Typing component props in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of typing component props in Vue?

Typing component props helps catch errors early by ensuring the data passed to components matches expected types. It improves code clarity and developer experience.

beginner

How do you define a prop with a specific type in Vue 3 using the Composition API?

You define props with types by using <code>defineProps&lt;Type&gt;()</code> inside the <code>&lt;script setup&gt;</code> block. For example: <code>const props = defineProps&lt;{ title: string }&gt;()</code>.

intermediate

What happens if a prop is passed with the wrong type in a typed Vue component?

If using TypeScript, the editor or build tool will show an error before running the app. At runtime, Vue may warn in the console if the prop type does not match the declared type.

intermediate

How can you make a prop optional with a default value in Vue 3 with TypeScript?

In <code>defineProps</code>, you can mark a prop as optional by adding <code>?</code> in the type and provide a default value using <code>withDefaults</code>. Example: <code>const props = withDefaults(defineProps&lt;{ count?: number }&gt;(), { count: 0 })</code>.

beginner

Why is typing props beneficial when working in larger Vue projects?

Typing props helps teams avoid bugs, improves code readability, and enables better tooling support like autocomplete and refactoring. It makes components easier to use and maintain.

Which Vue 3 feature allows you to type props directly in the <script setup> block?

A defineProps

B useProps

C propsType

D setProps

The defineProps function is used in Vue 3's <script setup> to declare and type component props.

How do you specify that a prop is optional with a default value in Vue 3 with TypeScript?

A Add <code>defaultValue</code> in the template

B Use <code>withDefaults</code> with <code>defineProps</code>

C Use <code>optionalProps</code> function

D Set <code>required: false</code> in the options object

Vue 3 uses withDefaults alongside defineProps to provide default values for optional props.

What type of error will you get if you pass a wrong type to a typed prop in Vue with TypeScript?

A No error, silently ignored

B Runtime crash

C Compile-time error

D Syntax error

TypeScript catches type mismatches at compile time, helping prevent runtime issues.

Which of these is NOT a benefit of typing props in Vue?

A Early error detection

B Improved code readability

C Better tooling support

D Automatic UI styling

Typing props does not affect UI styling; it helps with code quality and error checking.

In Vue 3, where do you typically write the prop type definitions when using the Composition API?

A Inside the <code>&lt;script setup&gt;</code> block

B In the template section

C In a separate CSS file

D In the main.js file

Prop types are declared inside the <script setup> block using defineProps.

Explain how to type component props in Vue 3 using the Composition API and how to provide default values.

Think about how you declare props inside <code>&lt;script setup&gt;</code> and how defaults are added.

Describe the benefits of typing props in Vue components for a team project.

Consider how typing helps both developers and the project quality.

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?