Typing component props in Vue - Mini Project: Build & Apply

Typing component props

📖 Scenario: You are building a simple Vue 3 component that shows a user's profile card. You want to make sure the component receives the correct types of data as props.

🎯 Goal: Create a Vue 3 component using the Composition API with <script setup> that accepts typed props for name (string), age (number), and isMember (boolean). Display these props in the template.

📋 What You'll Learn

Create a Vue 3 component with <script setup> syntax

Define props with explicit types: name as string, age as number, isMember as boolean

Use the props in the template to show the user's name, age, and membership status

Use the Composition API style for props typing

💡 Why This Matters

🌍 Real World

Typing props helps catch errors early and makes components easier to use and maintain in real projects.

💼 Career

Understanding how to type props is essential for Vue developers to build reliable and scalable applications.

1

Setup the component and define props object

Create a Vue 3 component with <script setup> and define a props object with keys name, age, and isMember without types yet.

Vue

// Define props object with keys name, age, isMember
// Your code here



  

Hint

Start by creating a props object with keys name, age, and isMember. You don't need to add types yet.

2

Add types to the props using defineProps

Replace the props object with defineProps and specify types: name as String, age as Number, and isMember as Boolean.

Vue

// Use defineProps with types for name, age, isMember
// Your code here



  

Hint

Use defineProps to declare the props with their types.

3

Display the props in the template

In the <template>, display the user's name, age, and isMember status inside paragraphs.

Vue

const props = defineProps({
  name: String,
  age: Number,
  isMember: Boolean
})



  
  

Hint

Use mustache syntax {{ }} to show each prop inside a paragraph.

4

Add default values for props

Modify the defineProps call to add default values: name default to "Guest", age default to 0, and isMember default to false.

Vue

// Add default values to props
const props = defineProps({
  name: String,
  age: Number,
  isMember: Boolean
})



  Name: {{ name }}
  Age: {{ age }}
  Member: {{ isMember }}

Hint

Use the object form for each prop to add a default value.

Practice

1. What is the main benefit of typing component props in Vue with TypeScript?

easy

A. It disables prop validation.

B. It automatically styles the component.

C. It makes the component load faster.

D. It helps catch errors early and improves code clarity.

Solution

1. Step 1: Understand the purpose of typing props

   Typing props allows the developer to specify what kind of data the component expects, which helps catch mistakes before running the code.

2. Step 2: Recognize the benefits of typing

   Typed props improve code clarity and reduce bugs by making expectations explicit.

3. Final Answer:

   It helps catch errors early and improves code clarity. -> Option D

4. Quick Check:

   Typing props = catch errors early [OK]

Hint: Typing props means fewer bugs and clearer code [OK]

Common Mistakes:

• Thinking typing changes component speed
• Confusing typing with styling
• Believing typing disables validation

2. Which is the correct syntax to type props in a Vue 3 <script setup lang="ts"> block?

easy

A. const props = defineProps('title: string')

B. const props = defineProps(["title: string"])

C. const props = defineProps<{ title: string }>()

D. const props = defineProps(title: string)

Solution

1. Step 1: Recall the correct typing syntax

   In Vue 3 with TypeScript, props are typed using defineProps<Type>() where Type is an object type.

2. Step 2: Identify the correct syntax

   const props = defineProps<{ title: string }>() uses the correct generic syntax with an object type describing the prop.

3. Final Answer:

   const props = defineProps<{ title: string }>() -> Option C

4. Quick Check:

   defineProps<Type>() syntax = const props = defineProps<{ title: string }>() [OK]

Hint: Use defineProps with angle brackets and object type [OK]

Common Mistakes:

• Passing a string instead of a type
• Using parentheses instead of angle brackets
• Not wrapping props in an object type

3. Given this Vue component code inside <script setup lang="ts">:

const props = defineProps<{ count?: number }>()

console.log(props.count ?? 0)

What will be logged if the component is used without passing count?

medium

A. 0

B. NaN

C. undefined

D. Error at runtime

Solution

1. Step 1: Understand the optional prop typing

   The prop count is optional (marked with ?), so it may be undefined if not passed.

2. Step 2: Evaluate the expression props.count ?? 0

   The nullish coalescing operator ?? returns the right side (0) if props.count is null or undefined.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Optional prop missing uses default 0 [OK]

Hint: Optional props may be undefined; use ?? for defaults [OK]

Common Mistakes:

• Expecting undefined instead of default
• Confusing ?? with || operator
• Assuming runtime error for missing optional prop

4. Identify the error in this Vue 3 TypeScript prop typing:

<script setup lang="ts">
const props = defineProps<{ name: string | undefined }>()
</script>

What is the problem?

medium

A. Props should not use union with undefined; use optional instead.

B. Missing default value for name prop.

C. defineProps must be called without type arguments.

D. The prop name must be a number, not string.

Solution

1. Step 1: Understand how to mark optional props

   In Vue with TypeScript, optional props should be marked with ? instead of union with undefined.

2. Step 2: Identify the incorrect typing

   Using string | undefined is allowed but not recommended; better to write name?: string for optional props.

3. Final Answer:

   Props should not use union with undefined; use optional instead. -> Option A

4. Quick Check:

   Use ? for optional props, not union with undefined [OK]

Hint: Use ? to mark optional props, not union with undefined [OK]

Common Mistakes:

• Using union with undefined instead of ?
• Forgetting to mark optional props
• Confusing default values with typing

5. You want to type a Vue component prop named items that accepts an array of objects with a required id (number) and optional label (string). Which is the correct TypeScript type for defineProps?

hard

A. defineProps<{ items?: Array<{ id: number; label: string }> }>()

B. defineProps<{ items: { id: number; label?: string }[] }>()

C. defineProps<{ items: { id: number; label: string | undefined } }>()

D. defineProps<{ items: Array<{ id: number; label: string }> }>()

Solution

1. Step 1: Understand the prop structure

   The prop items is an array of objects. Each object must have a required id (number) and an optional label (string).

2. Step 2: Choose the correct typing

   defineProps<{ items: { id: number; label?: string }[] }>() correctly types items as an array of objects with id: number and optional label?: string. The prop itself is required.

3. Final Answer:

   defineProps<{ items: { id: number; label?: string }[] }>() -> Option B

4. Quick Check:

   Array of objects with optional label = defineProps<{ items: { id: number; label?: string }[] }>() [OK]

Hint: Use ? inside object for optional fields in array props [OK]

Common Mistakes:

• Marking label as required when optional
• Making items optional when it should be required
• Using label: string | undefined instead of label?