Default prop values in Svelte - Mini Project: Build & Apply

Default prop values in Svelte

📖 Scenario: You are building a simple Svelte component to display a user's profile card. Sometimes the user might not provide all the information, so you want to set default values for some props.

🎯 Goal: Create a Svelte component called UserProfile.svelte that accepts props for name, age, and location. Set default values for age and location so the component still shows meaningful info if those props are not passed.

📋 What You'll Learn

Create a name prop without a default value

Create age and location props with default values

Render the props inside the component

Use Svelte syntax for default prop values

💡 Why This Matters

🌍 Real World

Setting default prop values is common in UI components to ensure they display useful information even when some data is missing.

💼 Career

Knowing how to handle default props is essential for frontend developers working with Svelte or similar frameworks to build robust, reusable components.

1

Create the name prop

In UserProfile.svelte, create a name prop using export let name; without assigning a default value.

Svelte

// Your code here


  // Create the name prop here

Hint

Use export let name; to declare a prop without a default value.

2

Add default values for age and location props

Add two more props in the <script> block: export let age = 30; and export let location = 'Unknown'; to set default values.

Svelte

  export let name;
  // Add age and location props with default values here

Hint

Assign default values directly when declaring the props.

3

Render the props in the component

Below the <script> block, add HTML to display the name, age, and location props inside <p> tags.

Svelte

  export let name;
  export let age = 30;
  export let location = 'Unknown';

Hint

Use curly braces {} to insert prop values inside HTML.

4

Complete the component with default prop values

Ensure the full component code includes the <script> block with all three props and the HTML rendering the props as paragraphs.

Svelte

  export let name;
  export let age = 30;
  export let location = 'Unknown';


Name: {name}
Age: {age}
Location: {location}

Hint

Check that all props and HTML are present exactly as specified.

Practice

1. What is the correct way to set a default value for a prop named color in a Svelte component?

easy

A. export let color = 'blue';

B. let color = 'blue';

C. prop color = 'blue';

D. export color = 'blue';

Solution

1. Step 1: Understand Svelte prop declaration

   In Svelte, props are declared with export let to make them accessible from outside.

2. Step 2: Assign default value

   Assigning = 'blue' after export let color sets the default value if no prop is passed.

3. Final Answer:

   export let color = 'blue'; -> Option A

4. Quick Check:

   Default prop syntax = export let prop = value [OK]

Hint: Use 'export let' with '=' to set default prop values [OK]

Common Mistakes:

• Forgetting 'export' keyword
• Using 'let' without export
• Using 'prop' keyword which doesn't exist
• Assigning default without 'let'

2. Which of the following is the correct syntax to declare a prop size with a default value of 10 in Svelte?

easy

A. export let size: 10;

B. export size = 10;

C. let export size = 10;

D. export let size = 10;

Solution

1. Step 1: Recognize correct export syntax

   The correct way to declare a prop with default is export let prop = value;.

2. Step 2: Identify correct option

   export let size = 10; matches this syntax exactly with size and default 10.

3. Final Answer:

   export let size = 10; -> Option D

4. Quick Check:

   Default prop syntax uses '=' not ':' or misplaced keywords [OK]

Hint: Remember: 'export let prop = value;' sets default props [OK]

Common Mistakes:

• Using ':' instead of '='
• Misplacing 'export' keyword
• Omitting 'let' keyword
• Assigning default without 'let'

3. Given this Svelte component code:

  <script>
    export let message = 'Hello';
  </script>

  <p>{message}</p>

What will be displayed if the component is used without passing any message prop?

medium

A. An empty paragraph

B. Hello

C. Undefined

D. Error: message not defined

Solution

1. Step 1: Check default prop value

   The prop message has a default value of 'Hello'.

2. Step 2: Understand usage without prop

   If no prop is passed, Svelte uses the default value automatically.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Default prop used when no value passed = Hello [OK]

Hint: No prop passed? Default value shows [OK]

Common Mistakes:

• Assuming empty string instead of default
• Expecting undefined or error
• Confusing default with required prop

4. Identify the error in this Svelte component code:

<script>
  export let count;
  if (count === undefined) {
    count = 5;
  }
</script>

<p>Count is {count}</p>

medium

A. The count prop must be a string, not a number.

B. The if condition should check for null, not undefined.

C. You cannot assign to a prop after export; default should be set in declaration.

D. The count variable should not be exported.

Solution

1. Step 1: Understand prop default assignment rules

   In Svelte, default prop values must be set during declaration, not by reassigning later.

2. Step 2: Identify why reassignment is wrong

   Reassigning an exported prop inside script causes runtime errors or unexpected behavior.

3. Final Answer:

   You cannot assign to a prop after export; default should be set in declaration. -> Option C

4. Quick Check:

   Default props set at declaration, not by reassignment [OK]

Hint: Set default with export let, not by reassigning later [OK]

Common Mistakes:

• Trying to assign default after export
• Checking wrong condition for default
• Confusing prop type requirements
• Not exporting the prop

5. You have a Svelte component that accepts a prop items which should default to an empty array if not provided. Which is the best way to declare this prop to avoid errors when iterating over items in the template?

hard

A. export let items = [];

B. export let items; if (!items) items = [];

C. let items = []; export { items };

D. export let items = null;

Solution

1. Step 1: Understand default prop for arrays

   Setting export let items = []; ensures items is always an array, avoiding errors in iteration.

2. Step 2: Evaluate other options

   export let items; if (!items) items = []; tries to assign after export which is incorrect. let items = []; export { items }; exports a local variable incorrectly. export let items = null; sets null which can cause errors when iterating.

3. Final Answer:

   export let items = []; -> Option A

4. Quick Check:

   Default array props set with export let prop = [] [OK]

Hint: Default arrays with export let prop = [] to avoid iteration errors [OK]

Common Mistakes:

• Assigning default after export
• Exporting local variables incorrectly
• Using null as default for arrays
• Not setting default causing runtime errors