Readonly props in Svelte - Mini Project: Build & Apply

Readonly props in Svelte

📖 Scenario: You are building a simple user profile card in Svelte. The card shows a user's name and age. The user's name should not be changed inside the profile card component once it is set from outside.

🎯 Goal: Create a Svelte component that receives a name as a readonly prop and an age as a normal prop. Display both on the card. Ensure the name cannot be changed inside the component.

📋 What You'll Learn

Create a Svelte component named UserProfile.svelte.

Declare a prop name that is readonly inside the component.

Declare a prop age that can be changed inside the component.

Display the name and age in the component's HTML.

Try to change name inside the component and ensure it causes an error or is prevented.

Change age inside the component and show the updated value.

💡 Why This Matters

🌍 Real World

Readonly props are useful when you want to protect data passed from a parent component so it cannot be accidentally changed inside a child component.

💼 Career

Understanding how to manage props and their mutability is important for building reliable and maintainable UI components in Svelte.

1

Setup props for user name and age

Create a Svelte component with two props: declare export let name and export let age.

Svelte

export let name;
export let age;

Hint

Use export let to declare props in Svelte.

2

Make the name prop readonly

Add const before name to make it readonly: write export const name and keep export let age as is.

Svelte

export const name;
export let age;

Hint

Use export const to make a prop readonly in Svelte.

3

Try to change name inside the component

Add a line inside the component that tries to assign name = 'New Name'. This should cause an error or be prevented because name is readonly.

Svelte

export const name;
export let age;

name = 'New Name'; // Try to change readonly prop

Hint

Readonly props cannot be reassigned inside the component.

4

Change age inside the component and display updated value

Add a line that changes age to age + 1 inside the component. Display the updated age below the original age.

Svelte

export const name;
export let age;

name = 'New Name'; // This will cause an error because name is readonly

age = age + 1; // Increase age by 1

Hint

Normal props declared with export let can be changed inside the component.

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