v-for with v-if precedence in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Output of v-for with v-if on the same element

Consider this Vue template snippet:

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

Given items = [{ id: 1, name: 'A', show: true }, { id: 2, name: 'B', show: false }, { id: 3, name: 'C', show: true }], what will be rendered?

Vue

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

A <ul><li>B</li></ul>

B <ul><li>A</li><li>B</li><li>C</li></ul>

C <ul><li>A</li><li>C</li></ul>

D <ul></ul>

💡 Hint

Remember that v-if on the same element as v-for filters items after the loop runs.

The v-for loops over all items, but v-if on the same element only renders those where item.show is true. So only items with show: true appear.

🧠 Conceptual

intermediate

Where to place v-if for better performance with v-for?

You want to render a list but only show items that meet a condition. Which approach is better for performance?

A Filter the array in the script and use <code>v-for</code> alone in the template.

B Use <code>v-if</code> on the same element as <code>v-for</code> to filter items.

C Use <code>v-if</code> on a parent element wrapping the <code>v-for</code> element.

D Use <code>v-show</code> on the <code>v-for</code> element instead of <code>v-if</code>.

💡 Hint

Think about how many times Vue has to evaluate the condition.

Filtering the array before rendering means Vue only loops over needed items, improving performance. Using v-if inside v-for runs the condition for every item.

📝 Syntax

advanced

Identify the syntax error with v-for and v-if usage

Which option contains a syntax error in Vue template using v-for and v-if?

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

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

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

D <li v-for="item in items" :key="item.id" v-if> {{ item.name }} </li>

💡 Hint

Check if all directives have valid expressions.

Option D uses v-if without a condition, which is invalid syntax and causes a compile error.

🔧 Debug

advanced

Why does this v-for with v-if not render any items?

Given this template:

<ul>
  <li v-for="item in items" v-if="item.visible" :key="item.id">{{ item.text }}</li>
</ul>

And data:

items = [{ id: 1, text: 'One', visible: false }, { id: 2, text: 'Two', visible: false }]

Why does nothing render?

Vue

<ul>
  <li v-for="item in items" v-if="item.visible" :key="item.id">{{ item.text }}</li>
</ul>

A Because the :key attribute is missing or invalid.

B Because all items have visible set to false, so v-if filters them all out.

C Because v-for cannot be used with v-if on the same element.

D Because the items array is empty.

💡 Hint

Check the values of the visible property in the data.

All items have visible: false, so v-if prevents rendering any list item.

❓ state_output

expert

What is the rendered output with nested v-for and v-if precedence?

Consider this Vue template:

<ul>
  <li v-for="group in groups" :key="group.id">
    <h3>{{ group.name }}</h3>
    <ul>
      <li v-for="item in group.items" v-if="item.active" :key="item.id">{{ item.label }}</li>
    </ul>
  </li>
</ul>

Given data:

groups = [
  { id: 1, name: 'Group 1', items: [
    { id: 'a', label: 'Item A', active: true },
    { id: 'b', label: 'Item B', active: false }
  ]},
  { id: 2, name: 'Group 2', items: [
    { id: 'c', label: 'Item C', active: false },
    { id: 'd', label: 'Item D', active: true }
  ]}
]

What will be the rendered HTML?

Vue

<ul>
  <li v-for="group in groups" :key="group.id">
    <h3>{{ group.name }}</h3>
    <ul>
      <li v-for="item in group.items" v-if="item.active" :key="item.id">{{ item.label }}</li>
    </ul>
  </li>
</ul>

A <ul><li><h3>Group 1</h3><ul><li>Item A</li></ul></li><li><h3>Group 2</h3><ul><li>Item D</li></ul></li></ul>

B <ul><li><h3>Group 1</h3><ul><li>Item A</li><li>Item B</li></ul></li><li><h3>Group 2</h3><ul><li>Item C</li><li>Item D</li></ul></li></ul>

C <ul><li><h3>Group 1</h3><ul></ul></li><li><h3>Group 2</h3><ul></ul></li></ul>

D <ul><li><h3>Group 1</h3><ul><li>Item B</li></ul></li><li><h3>Group 2</h3><ul><li>Item C</li></ul></li></ul>

💡 Hint

Remember v-if filters items inside each group's items array.

Only items with active: true render inside each group's list. Group 1 shows Item A, Group 2 shows Item D.

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