Optional parameters in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when using an optional parameter in a Svelte component?

Consider a Svelte component that accepts a prop with a default value (an optional parameter). What will be rendered if the prop is not passed?

Svelte

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

<h1>Hello, {name}!</h1>

A <h1>Hello, undefined!</h1>

B <h1>Hello, Guest!</h1>

C <h1>Hello, !</h1>

D Component throws an error

💡 Hint

Think about what default values do when no prop is passed.

When a prop has a default value, Svelte uses that value if the prop is not provided. Here, name defaults to "Guest".

📝 Syntax

intermediate

Which syntax correctly defines an optional parameter with a default value in a Svelte component?

Choose the correct way to define an optional prop count with a default value of 10 in Svelte.

A let count: number = 10;

B let count = 10; export count;

C export let count = 10;

D export count = 10;

💡 Hint

Remember how Svelte exports props with default values.

In Svelte, export let count = 10; declares a prop count with a default value of 10.

❓ state_output

advanced

What is the output when an optional parameter is updated dynamically?

Given this Svelte component, what will be displayed after clicking the button twice?

Svelte

<script>
  export let start = 5;
  let count = start;
  function increment() {
    count += 1;
  }
</script>

<p>Count: {count}</p>
<button on:click={increment}>Add</button>

A Count: 7

B Count: 5

C Count: 6

D Count: 0

💡 Hint

Count starts at start and increments by 1 each click.

Initial count is 5. Clicking twice adds 2, so count becomes 7.

🔧 Debug

advanced

Why does this Svelte component fail to use the optional parameter default?

Identify the reason why the default value for title is ignored and undefined is shown.

Svelte

<script>
  export let title;
  if (!title) {
    title = "Default Title";
  }
</script>

<h2>{title}</h2>

A Because reassigning an exported prop inside the script does not update the reactive value

B Because the default value must be set in the markup, not script

C Because Svelte does not allow conditional logic in script

D Because the prop must be declared with const, not let

💡 Hint

Think about how Svelte handles exported props and reactivity.

Reassigning an exported prop inside the script does not update the reactive value. The correct way is to assign a default value when exporting.

🧠 Conceptual

expert

How does Svelte handle optional parameters when a parent component passes undefined explicitly?

If a parent component passes undefined explicitly to an optional prop with a default value, what will the child component receive?

A The child receives <code>null</code> instead of <code>undefined</code>

B The child uses the default value instead of <code>undefined</code>

C Svelte throws a runtime error for undefined prop

D The child receives <code>undefined</code>, ignoring the default value

💡 Hint

Consider how JavaScript treats explicit undefined versus missing props.

If a parent passes undefined explicitly, the child receives undefined and does not use the default value. Defaults apply only when the prop is missing.

Practice

1. In Svelte, how do you make a component prop optional?

easy

A. Use export let propName = defaultValue;

B. Use let propName; without export

C. Use export const propName; without a value

D. Use function propName() {} inside the component

Solution

1. Step 1: Understand Svelte prop declaration

   Props are declared with export let to receive values from outside.

2. Step 2: Assign default value to make prop optional

   Assigning a default value like = defaultValue means the prop is optional and uses that value if none is passed.

3. Final Answer:

   Use export let propName = defaultValue; -> Option A

4. Quick Check:

   Optional prop = export let with default [OK]

Hint: Add = defaultValue after export let to make prop optional [OK]

Common Mistakes:

• Forgetting to export the prop
• Not assigning a default value
• Using const instead of let for props

2. Which of the following is the correct syntax to define an optional parameter in a JavaScript function inside a Svelte component?

easy

A. function greet(name: 'Guest') { console.log(name); }

B. function greet(name) { console.log(name || 'Guest'); }

C. function greet(name?) { console.log(name); }

D. function greet(name = 'Guest') { console.log(name); }

Solution

1. Step 1: Recall JavaScript default parameter syntax

   Default values are assigned using = value in the parameter list.

2. Step 2: Identify correct syntax

   function greet(name = 'Guest') { console.log(name); } uses name = 'Guest', which is valid and makes the parameter optional.

3. Final Answer:

   function greet(name = 'Guest') { console.log(name); } -> Option D

4. Quick Check:

   Default parameter = param = value [OK]

Hint: Use param = defaultValue in function signature for optional params [OK]

Common Mistakes:

• Using TypeScript syntax like name?
• Using colon instead of equals for default
• Using logical OR inside function body instead of default param

3. What will be the output of this Svelte component code?

 <script>
  export let greeting = 'Hello';
  export let name;
</script>

<p>{greeting}, {name || 'Friend'}!</p>

If used as <Component /> with no props passed?

medium

A. Hello, Friend!

B. Hello, undefined!

C. , Friend!

D. undefined, Friend!

Solution

1. Step 1: Check default values of props

   greeting has default 'Hello', name is undefined because no prop passed.

2. Step 2: Evaluate expression in template

   {name || 'Friend'} uses 'Friend' because name is falsy (undefined).

3. Final Answer:

   Hello, Friend! -> Option A

4. Quick Check:

   Default greeting + fallback name = Hello, Friend! [OK]

Hint: Use || to provide fallback for undefined props [OK]

Common Mistakes:

• Expecting undefined to print as string
• Forgetting default value for greeting
• Assuming name defaults to empty string

4. Identify the error in this Svelte component code that tries to make a prop optional:

<script>
  export let count;
  if (count === undefined) {
    count = 10;
  }
</script>

medium

A. The if condition should check for null instead of undefined

B. The default value must be assigned during export, not later

C. You cannot assign to an exported prop inside the script after declaration

D. There is no error; this code works fine

Solution

1. Step 1: Understand Svelte prop reactivity rules

   Exported props are reactive and should not be reassigned directly inside the script after declaration.

2. Step 2: Identify correct way to set default

   Default values must be assigned during export, like export let count = 10;, not by reassigning later.

3. Final Answer:

   You cannot assign to an exported prop inside the script after declaration -> Option C

4. Quick Check:

   Default props assigned during export only [OK]

Hint: Assign default value in export let, not by reassigning later [OK]

Common Mistakes:

• Trying to reassign exported props inside script
• Checking wrong condition for default
• Assuming reassignment updates prop externally

5. You want to create a Svelte component that accepts an optional size prop with default 'medium' and an optional color prop with default 'blue'. Which code correctly implements this and uses both props in the template?

hard

A. <script>
let size = 'medium';
let color = 'blue';
</script>

<div style="font-size: {size}; color: {color};">Styled Text</div>

B. <script>
export let size = 'medium';
export let color = 'blue';
</script>

<div style="font-size: {size}; color: {color};">Styled Text</div>

C. <script>
export const size = 'medium';
export const color = 'blue';
</script>

<div style="font-size: {size}; color: {color};">Styled Text</div>

D. <script>
export let size;
export let color;
size ||= 'medium';
color ||= 'blue';
</script>

<div style="font-size: {size}; color: {color};">Styled Text</div>

Solution

1. Step 1: Check how to declare optional props with defaults

   Props must be declared with export let and assigned default values directly.

2. Step 2: Verify usage in template

   Using {size} and {color} inside style attribute is correct.

3. Final Answer:

   <script> export let size = 'medium'; export let color = 'blue'; </script> <div style="font-size: {size}; color: {color};">Styled Text</div> -> Option B

4. Quick Check:

   export let with default + use in template = correct [OK]

Hint: Use export let with defaults and reference props in template [OK]

Common Mistakes:

• Not exporting props
• Using const instead of let for props
• Trying to assign defaults after declaration