Readonly props in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a readonly prop in a Svelte component.

Svelte

<script>
  export let name[1];
</script>

Drag options to blanks, or click blank then click option'

Areadonly

Bconst

Cimmutable

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' instead of 'readonly' for props.

Trying to use 'immutable' which is not valid in Svelte.

In Svelte, you mark a prop as readonly by adding the readonly keyword after let.

2fill in blank

medium

Complete the code to prevent modification of a prop inside the component.

Svelte

<script>
  export let count[1];

  // Trying to change count here will cause an error
</script>

Drag options to blanks, or click blank then click option'

Aconst

Breadonly

Clet

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' which is not valid for exported props in Svelte.

Using 'var' which is outdated and not recommended.

Using readonly after let makes the prop immutable inside the component.

3fill in blank

hard

Fix the error in the code by making the prop readonly.

Svelte

<script>
  export let message[1];

  message = 'New message'; // This should cause an error
</script>

Drag options to blanks, or click blank then click option'

Areadonly

Bconst

Clet

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' which is not allowed for exported props.

Not adding any keyword and trying to reassign the prop.

Adding readonly after let prevents reassignment of the prop inside the component.

4fill in blank

hard

Fill both blanks to declare a readonly prop and use it in markup.

Svelte

<script>
  export let title[1];
</script>

<h1>[2]</h1>

Drag options to blanks, or click blank then click option'

Areadonly

Btitle

Cname

Dconst

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' instead of 'readonly' for the prop.

Using a wrong variable name in the markup.

The prop is declared readonly with readonly. The markup uses the prop title to display it.

5fill in blank

hard

Fill all three blanks to declare multiple readonly props and use them in markup.

Svelte

<script>
  export let firstName[1];
  export let lastName[2];
</script>

<p>Hello, [3] {{lastName}}!</p>

Drag options to blanks, or click blank then click option'

Areadonly

Bconst

CfirstName

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' or 'let' instead of 'readonly' for props.

Using a wrong variable name in the markup.

Both props are declared readonly with readonly. The markup uses firstName and lastName to greet the user.

Practice

1. In Svelte, what does it mean when we say props are readonly inside a child component?

easy

A. Props are only readonly if explicitly declared as such.

B. The parent component cannot send new values to the child.

C. The child component cannot change the value of props it receives from the parent.

D. Props can be changed freely inside the child component.

Solution

1. Step 1: Understand prop behavior in Svelte

   Props passed from parent to child are fixed values inside the child and cannot be reassigned there.

2. Step 2: Identify what readonly means here

   Readonly means the child cannot modify these props, ensuring data flows down safely.

3. Final Answer:

   The child component cannot change the value of props it receives from the parent. -> Option C

4. Quick Check:

   Props are readonly = child cannot modify [OK]

Hint: Props from parent are fixed inside child, no changes allowed [OK]

Common Mistakes:

• Thinking child can modify props directly
• Confusing readonly with immutable everywhere
• Believing parent cannot update props

2. Which of the following is the correct way to declare a prop in a Svelte component?

easy

A. export const count;

B. let export count;

C. prop count;

D. export let count;

Solution

1. Step 1: Recall Svelte prop syntax

   Props are declared using export let to allow parent to pass values.

2. Step 2: Check each option

   Only export let count; is valid syntax for a prop declaration.

3. Final Answer:

   export let count; -> Option D

4. Quick Check:

   Props declared with export let = A [OK]

Hint: Props always start with 'export let' in Svelte [OK]

Common Mistakes:

• Using 'let export' instead of 'export let'
• Trying to use 'prop' keyword
• Using 'export const' which is not for props

3. Given this Svelte component code:

  <script>
  export let name = 'Guest';
  name = 'Alice';
  </script>
  <p>Hello {name}!</p>

What will happen when this component runs?

medium

A. It will display 'Hello Alice!' without errors.

B. It will cause a runtime error because props are readonly.

C. It will display 'Hello Guest!' and ignore the assignment.

D. It will display 'Hello undefined!' because of reassignment.

Solution

1. Step 1: Identify the prop and reassignment

   The prop name is declared with default 'Guest' but reassigned to 'Alice' inside the component.

2. Step 2: Understand readonly prop behavior

   Props are readonly inside the component, so reassigning name causes an error.

3. Final Answer:

   It will cause a runtime error because props are readonly. -> Option B

4. Quick Check:

   Reassigning prop inside component = error [OK]

Hint: Props cannot be reassigned inside component, causes error [OK]

Common Mistakes:

• Assuming default prop value can be changed inside
• Thinking reassignment silently fails
• Confusing local variables with props

4. You have this Svelte component:

<script>
  export let count = 0;
  function increment() {
    count += 1;
  }
</script>
<button on:click={increment}>Increment</button>
<p>Count: {count}</p>

What is the problem here?

medium

A. Error because you cannot reassign a readonly prop inside the component.

B. No problem, count will increment on button click.

C. The button click event is not set up correctly.

D. The initial value of count must be passed from parent.

Solution

1. Step 1: Analyze prop reassignment in increment()

   The function tries to increase count, which is a prop and readonly inside the component.

2. Step 2: Understand Svelte readonly prop rules

   Reassigning a prop inside the component causes an error because props are readonly.

3. Final Answer:

   Error because you cannot reassign a readonly prop inside the component. -> Option A

4. Quick Check:

   Modifying prop inside component = error [OK]

Hint: Props cannot be changed inside component, use local state instead [OK]

Common Mistakes:

• Thinking event handler fixes reassignment issue
• Assuming default prop value means local variable
• Ignoring readonly prop error

5. How can you allow a Svelte child component to update a value originally passed as a readonly prop from its parent?

hard

A. Use a local variable inside the child and dispatch an event to notify the parent of changes.

B. Directly reassign the prop inside the child component to update it.

C. Declare the prop with export const to make it writable.

D. Use bind:this on the prop to make it mutable.

Solution

1. Step 1: Recognize props are readonly in child

   Props cannot be changed inside the child component directly.

2. Step 2: Use event dispatch to update parent

   Child can keep a local copy and send events to parent to update the original value.

3. Final Answer:

   Use a local variable inside the child and dispatch an event to notify the parent of changes. -> Option A

4. Quick Check:

   Update via event dispatch and local state = C [OK]

Hint: Use local state + event dispatch to update parent from child [OK]

Common Mistakes:

• Trying to reassign props directly in child
• Misusing export const for writable props
• Confusing bind:this with prop binding