JavaScript expressions in templates in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Output of JavaScript expression in Vue template interpolation

What will be displayed inside the <div> when this Vue component renders?

Vue

<template>
  <div>{{ message.split('').reverse().join('') }}</div>
</template>

<script setup>
const message = 'hello'
</script>

A hello

B olleh

C h e l l o

D undefined

💡 Hint

Think about what the split, reverse, and join methods do to a string.

The expression splits the string into characters, reverses the array, then joins it back into a string, resulting in the reversed word 'olleh'.

📝 Syntax

intermediate

Valid JavaScript expression in Vue template

Which option is a valid JavaScript expression inside a Vue template interpolation to show the square of num?

Vue

<template>
  <div>{{ /* expression here */ }}</div>
</template>

<script setup>
const num = 4
</script>

A num *

B Math.pow(num)

C num ** 2

D square(num)

💡 Hint

Remember the exponentiation operator in JavaScript.

Option C correctly uses the exponentiation operator '**' to square the number. Option C misses the second argument for Math.pow. Option C is incomplete syntax. Option C calls an undefined function.

🔧 Debug

advanced

Why does this Vue template expression cause an error?

Consider this Vue template snippet:

<div>{{ user.name.toUpperCase() }}</div>

If user is null, what error will occur and why?

Vue

<template>
  <div>{{ user.name.toUpperCase() }}</div>
</template>

<script setup>
const user = null
</script>

A TypeError: Cannot read properties of null (reading 'name')

B ReferenceError: user is not defined

C SyntaxError: Unexpected token '.'

D No error, displays empty string

💡 Hint

Think about what happens when you try to access a property on null.

Since user is null, trying to access user.name causes a TypeError because null has no properties.

❓ state_output

advanced

Output after reactive state change in Vue template expression

Given this Vue component, what will be the displayed output after the button is clicked once?

Vue

<template>
  <div>{{ count.value * 3 }}</div>
  <button @click="increment">Add</button>
</template>

<script setup>
import { ref } from 'vue'
const count = ref(2)
function increment() {
  count.value++
}
</script>

A 9

B 6

C 3

D undefined

💡 Hint

Remember that count is reactive and starts at 2, then increments by 1.

Initially count is 2, after clicking increment once, count becomes 3. The template shows count * 3, so 3 * 3 = 9.

🧠 Conceptual

expert

Understanding expression evaluation order in Vue templates

In a Vue template, what is the result of this expression if a = 5 and b = 0?

{{ a && b || a }}

Vue

<template>
  <div>{{ a && b || a }}</div>
</template>

<script setup>
const a = 5
const b = 0
</script>

A false

B 0

C undefined

D 5

💡 Hint

Recall how JavaScript evaluates && and || operators from left to right.

The expression evaluates as (a && b) || a. Since a=5 (truthy), a && b returns b which is 0 (falsy). Then 0 || a returns 5. So the final result is 5.

Practice

1. In Vue templates, which syntax correctly inserts a JavaScript expression to display a variable named message?

easy

A. [[ message ]]

B. { message }

C. {{ message }}

D. ${ message }

Solution

1. Step 1: Recognize Vue interpolation syntax

   Vue uses double curly braces {{ }} to insert JavaScript expressions in templates.

2. Step 2: Match the correct syntax for variable display

   The correct way to show the value of message is {{ message }}.

3. Final Answer:

   <code>{{ message }}</code> -> Option C

4. Quick Check:

   Vue interpolation = {{ }} [OK]

Hint: Use double curly braces {{ }} to insert expressions [OK]

Common Mistakes:

• Using single curly braces or square brackets
• Trying to use template literals syntax inside templates
• Forgetting to use any braces

2. Which of the following is the correct way to write a simple math expression inside a Vue template to add 5 and 3?

easy

A. {{ 5 & 3 }}

B. {{ 5 + 3 }}

C. {{ add(5, 3) }}

D. {{ '5' + '3' }}

Solution

1. Step 1: Understand expression evaluation in Vue templates

   Vue evaluates JavaScript expressions inside {{ }} as normal JavaScript.

2. Step 2: Identify the correct math addition expression

   5 + 3 correctly adds numbers to get 8. {{ '5' + '3' }} concatenates strings, {{ add(5, 3) }} calls an undefined function, and {{ 5 & 3 }} uses bitwise AND.

3. Final Answer:

   <code>{{ 5 + 3 }}</code> -> Option B

4. Quick Check:

   Math addition uses + inside {{ }} [OK]

Hint: Use normal JavaScript math inside {{ }} [OK]

Common Mistakes:

• Using string quotes for numbers causing concatenation
• Calling undefined functions inside templates
• Using bitwise operators instead of arithmetic

3. Given the Vue data property count: 4, what will be displayed by this template snippet?

<div>{{ count * 2 }}</div>

medium

A. 8

B. count * 2

C. 4 * 2

D. undefined

Solution

1. Step 1: Evaluate the expression inside the template

   The expression count * 2 multiplies the value of count (which is 4) by 2.

2. Step 2: Calculate the result

   4 multiplied by 2 equals 8, so the template will display 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   4 * 2 = 8 [OK]

Hint: Multiply variables directly inside {{ }} [OK]

Common Mistakes:

• Expecting the expression to print as text
• Confusing variable name with string
• Assuming undefined if variable is not reactive

4. What is wrong with this Vue template expression?

<div>{{ user.name.toUpperCase }}</div>

Assuming user.name is a string.

medium

A. toUpperCase is not a valid method

B. Cannot use functions inside templates

C. user.name is undefined

D. Missing parentheses to call the function

Solution

1. Step 1: Identify the method usage error

   toUpperCase is a function and must be called with parentheses ().

2. Step 2: Correct the expression

   The correct expression is user.name.toUpperCase() to get the uppercase string.

3. Final Answer:

   Missing parentheses to call the function -> Option D

4. Quick Check:

   Call methods with () inside {{ }} [OK]

Hint: Always add () when calling functions in templates [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Trying to use unsupported functions
• Assuming properties auto-call functions

5. You want to display a list of numbers doubled inside a Vue template using numbers = [1, 2, 3]. Which expression correctly creates a comma-separated string of doubled numbers?

hard

A. {{ numbers.map(n => n * 2).join(', ') }}

B. {{ numbers * 2 }}

C. {{ numbers.forEach(n => n * 2) }}

D. {{ numbers.map(n => n * 2) }}

Solution

1. Step 1: Understand array transformation in templates

   To double each number, use map with a function n => n * 2.

2. Step 2: Convert the array to a string

   Using join(', ') converts the array to a comma-separated string for display.

3. Final Answer:

   <code>{{ numbers.map(n => n * 2).join(', ') }}</code> -> Option A

4. Quick Check:

   Use map + join for array display [OK]

Hint: Use map() then join() to show transformed arrays [OK]

Common Mistakes:

• Trying to multiply array directly
• Using forEach which returns undefined
• Not joining array to string for display