v-model modifiers (lazy, number, trim) in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does the lazy modifier affect v-model updates?

Consider a Vue 3 component with an input bound using v-model.lazy. When does the component update its bound data compared to a normal v-model?

Vue

<template>
  <input v-model.lazy="text" />
  <p>{{ text }}</p>
</template>

<script setup>
import { ref } from 'vue'
const text = ref('')
</script>

A The data updates only when the input is cleared.

B The data updates only when the input loses focus (on blur event).

C The data updates immediately on every keystroke.

D The data updates only when the Enter key is pressed.

💡 Hint

Think about when the input event triggers versus the change event.

The lazy modifier delays updating the bound data until the input loses focus, which means the update happens on the change event instead of the input event.

❓ component_behavior

intermediate

What effect does the number modifier have on v-model?

Given an input bound with v-model.number, what type will the bound data have after user input?

Vue

<template>
  <input v-model.number="age" />
  <p>{{ typeof age }}</p>
</template>

<script setup>
import { ref } from 'vue'
const age = ref('')
</script>

A 'number', because the modifier converts input to a number.

B 'string', because input values are always strings.

C 'boolean', because the modifier converts input to true or false.

D 'object', because the modifier wraps the value.

💡 Hint

Consider what happens when you type '42' in the input.

The number modifier automatically converts the input string to a number type before updating the bound data.

📝 Syntax

advanced

Which v-model usage correctly trims whitespace from user input?

Select the correct Vue 3 template syntax that trims whitespace from the input value using v-model modifiers.

A <input v-model="trim(username)" />

B <input v-model.lazy.trim="username" />

C <input v-model="username.trim" />

D <input v-model.trim="username" />

💡 Hint

Modifiers are added after v-model separated by dots.

The trim modifier removes leading and trailing whitespace from the input before updating the bound data. It is used as v-model.trim.

🔧 Debug

advanced

Why does this v-model.number input not convert to a number?

Given this code, the bound data remains a string after input. What is the likely cause?

Vue

<template>
  <input v-model.number.lazy="score" />
  <p>{{ typeof score }}</p>
</template>

<script setup>
import { ref } from 'vue'
const score = ref('')
</script>

A Because <code>number</code> modifier does not work with <code>lazy</code> modifier.

B Because <code>score</code> is initialized as a string, it cannot convert to number.

C Because <code>lazy</code> delays update until blur, the number conversion happens only then.

D Because <code>v-model.number.lazy</code> syntax is invalid and ignored.

💡 Hint

Check when the data updates and when the conversion applies.

The lazy modifier delays the update until the input loses focus. The number modifier converts the value at update time, so the type remains string until blur.

🧠 Conceptual

expert

What is the combined effect of v-model.lazy.number.trim modifiers on an input?

Explain what happens to the bound data when using v-model.lazy.number.trim on an input element.

Vue

<template>
  <input v-model.lazy.number.trim="value" />
  <p>{{ value }} ({{ typeof value }})</p>
</template>

<script setup>
import { ref } from 'vue'
const value = ref('')
</script>

A The input updates the bound data only on blur, trims whitespace, and converts the value to a number.

B The input updates on every keystroke, trims whitespace, but keeps the value as a string.

C The input updates only on blur, but does not trim or convert the value.

D The input updates on every keystroke, converts to number, but does not trim whitespace.

💡 Hint

Consider the order and effect of each modifier on update timing and value transformation.

The lazy modifier delays update until blur, trim removes whitespace before update, and number converts the trimmed string to a number before updating the bound data.

Practice

1. What does the .lazy modifier do when used with v-model in Vue?

easy

A. Converts the input value to a number immediately

B. Automatically trims whitespace from the input value

C. Prevents the input from updating the bound data

D. Updates the bound data only when the input loses focus or on change event

Solution

1. Step 1: Understand the default v-model behavior

   By default, v-model updates the bound data on every input event (each keystroke).

2. Step 2: Effect of .lazy modifier

   The .lazy modifier changes this behavior to update only when the input loses focus or on the change event.

3. Final Answer:

   Updates the bound data only when the input loses focus or on change event -> Option D

4. Quick Check:

   .lazy delays update until change [OK]

Hint: Lazy means update on change or blur, not every keystroke [OK]

Common Mistakes:

• Thinking .lazy trims spaces
• Assuming .lazy converts to number
• Believing .lazy prevents updates

2. Which of the following is the correct syntax to use v-model with the .number modifier on an input element?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall correct modifier syntax

   Vue modifiers are added after v-model with a dot, like v-model.number.

2. Step 2: Identify correct syntax

   The correct syntax is <input v-model.number="age">. Placing the modifier inside quotes, using a function call, or adding it as a separate attribute is incorrect.

3. Final Answer:

   <input v-model.number="age"> -> Option A

4. Quick Check:

   Modifier syntax = dot after v-model [OK]

Hint: Modifiers go right after v-model with a dot [OK]

Common Mistakes:

• Placing modifier inside quotes
• Using modifier as a property of variable
• Adding modifier as a separate attribute

3. Given this Vue template snippet:

<input v-model.trim="username">
<p>User: {{ username }}</p>

If the user types alice (with spaces before and after) and then the input loses focus, what will be displayed inside the paragraph?

medium

A. User: alice

B. User: alice

C. ecila :resU

D. ecila :resU

Solution

1. Step 1: Understand .trim modifier effect

   The .trim modifier removes whitespace from the start and end of the input value before updating the bound data.

2. Step 2: Apply to input value

   The user input alice will be trimmed to alice before updating username.

3. Final Answer:

   User: alice -> Option A

4. Quick Check:

   .trim removes spaces around input [OK]

Hint: Trim removes spaces around input before update [OK]

Common Mistakes:

• Assuming spaces remain in bound data
• Confusing trim with lazy or number
• Expecting trimming only on submit

4. Consider this Vue input:

<input v-model.number.lazy="score">

What is the main issue if the user types 42 but the bound score variable remains a string instead of a number?

medium

A. The .number modifier does not work with .lazy

B. The input event is not triggered because .lazy waits for change event

C. The score variable was initialized as a string and cannot change type

D. The modifiers are applied in the wrong order

Solution

1. Step 1: Understand .lazy event timing

   The .lazy modifier updates the bound data only on the change event (when input loses focus), not on every input event.

2. Step 2: Reason about .number conversion timing

   The .number modifier converts the value when the update happens. If the update hasn't happened yet (input still focused), score remains unchanged.

3. Final Answer:

   The input event is not triggered because .lazy waits for change event -> Option B

4. Quick Check:

   .lazy delays update, so number conversion waits [OK]

Hint: Lazy delays update; number converts only on update [OK]

Common Mistakes:

• Thinking .number fails with .lazy
• Believing variable type is fixed
• Assuming modifier order matters here

5. You want to create a form input that updates the bound variable only when the user finishes typing and leaves the field, trims spaces, and converts the input to a number. Which of the following is the correct v-model usage?

hard

A.

B.

C.

D.

Solution

1. Step 1: Understand modifier order importance

   Vue applies modifiers from left to right. The .lazy modifier must come first to delay update until change event.

2. Step 2: Apply .trim and .number after .lazy

   After update triggers, .trim removes spaces, then .number converts to number before setting the variable.

3. Final Answer:

   <input v-model.lazy.trim.number="value"> -> Option C

4. Quick Check:

   Lazy first, then trim, then number [OK]

Hint: Put .lazy first, then .trim, then .number [OK]

Common Mistakes:

• Placing .lazy last so update happens too early
• Mixing modifier order causing wrong behavior
• Assuming order does not affect result