v-for with v-if precedence in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to correctly loop over items using v-for.

Vue

<ul>
  <li [1]="item in items">{{ item }}</li>
</ul>

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

Av-for

Bv-if

Cv-bind

Dv-show

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-if instead of v-for for looping.

Using v-bind which is for binding attributes.

Using v-show which controls visibility but does not loop.

The v-for directive is used to loop over arrays in Vue templates.

2fill in blank

medium

Complete the code to conditionally show list items only if item.active is true, using v-if.

Vue

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

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

Av-for

Bv-bind

Cv-if

Dv-show

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-for instead of v-if for condition.

Using v-show which only toggles visibility but keeps elements in DOM.

Using v-bind which is for attribute binding.

The v-if directive conditionally renders elements based on the expression.

3fill in blank

hard

Fix the error in the code by placing v-if with correct precedence to filter items inside the loop.

Vue

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

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

Av-for

Bv-if

Cv-bind

Dv-show

Attempts:

3 left

💡 Hint

Common Mistakes

Placing v-if before v-for causing unexpected behavior.

Using v-show instead of v-if for filtering.

Omitting v-if and filtering outside the loop.

When using v-for and v-if on the same element, v-for should come first and v-if after to ensure correct filtering.

4fill in blank

hard

Fill both blanks to correctly loop over items and conditionally render only active items.

Vue

<ul>
  <li [1]="item in items" [2]="item.active">{{ item.name }}</li>
</ul>

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

Av-for

Bv-show

Cv-if

Dv-bind

Attempts:

3 left

💡 Hint

Common Mistakes

Using v-show instead of v-if for filtering.

Reversing the order of v-for and v-if.

Using v-bind which is for attribute binding.

Use v-for to loop and v-if to conditionally render active items inside the loop.

5fill in blank

hard

Fill all three blanks to create a list that loops over items, shows only active items, and binds a unique key.

Vue

<ul>
  <li [1]="item in items" [2]="item.active" [3]="item.id">{{ item.name }}</li>
</ul>

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

Av-for

Bv-if

C:key

Dv-show

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the key binding causing rendering issues.

Using v-show instead of v-if for filtering.

Placing directives in wrong order.

Use v-for to loop, v-if to filter active items, and :key to bind a unique key for Vue's rendering optimization.

Practice

1. In Vue, when using v-for and v-if on the same element, which happens first?

easy

A. v-if runs first, then v-for loops over the filtered items.

B. Both run simultaneously.

C. v-for runs first, then v-if filters the rendered items.

D. Neither runs; Vue throws an error.

Solution

1. Step 1: Understand Vue's rendering order

   Vue processes v-for first to create the list of items.

2. Step 2: Apply v-if filtering

   After the list is created, v-if decides which items to actually render.

3. Final Answer:

   v-for runs first, then v-if filters the rendered items. -> Option C

4. Quick Check:

   v-for before v-if = B [OK]

Hint: Remember: loop first, then filter with v-if [OK]

Common Mistakes:

• Thinking v-if filters before looping
• Assuming simultaneous execution
• Believing Vue throws error with both directives

2. Which of the following is the correct syntax to use v-for with v-if on the same element in Vue?

easy

A. <li v-for="item in items"> <template v-if="item.active">{{ item.name }}</template> </li>

B. <li v-for="item in items" v-if="item.active">{{ item.name }}</li>

C. <li v-if="item.active" v-for=item in items>{{ item.name }}</li>

D. <li v-if="item.active"> <template v-for="item in items">{{ item.name }}</template> </li>

Solution

1. Step 1: Check directive order on the same element

   Vue allows v-for and v-if on the same element; v-for should come first logically but order in code doesn't affect behavior.

2. Step 2: Validate syntax correctness

   <li v-for="item in items" v-if="item.active">{{ item.name }}</li> correctly places both directives on the <li> element with proper syntax.

3. Final Answer:

   <li v-for="item in items" v-if="item.active">{{ item.name }}</li> -> Option B

4. Quick Check:

   Correct syntax with both directives on same element = C [OK]

Hint: Put v-for and v-if on same tag; v-for first is common [OK]

Common Mistakes:

• Swapping v-for and v-if causing confusion
• Using v-if inside v-for incorrectly
• Placing v-for inside v-if template incorrectly

3. Given the Vue template:

<ul>
  <li v-for="item in items" v-if="item.show">{{ item.name }}</li>
</ul>

and data:

items = [
  { name: 'A', show: true },
  { name: 'B', show: false },
  { name: 'C', show: true }
]

What will be rendered inside the <ul>?

medium

A. AC

B. ABC

C. B

D. No items rendered

Solution

1. Step 1: Loop through all items with v-for

   Vue creates list items for all three objects: A, B, and C.

2. Step 2: Apply v-if to filter items

   Only items with show: true are rendered, so B is skipped.

3. Final Answer:

   <li>A</li><li>C</li> -> Option A

4. Quick Check:

   v-if filters out B, so only A and C show [OK]

Hint: v-for creates all, v-if hides unwanted [OK]

Common Mistakes:

• Assuming v-if filters before v-for
• Expecting all items to render
• Confusing property names in data

4. What is the main issue with this Vue template?

<div v-if="item.active" v-for="item in items">{{ item.name }}</div>

Assuming items is an array of objects with active boolean property.

medium

A. There is no issue; this is the recommended pattern.

B. The v-if should be inside the loop, not on the same element.

C. The order of v-if and v-for is invalid and causes syntax error.

D. Using v-if and v-for on the same element can cause performance issues.

Solution

1. Step 1: Understand v-for and v-if on same element

   Vue processes v-for first, then v-if filters items, but this means all items are looped before filtering.

2. Step 2: Identify performance concern

   Using v-if with v-for on the same element can cause unnecessary loops and slow rendering.

3. Final Answer:

   Using v-if and v-for on the same element can cause performance issues. -> Option D

4. Quick Check:

   v-for then v-if may hurt performance [OK]

Hint: Avoid v-if on same element as v-for for better speed [OK]

Common Mistakes:

• Thinking order causes syntax error
• Believing no performance impact
• Assuming v-if runs before v-for

5. You want to display only active users from a large list efficiently. Which Vue pattern is best to combine v-for and v-if for performance?

hard

A. Filter the list in a computed property, then use v-for without v-if.

B. Use v-for with v-if on the same element to filter active users.

C. Use v-if outside the v-for to filter the entire list.

D. Use nested v-for loops with v-if inside.

Solution

1. Step 1: Understand performance impact of v-if with v-for

   Using v-if inside v-for causes Vue to loop all items before filtering, which is inefficient for large lists.

2. Step 2: Use computed property to pre-filter data

   Filtering the list in a computed property returns only active users, so v-for loops only needed items without v-if.

3. Final Answer:

   Filter the list in a computed property, then use v-for without v-if. -> Option A

4. Quick Check:

   Pre-filter data before v-for for best speed [OK]

Hint: Filter data first, then loop with v-for only [OK]

Common Mistakes:

• Using v-if inside v-for causing slow loops
• Trying to filter with v-if outside v-for incorrectly
• Using nested loops unnecessarily