Typing component props in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a prop named title of type String.

Vue

<script setup lang="ts">
defineProps<{ title: [1] }>()
</script>

Drag options to blanks, or click blank then click option'

Anumber

Bstring

Cboolean

DArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'String' constructor instead of 'string' primitive.

Confusing prop type with default value.

The title prop is typed as string to accept string values.

2fill in blank

medium

Complete the code to type a prop named count as a Number.

Vue

<script setup lang="ts">
const props = defineProps<{ count: [1] }>()
</script>

Drag options to blanks, or click blank then click option'

Aboolean

Bstring

CObject

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Number' constructor instead of 'number'.

Typing the prop as string by mistake.

The count prop is typed as number to accept numeric values.

3fill in blank

hard

Fix the error in typing the isActive prop as a boolean.

Vue

<script setup lang="ts">
const props = defineProps<{ isActive: [1] }>()
</script>

Drag options to blanks, or click blank then click option'

Aboolean

BBoolean

Cbool

DBool

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Boolean' constructor instead of 'boolean'.

Using invalid types like 'bool' or 'Bool'.

Use the primitive type boolean to correctly type boolean props. The constructor Boolean types as a wrapper object.

4fill in blank

hard

Fill both blanks to type props name as String and age as Number.

Vue

<script setup lang="ts">
const props = defineProps<{ name: [1], age: [2] }>()
</script>

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the types for name and age.

Using uppercase constructors instead of lowercase primitives.

The name prop is typed as string and age as number to accept text and numeric values respectively.

5fill in blank

hard

Fill all three blanks to type props title as String, visible as Boolean, and items as Array.

Vue

<script setup lang="ts">
const props = defineProps<{ title: [1], visible: [2], items: [3] }>()
</script>

Drag options to blanks, or click blank then click option'

Anumber

Bboolean

CArray

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase constructors instead of lowercase primitives for basic types.

Confusing Array with Object.

The title prop is typed as string, visible as boolean, and items as Array to accept text, true/false, and list values respectively.

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?