Readonly props in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a child tries to modify a readonly prop in Svelte?

Consider a Svelte component that receives a prop. What will happen if the child component tries to change that prop's value?

Svelte

<script>
  export let count;
</script>

<button on:click={() => count = count + 1}>Increment</button>
<p>{count}</p>

A Svelte throws a compile-time error preventing the assignment.

B The component updates the count and re-renders with the new value.

C The assignment is ignored silently; count remains unchanged.

D A runtime error occurs because props are immutable inside the child.

💡 Hint

Think about how Svelte treats props inside child components.

In Svelte, props are readonly inside the child component. Trying to assign a new value to a prop inside the child causes a runtime error because props are immutable from the child's perspective.

❓ state_output

intermediate

What is the output when a readonly prop is updated in the parent?

Given a parent component passing a prop to a child, what will the child display if the parent updates the prop value?

Svelte

<!-- Parent.svelte -->
<script>
  import Child from './Child.svelte';
  let message = 'Hello';
  function update() {
    message = 'Hi';
  }
</script>
<button on:click={update}>Update Message</button>
<Child text={message} />

<!-- Child.svelte -->
<script>
  export let text;
</script>
<p>{text}</p>

A Always shows 'Hello' because props are readonly and cannot update.

B Shows 'Hi' immediately without clicking the button.

C Initially 'Hello', then changes to 'Hi' after clicking the button.

D Throws an error because the child tries to modify the prop.

💡 Hint

Consider how data flows from parent to child in Svelte.

Props are readonly inside the child, but the parent can update the value it passes. The child will reactively update its display when the parent changes the prop.

📝 Syntax

advanced

Which code correctly prevents a child from modifying a prop in Svelte?

Choose the code snippet that enforces the child component cannot modify the prop value.

A <script>export let value; // no reassignment allowed by Svelte</script>

B <script>export let value; Object.freeze(value);</script>

C <script>export let value; value = 10;</script>

D <script>export let value; value += 1;</script>

💡 Hint

Think about how Svelte treats props by default.

Svelte props are readonly by default inside the child. No special code is needed to prevent reassignment; attempting to assign to a prop causes a runtime error.

🔧 Debug

advanced

Why does this Svelte child component throw an error?

Examine the child component code below. Why does it throw an error when the button is clicked?

Svelte

<script>
  export let score;
  function increase() {
    score += 1;
  }
</script>
<button on:click={increase}>Increase</button>
<p>{score}</p>

A Because the button element is missing an aria-label.

B Because props are readonly and cannot be reassigned inside the child.

C Because score is not initialized with a default value.

D Because the function increase is not bound to the button click event.

💡 Hint

Recall how Svelte treats props inside child components.

Props in Svelte are readonly inside the child. Trying to reassign 'score' inside the child causes a runtime error.

🧠 Conceptual

expert

How can a Svelte child component communicate changes back to the parent when props are readonly?

Since props are readonly inside a Svelte child component, how can the child notify the parent to update a prop value?

A The child dispatches a custom event that the parent listens to and updates the prop accordingly.

B The child directly modifies the prop value using assignment.

C The child uses a global variable to change the prop value.

D The child imports the parent component and calls a method to update the prop.

💡 Hint

Think about how components communicate in Svelte without direct prop mutation.

In Svelte, children cannot modify props directly. Instead, they dispatch custom events to inform the parent, which can then update the prop value.

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