Typing component props in Vue

Introduction

Typing component props helps you check that the right kind of data is passed to your Vue components. This makes your app more reliable and easier to understand.

When you want to make sure a component only receives specific types of data, like a number or string.

When you want to catch mistakes early by checking props during development.

When you want to document what kind of data a component expects for better teamwork.

When you want to provide default values and validations for props.

When you want to improve code readability and maintainability.

Syntax

Vue

defineProps<{ propName: Type }>()

Use defineProps with a TypeScript type to specify prop types in <script setup>.

This syntax works only in Vue 3 with the Composition API and <script setup>.

Examples

This example types a prop named title as a string.

Vue

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

This example types an optional prop count as a number.

Vue

<script setup lang="ts">
const props = defineProps<{ count?: number }>()
</script>

This example types two props: isActive as boolean and label as string.

Vue

<script setup lang="ts">
const props = defineProps<{ isActive: boolean; label: string }>()
</script>

Sample Program

This component expects a required title prop of type string and an optional count prop of type number. It displays them in the template.

Vue

<template>
  <h1>{{ title }}</h1>
  <p>Count: {{ count }}</p>
</template>

<script setup lang="ts">
const props = defineProps<{ title: string; count?: number }>()
</script>

Important Notes

If you pass a prop with the wrong type, TypeScript will show an error during development.

Optional props can be marked with ? in the type.

Always use lang="ts" in <script setup> to enable TypeScript.

Summary

Typing props helps catch errors early and makes your code clearer.

Use defineProps<Type>() inside <script setup lang="ts"> to type props.

Mark props optional with ? if they are not required.

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?