Defining components in Vue

Introduction

Components let you build small, reusable parts of a webpage. They help organize your code and make it easier to manage.

You want to create a button that is used in many places with the same style and behavior.

You need to split a large webpage into smaller, manageable pieces.

You want to reuse a form input with validation in different parts of your app.

You want to separate the header, footer, and main content into different parts.

You want to share logic and layout between similar UI elements.

Syntax

Vue

<script setup>
// Define your component logic here
</script>

<template>
  <!-- Your HTML structure here -->
</template>

<style scoped>
/* Your component styles here */
</style>

Use <script setup> for a simple and clean way to write component logic in Vue 3.4+.

The scoped attribute in <style> makes styles apply only to this component.

Examples

A simple component showing a message using a variable.

Vue

<script setup>
const message = 'Hello from component!'
</script>

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

A button component that counts how many times it is clicked.

Vue

<script setup>
import { ref } from 'vue'
const count = ref(0)
function increment() {
  count.value++
}
</script>

<template>
  <button @click="increment">Clicked {{ count }} times</button>
</template>

A component that accepts a title prop to display a heading.

Vue

<script setup>
const props = defineProps({
  title: String
})
</script>

<template>
  <h1>{{ title }}</h1>
</template>

Sample Program

This component shows a button that counts how many times you click it. It uses ref to keep track of the count. The button has an accessible label and changes color when hovered or focused. The styles are scoped so they only affect this component.

Vue

<script setup>
import { ref } from 'vue'
const count = ref(0)
function increment() {
  count.value++
}
</script>

<template>
  <section>
    <h2>Counter Component</h2>
    <button @click="increment" aria-label="Increment counter">
      Clicked {{ count }} times
    </button>
  </section>
</template>

<style scoped>
section {
  padding: 1rem;
  border: 2px solid #4a90e2;
  border-radius: 0.5rem;
  max-width: 200px;
  text-align: center;
  font-family: Arial, sans-serif;
}
button {
  background-color: #4a90e2;
  color: white;
  border: none;
  padding: 0.5rem 1rem;
  font-size: 1rem;
  border-radius: 0.25rem;
  cursor: pointer;
}
button:hover, button:focus {
  background-color: #357ABD;
  outline: none;
}
</style>

Important Notes

Always use ref or reactive to make data reactive in Vue components.

Use @click to listen for user clicks on buttons or other elements.

Scoped styles help keep your component styles from affecting other parts of the page.

Summary

Components are reusable building blocks for your Vue app.

Use <script setup> and <template> to define component logic and layout.

Props let you pass data into components, and reactive variables track changing data.

Practice

1. What is the main purpose of defining components in Vue?

easy

A. To handle server-side rendering only

B. To create reusable pieces of UI that can be used throughout the app

C. To connect the app to a database

D. To write CSS styles for the app

Solution

1. Step 1: Understand what components do in Vue

   Components are small, reusable parts of the user interface that can be used multiple times in an app.

2. Step 2: Identify the correct purpose

   Among the options, only creating reusable UI pieces matches the role of components.

3. Final Answer:

   To create reusable pieces of UI that can be used throughout the app -> Option B

4. Quick Check:

   Components = reusable UI blocks [OK]

Hint: Components build reusable UI parts in Vue apps [OK]

Common Mistakes:

• Confusing components with styling or backend logic
• Thinking components handle database connections
• Believing components only work on the server side

2. Which of the following is the correct way to start a Vue component using the Composition API with <script setup>?

easy

A.

{{ message }}

B.

{{ message }}

C.
{{ message }}

D.

{{ message }}

Solution

1. Step 1: Recognize the <script setup> syntax

   In Vue 3, <script setup> is a special syntax that allows direct use of variables without export default.

2. Step 2: Check which option uses <script setup> correctly

   <script setup> const message = 'Hello' </script> <template> <p>{{ message }}</p> </template> declares a variable inside <script setup> and uses it in the template, which is correct.

3. Final Answer:

   <script setup> const message = 'Hello' </script> <template> <p>{{ message }}</p> </template> -> Option A

4. Quick Check:

   <script setup> uses direct variable declaration [OK]

Hint: Use

Common Mistakes:

• Using export default inside
• Declaring variables outside
• Placing <script> after <template> incorrectly

3. Given this Vue component code, what will be displayed when rendered?

<script setup>
import { ref } from 'vue'
const count = ref(0)
function increment() {
  count.value++
}
</script>

<template>
  <button @click="increment">Clicked {{ count }} times</button>
</template>

medium

A. A button showing 'Clicked 0 times' initially, increasing count on each click

B. A button showing 'Clicked 1 times' initially, increasing count on each click

C. A button showing 'Clicked count times' as a literal string

D. An error because count is not reactive

Solution

1. Step 1: Understand ref and reactivity

   The variable count is a reactive reference starting at 0. Using count.value++ updates it reactively.

2. Step 2: Check the template interpolation

   The template uses {{ count }}, which Vue unwraps automatically to show the current value of count.

3. Final Answer:

   A button showing 'Clicked 0 times' initially, increasing count on each click -> Option A

4. Quick Check:

   ref count starts at 0 and updates on click [OK]

Hint: ref variables start at initial value and update reactively [OK]

Common Mistakes:

• Thinking count starts at 1 instead of 0
• Expecting literal 'count' string instead of value
• Assuming count is not reactive without .value

4. Identify the error in this Vue component code:

<script setup>
const props = defineProps({
  title: String
})

function showTitle() {
  console.log(props.title.toUpperCase())
}

showTitle()
</script>

<template>
  <h1>{{ title }}</h1>
</template>

medium

A. Missing import of defineProps

B. Calling showTitle() before props are available

C. Using title in template instead of props.title

D. Using toUpperCase() on a possibly undefined prop

Solution

1. Step 1: Analyze prop definition and usage

   defineProps({ title: String }) declares title without required: true or default value, so props.title can be undefined.

2. Step 2: Locate the error in JS code

   In showTitle(), props.title.toUpperCase() assumes title is a string, but calling toUpperCase() on undefined causes a runtime error.

3. Final Answer:

   Using toUpperCase() on a possibly undefined prop -> Option D

4. Quick Check:

   Validate props before string operations [OK]

Hint: Guard undefined props before methods like toUpperCase [OK]

Common Mistakes:

• Applying string methods to props that may be undefined
• Forgetting to import defineProps (though auto-imported in some setups)
• Assuming props are reactive variables directly

5. You want to create a Vue component that accepts a user prop object with name and age, and displays "Name: [name], Age: [age]". Which is the best way to define and use this prop in a <script setup> component?

hard

A.

Name: {{ user.name }}, Age: {{ user.age }}

B.

Name: {{ props.name }}, Age: {{ props.age }}

C.

Name: {{ props.user.name }}, Age: {{ props.user.age }}

D.

Name: {{ user.name }}, Age: {{ user.age }}

Solution

1. Step 1: Define the prop as an object with expected properties

   The prop user is an object containing name and age, so define it as an Object type and mark required if needed.

2. Step 2: Access the nested properties correctly in the template

   Use props.user.name and props.user.age to display the values inside the template.

3. Final Answer:

   Define user as an object prop and access nested properties via props.user.name and props.user.age -> Option C

4. Quick Check:

   Object prop with nested access = <script setup> const props = defineProps({ user: { type: Object, required: true } }) </script> <template> <p>Name: {{ props.user.name }}, Age: {{ props.user.age }}</p> </template> [OK]

Hint: Define object prop fully and access nested fields via props [OK]

Common Mistakes:

• Defining nested fields as separate props instead of one object
• Destructuring props incorrectly in
• Using defineProps with wrong syntax for objects