JavaScript expressions in templates in Vue

Introduction

JavaScript expressions in templates let you show dynamic content easily. They help your webpage change based on data or user actions.

Showing a user's name or age that changes over time

Calculating and displaying a total price from item quantities

Changing text style or content based on a condition

Displaying the current date or time in a readable format

Syntax

Vue

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

Use double curly braces {{ }} to insert JavaScript expressions inside templates.

Expressions can be simple variables, math operations, or function calls.

Examples

Displays the value of the message variable.

Vue

<template>
  <p>{{ message }}</p>
</template>

Shows the value of count plus one.

Vue

<template>
  <p>{{ count + 1 }}</p>
</template>

Shows 'Active' if isActive is true, otherwise 'Inactive'.

Vue

<template>
  <p>{{ isActive ? 'Active' : 'Inactive' }}</p>
</template>

Calls the greet method with name and shows the result.

Vue

<template>
  <p>{{ greet(name) }}</p>
</template>

Sample Program

This Vue component uses JavaScript expressions inside the template to show a greeting, add numbers, and display a message based on a condition.

Vue

<script setup>
import { ref } from 'vue'

const name = ref('Alice')
const count = ref(3)

function greet(person) {
  return `Hello, ${person}!`
}
</script>

<template>
  <div>
    <p>{{ greet(name) }}</p>
    <p>You have {{ count + 2 }} new messages.</p>
    <p>Status: {{ count > 0 ? 'You have messages' : 'No messages' }}</p>
  </div>
</template>

Important Notes

Expressions inside {{ }} should be simple and fast to avoid slowing down rendering.

Avoid complex logic in templates; use methods or computed properties instead.

Summary

Use {{ }} to insert JavaScript expressions in Vue templates.

Expressions can show variables, do math, or run simple functions.

Keep expressions simple for better performance and readability.

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