Prop types and validation in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Vue component receives a prop with the wrong type?

Consider a Vue 3 component that declares a prop with type Number. What will happen if the parent passes a String instead?

Vue

<script setup>
const props = defineProps({
  age: Number
})
</script>
<template>
  <p>Age: {{ age }}</p>
</template>

A Vue will automatically convert the string to a number before rendering.

B Vue will show a console warning about the type mismatch but still render the component with the passed string value.

C Vue will throw a runtime error and stop rendering the component.

D Vue will ignore the prop and use a default value of 0.

💡 Hint

Vue's prop validation warns but does not block rendering.

Vue checks prop types during development and logs warnings in the console if types don't match. It does not stop rendering or convert types automatically.

📝 Syntax

intermediate

Which prop declaration correctly validates a required string with a custom validator?

You want a prop named username that must be a string, is required, and must be at least 3 characters long. Which option is correct?

A username: { type: String, required: true, validator: val => val > 3 }

B username: { type: String, required: true, validate: val => val.length >= 3 }

C username: { type: String, required: true, validator: val => val.length >= 3 }

D username: { type: String, required: true, validator: val => val.length > 3 }

💡 Hint

The validator function receives the prop value and returns true or false.

The correct key is validator. The function checks val.length >= 3 to allow strings with 3 or more characters. Option C excludes length 3, which is not correct.

🔧 Debug

advanced

Why does this prop validation not warn when passing a number?

This Vue component declares a prop with type [String, Number]. But when passing a boolean, no warning appears. Why?

Vue

<script setup>
const props = defineProps({
  value: {
    type: [String, Number],
    required: true
  }
})
</script>

A Vue does not warn because the prop type array allows multiple types, but boolean is not one of them, so it should warn but the warning is suppressed in production mode.

B Vue does not warn because the prop is required, so any value is accepted without warning.

C Vue only warns for types declared in the array; boolean is not included, so it should warn but doesn't due to a bug.

D Vue does not warn because the prop type array only checks the first type, ignoring others.

💡 Hint

Vue warnings only appear in development mode, not production.

Vue warns about type mismatches only in development mode. If running in production, warnings are suppressed. The prop type array does not include boolean, so passing boolean should warn in development.

❓ state_output

advanced

What is the value of props.count after mounting?

Given this Vue component, what will props.count be if the parent passes no value?

Vue

<script setup>
const props = defineProps({
  count: {
    type: Number,
    default: 10
  }
})
</script>
<template>
  <p>{{ count }}</p>
</template>

A undefined

B 0

C null

D 10

💡 Hint

Default values are used when no prop is passed.

If the parent does not pass a value, Vue uses the default value specified in the prop declaration.

🧠 Conceptual

expert

Why use defineProps with TypeScript types instead of runtime validators in Vue 3?

In Vue 3 with TypeScript, what is the main advantage of using defineProps<Type>() over runtime prop validation objects?

A TypeScript types provide compile-time checking and better editor support without runtime overhead.

B Runtime validators are faster and more reliable than TypeScript types.

C Using <code>defineProps&lt;Type&gt;()</code> disables all runtime prop validation.

D TypeScript types automatically convert prop values to the correct type at runtime.

💡 Hint

Think about when TypeScript checks types versus when Vue validates props.

TypeScript types help catch errors before running the app and improve developer experience. Runtime validators check types during execution but add overhead.

Practice

1. What is the main purpose of defining props with types in a Vue component?

easy

A. To ensure the component receives data in the expected format

B. To style the component automatically

C. To make the component load faster

D. To prevent the component from rendering

Solution

1. Step 1: Understand what props do in Vue

   Props allow passing data from a parent to a child component.

2. Step 2: Recognize the role of prop types

   Defining types helps Vue check if the data matches the expected format, catching errors early.

3. Final Answer:

   To ensure the component receives data in the expected format -> Option A

4. Quick Check:

   Prop types = data format check [OK]

Hint: Prop types check data format passed to components [OK]

Common Mistakes:

• Thinking prop types affect styling
• Believing prop types improve speed
• Assuming prop types stop rendering

2. Which of the following is the correct syntax to define a required string prop named title in a Vue component?

easy

A. props: { title: String, required: true }

B. props: { title: { type: String, required: true } }

C. props: { title: { required: true type: String } }

D. props: { title: { String, required: true } }

Solution

1. Step 1: Recall Vue prop syntax for validation

   Props with validation use an object with keys like type and required.

2. Step 2: Check the correct order and structure

   The correct syntax is { type: String, required: true } inside the prop definition.

3. Final Answer:

   props: { title: { type: String, required: true } } -> Option B

4. Quick Check:

   Prop validation uses object with type and required [OK]

Hint: Use object with type and required keys for prop validation [OK]

Common Mistakes:

• Placing required outside the prop object
• Using array instead of object for validation
• Wrong order or missing keys in prop definition

3. Given this Vue component prop definition:

props: { count: { type: Number, default: 0 } }

What will be the value of count inside the component if the parent does NOT pass any count prop?

medium

A. 0

B. null

C. undefined

D. NaN

Solution

1. Step 1: Understand default prop values

   If a prop is not passed, Vue uses the default value if defined.

2. Step 2: Check the default value for count

   The default is set to 0, so count will be 0 inside the component.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Missing prop uses default value [OK]

Hint: Default prop value used if parent omits prop [OK]

Common Mistakes:

• Assuming undefined if prop is missing
• Confusing null with default value
• Expecting error when prop is missing

4. Consider this prop validation in a Vue component:

props: { age: { type: Number, required: true } }

What will happen if the parent passes age="twenty" (a string) instead of a number?

medium

A. Vue will automatically convert the string to a number

B. Vue will throw a runtime error and stop rendering

C. Vue will show a console warning about the type mismatch

D. Vue will ignore the prop and use the default value

Solution

1. Step 1: Understand Vue's prop type checking behavior

   Vue checks prop types and warns in the console if types don't match but does not stop rendering.

2. Step 2: Check what happens with wrong type

   Passing a string instead of a number triggers a console warning but no error or conversion.

3. Final Answer:

   Vue will show a console warning about the type mismatch -> Option C

4. Quick Check:

   Wrong prop type triggers warning, not error [OK]

Hint: Vue warns on wrong prop types but keeps rendering [OK]

Common Mistakes:

• Expecting runtime errors instead of warnings
• Thinking Vue auto-converts prop types
• Assuming default value is used on type mismatch

5. You want to validate a prop status that only accepts the strings 'active', 'inactive', or 'pending'. Which is the correct way to add this validation in Vue?

hard

A. props: { status: { type: String, validate: value => ['active', 'inactive', 'pending'].includes(value) } }"

B. props: { status: { type: String, required: true, default: 'active' } }

C. props: { status: { type: String, enum: ['active', 'inactive', 'pending'] } }

D. props: { status: { type: String, validator: value => ['active', 'inactive', 'pending'].includes(value) } }

Solution

1. Step 1: Recall how to restrict prop values in Vue

   Vue uses a validator function to check if a prop value is allowed.

2. Step 2: Check the correct validator syntax

   The validator is a function returning true if the value is valid, like value => ['active', 'inactive', 'pending'].includes(value).

3. Final Answer:

   props: { status: { type: String, validator: value => ['active', 'inactive', 'pending'].includes(value) } } -> Option D

4. Quick Check:

   Use validator function to restrict prop values [OK]

Hint: Use validator function to limit allowed prop values [OK]

Common Mistakes:

• Using non-existent 'enum' key for props
• Confusing 'validate' with 'validator'
• Relying only on default or required without value check