with event listeners
• Thinking it makes content reactive
• Assuming it delays rendering

2. Which of the following is the correct syntax to apply v-once to a paragraph element?

easy

A. Static text here

B. Static text here

C. Static text here

D. Static text here

Solution

1. Step 1: Recall Vue directive syntax

   Vue directives like v-once are used as attributes without values unless they accept arguments or expressions.

2. Step 2: Identify correct usage

   The correct syntax is simply v-once without any value or binding syntax.

3. Final Answer:

   <p v-once>Static text here</p> -> Option A

4. Quick Check:

   Directive attribute without value =

   Static text here

   [OK]

Hint: Use v-once as a plain attribute without value [OK]

Common Mistakes:

• Adding ="true" which is unnecessary
• Using binding syntax like :v-once
• Trying to call it like a function

3. Consider this Vue template snippet:

<template>
  <div>
    <h1 v-once>Title: {{ title }}</h1>
    <button @click="changeTitle">Change Title</button>
  </div>
</template>

<script setup>
import { ref } from 'vue'
const title = ref('Hello')
function changeTitle() {
  title.value = 'Goodbye'
}
</script>

What will be displayed in the <h1> after clicking the button?

medium

A. An error occurs

B. Title: Goodbye

C. Title: {{ title }}

D. Title: Hello

Solution

1. Step 1: Understand v-once effect on rendering

   The v-once directive renders the element only once and skips updates, so the initial value of title is used.

2. Step 2: Analyze what happens on button click

   When the button is clicked, title.value changes to 'Goodbye', but the <h1> does not update because of v-once.

3. Final Answer:

   Title: Hello -> Option D

4. Quick Check:

   Static render with v-once = unchanged text [OK]

Hint: Remember v-once freezes initial render [OK]

Common Mistakes:

• Expecting reactive update despite v-once
• Thinking interpolation shows raw text
• Assuming runtime error on update

4. You wrote this Vue template:

<div v-once>
  <p>Count: {{ count }}</p>
</div>

<script setup>
import { ref } from 'vue'
const count = ref(0)
setInterval(() => { count.value++ }, 1000)
</script>

But the displayed count never changes. What is the problem?

medium

A. The count variable is not reactive

B. Using v-once prevents updates to the count display

C. The setInterval function is not running

D. The interpolation syntax is incorrect

Solution

1. Step 1: Identify effect of v-once on updates

   v-once renders the element once and disables future updates, so changes to count won't reflect.

2. Step 2: Check reactivity and code correctness

   count is reactive and setInterval updates it correctly; interpolation syntax is valid.

3. Final Answer:

   Using v-once prevents updates to the count display -> Option B

4. Quick Check:

   v-once disables updates = Using v-once prevents updates to the count display [OK]

Hint: Remember v-once disables reactive updates [OK]

Common Mistakes:

• Assuming reactivity is broken
• Thinking setInterval is not called
• Blaming interpolation syntax

5. You want to optimize a Vue app by marking static parts with v-once. Which of these is the best use case for v-once?

hard

A. A static footer that never changes after page load

B. A user profile name that updates when user edits it

C. A live clock showing current time

D. A list of messages that updates when new messages arrive

Solution

1. Step 1: Understand when v-once is useful

   v-once is best for content that never changes after initial render to save update costs.

2. Step 2: Evaluate each option

   Footer is static, so v-once fits. User profile, live clock, and message list all change and need updates, so v-once is inappropriate.

3. Final Answer:

   A static footer that never changes after page load -> Option A

4. Quick Check:

   Static content = use v-once [OK]

Hint: Use v-once only for truly static content [OK]

Common Mistakes:

• Applying v-once to dynamic data
• Expecting reactive updates with v-once
• Using v-once on frequently changing UI