Declaring props with export let in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Svelte component display?

Consider this Svelte component code:

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

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

If you use this component without passing any prop, what will be shown on the page?

Svelte

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

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

A Hello name!

B Hello !

C Hello undefined!

D Hello Guest!

💡 Hint

Think about the default value assigned to the prop.

The prop name has a default value of 'Guest'. If no value is passed, it uses this default, so the output is 'Hello Guest!'.

📝 Syntax

intermediate

Which option correctly declares a prop named count with default 0?

Choose the correct Svelte syntax to declare a prop count with a default value of 0.

A export let count = 0;

B let export count = 0;

C let count export = 0;

D export count let = 0;

💡 Hint

Remember the order: export then let.

The correct syntax is export let count = 0;. The other options have invalid order or keywords.

❓ state_output

advanced

What is the output after updating a prop inside the component?

Given this Svelte component:

<script>
  export let value = 5;
  value = value + 3;
</script>

<p>Value is {value}</p>

What will be displayed when the component is used with <Component value={10} />?

Svelte

<script>
export let value = 5;
value = value + 3;
</script>

<p>Value is {value}</p>

A Value is 13

B Value is 8

C Value is 5

D Value is 10

💡 Hint

Remember the prop value is overwritten inside the component.

The prop value is passed as 10, then inside the component it is increased by 3, so the final value is 13.

🔧 Debug

advanced

Why does this Svelte component throw an error?

Look at this code snippet:

<script>
  export let count;
  count = count + 1;
</script>

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

When used without passing count, it throws an error. Why?

Svelte

<script>
export let count;
count = count + 1;
</script>

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

A Because the component is missing a return statement

B Because count is undefined and adding 1 to undefined causes an error

C Because count must be declared with const, not let

D Because export let cannot be reassigned inside the component

💡 Hint

Think about what happens when you add a number to undefined.

If count is not passed, it is undefined. Adding 1 to undefined results in NaN in JavaScript, but Svelte treats this as an error in this context.

🧠 Conceptual

expert

What happens if you declare a prop without export in Svelte?

Consider this Svelte component snippet:

<script>
  let title = 'Hello';
</script>

<h1>{title}</h1>

If you try to pass a prop named title from a parent component, what will happen?

Svelte

<script>
let title = 'Hello';
</script>

<h1>{title}</h1>

A The component will receive and display the passed title prop

B Svelte will throw a syntax error for missing export

C The prop will not be received; the component uses its internal title variable

D The component will crash at runtime

💡 Hint

Think about how Svelte knows which variables are props.

Only variables declared with export let become props. Without export, the variable is internal and cannot receive values from outside.

Practice

1. What does export let do inside a Svelte component's <script> block?

easy

A. It creates a local variable that cannot be changed.

B. It imports a module from another file.

C. It declares a prop that can be set from outside the component.

D. It defines a function inside the component.

Solution

1. Step 1: Understand the role of export let

   In Svelte, export let declares a property (prop) that the parent component can pass in.

2. Step 2: Differentiate from other uses

   Unlike local variables or imports, export let specifically exposes variables as props for customization.

3. Final Answer:

   It declares a prop that can be set from outside the component. -> Option C

4. Quick Check:

   Props = export let [OK]

Hint: Remember: export let means external prop [OK]

Common Mistakes:

• Confusing export let with import statements
• Thinking export let creates constants
• Assuming export let defines functions

2. Which of the following is the correct syntax to declare a prop named title with a default value of "Hello" in Svelte?

easy

A. export let title = "Hello";

B. let export title = "Hello";

C. export const title = "Hello";

D. let title = export "Hello";

Solution

1. Step 1: Recall correct syntax for props

   In Svelte, props are declared with export let followed by the variable name and optional default value.

2. Step 2: Check each option

   export let title = "Hello"; matches the correct syntax: export let title = "Hello";. Others have wrong order or keywords.

3. Final Answer:

   export let title = "Hello"; -> Option A

4. Quick Check:

   export let + name + = default [OK]

Hint: Use 'export let' before variable name for props [OK]

Common Mistakes:

• Swapping 'let' and 'export' keywords
• Using 'const' instead of 'let' for props
• Placing export after variable name

3. Given this Svelte component code:

<script>
  export let count = 5;
</script>

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

What will be displayed if the component is used as <MyComponent count={10} />?

medium

A. Count is 5

B. Count is 10

C. Count is undefined

D. Syntax error

Solution

1. Step 1: Understand default and passed props

   The component sets a default count of 5, but the parent passes 10, which overrides the default.

2. Step 2: Determine rendered output

   The displayed text uses the passed value 10, so it shows "Count is 10".

3. Final Answer:

   Count is 10 -> Option B

4. Quick Check:

   Passed prop overrides default [OK]

Hint: Passed props override defaults [OK]

Common Mistakes:

• Assuming default always shows ignoring passed value
• Thinking undefined appears if prop is passed
• Confusing syntax error with valid code

4. Identify the error in this Svelte component code:

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

<p>Hello {name}</p>

medium

A. You cannot assign a value to a prop inside the component after declaration.

B. Missing default value for the prop name.

C. The export keyword should be after let.

D. The <p> tag cannot use curly braces.

Solution

1. Step 1: Understand prop assignment rules

   Props declared with export let should not be reassigned inside the component; they come from outside.

2. Step 2: Check the code

   The code tries to assign name = "John" after declaration, which is not allowed and causes an error.

3. Final Answer:

   You cannot assign a value to a prop inside the component after declaration. -> Option A

4. Quick Check:

   Props are read-only inside component [OK]

Hint: Don't reassign props inside component [OK]

Common Mistakes:

• Trying to set default by assignment after export let
• Confusing export let order
• Misunderstanding template syntax

5. You want to create a Svelte component that accepts a prop items which is an array, but if no prop is passed, it should default to an empty array. Which code correctly declares this prop?

hard

A. export const items = [];

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

C. let items = [];

D. export let items = [];

Solution

1. Step 1: Set default value in prop declaration

   In Svelte, you can assign a default value directly when declaring a prop with export let.

2. Step 2: Evaluate options

   export let items = []; correctly declares items with default empty array. export let items;
if (!items) items = []; tries to assign after declaration, which is not allowed. let items = []; is local variable, not prop. export const items = []; uses const which is incorrect for props.

3. Final Answer:

   export let items = []; -> Option D

4. Quick Check:

   Default values go in export let declaration [OK]

Hint: Assign default value in export let line [OK]

Common Mistakes:

• Assigning default after declaration
• Using const instead of let for props
• Not exporting the variable