Default prop values in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the rendered output with default prop values?

Consider this Svelte component that uses a default prop value:

  <script>
    export let name = 'Guest';
  </script>

  <p>Hello, {name}!</p>

What will be the output if this component is used without passing any prop?

Svelte

  <script>
    export let name = 'Guest';
  </script>

  <p>Hello, {name}!</p>

A Hello, undefined!

B Hello, !

C Hello, null!

D Hello, Guest!

💡 Hint

Think about what happens when no value is passed for a prop with a default.

Since name has a default value of 'Guest', it is used when no prop is passed.

📝 Syntax

intermediate

Which option correctly sets a default prop value in Svelte?

Which of the following code snippets correctly sets a default value for a prop count in a Svelte component?

A <script>let count = 10;</script>

B <script>export let count = 10;</script>

C <script>export count = 10;</script>

D <script>export let count;</script><script>count = 10;</script>

💡 Hint

Remember how to export props with default values in Svelte.

Only option B correctly exports a prop with a default value. Option B has invalid syntax with multiple script tags and does not set a default, B does not export, C has invalid syntax.

❓ state_output

advanced

What is the output when a prop is passed as undefined?

Given this Svelte component:

<script>
  export let title = 'Default Title';
</script>

<h1>{title}</h1>

What will be rendered if the component is used like this?

<Component title={undefined} />

Svelte

<script>
  export let title = 'Default Title';
</script>

<h1>{title}</h1>

A <h1>undefined</h1>

B <h1>Default Title</h1>

C <h1></h1>

D <h1>null</h1>

💡 Hint

Think about how Svelte treats explicitly passed undefined props versus missing props.

Passing undefined explicitly overrides the default value, so title becomes undefined and renders as the string 'undefined'.

🔧 Debug

advanced

Why does this default prop value not work as expected?

Look at this Svelte component:

<script>
  export let items = [];
</script>

<ul>
  {#each items as item}
    <li>{item}</li>
  {/each}
</ul>

When used without passing items, the list is empty. But when used multiple times, all instances share the same items array. Why?

Svelte

<script>
  export let items = [];
</script>

<ul>
  {#each items as item}
    <li>{item}</li>
  {/each}
</ul>

A Because the items prop must be initialized inside onMount.

B Because Svelte does not allow arrays as default props.

C Because the default array is shared across all instances, causing shared state.

D Because the export keyword is missing.

💡 Hint

Think about how default values for objects or arrays behave in JavaScript.

Default prop values that are objects or arrays are shared references, so all component instances share the same array, causing unexpected shared state.

🧠 Conceptual

expert

How does Svelte handle default prop values during hydration?

When a Svelte component with default prop values is server-rendered and then hydrated on the client, how are the default prop values treated if the client does not pass those props?

A The default prop values are used during hydration if the client does not pass props.

B The default prop values are ignored during hydration; only server-rendered values are used.

C Hydration fails if default prop values are not explicitly passed by the client.

D The component re-renders with empty props, ignoring defaults.

💡 Hint

Consider how Svelte merges server and client props during hydration.

Svelte uses default prop values on the client during hydration if no props are passed, ensuring consistent behavior between server and client.

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