Slot basics (default slot) in Svelte - Mini Project: Build & Apply

Slot basics (default slot)

📖 Scenario: You are building a simple Svelte component that can display a message inside a box. You want to allow users of your component to put any content inside this box using Svelte's default slot feature.

🎯 Goal: Create a Svelte component called MessageBox.svelte that uses a default slot to display any content passed inside it. Then use this component in an App.svelte file to show a message inside the box.

📋 What You'll Learn

Create a Svelte component named MessageBox.svelte with a default slot

Use a <slot> tag inside a container element in MessageBox.svelte

Create an App.svelte file that imports MessageBox.svelte

Use the <MessageBox> component in App.svelte and pass some text inside it

Ensure the text inside <MessageBox> appears inside the box

💡 Why This Matters

🌍 Real World

Default slots let you create reusable UI components that can display any content inside them, like cards, modals, or message boxes.

💼 Career

Understanding slots is key for building flexible, maintainable components in Svelte, a popular modern frontend framework.

1

Create the MessageBox component with a container

Create a Svelte component file named MessageBox.svelte. Inside it, write a <div> element with a class box to act as a container.

Svelte

Hint

Use the <slot> tag inside the <div> to allow content insertion.

2

Add basic styling to the MessageBox container

In MessageBox.svelte, add a <style> block below the HTML. Inside it, style the .box class with a border of 2px solid black, padding of 1rem, and a max-width of 20rem.

Svelte

  



  /* Your code here */

Hint

Use CSS inside the <style> block to style the container.

3

Create the App component and import MessageBox

Create a new Svelte component file named App.svelte. Import the MessageBox component from ./MessageBox.svelte at the top of the file.

Svelte

import MessageBox from './MessageBox.svelte';

Hint

Use the import statement and then use the component as a tag.

4

Use the MessageBox component with default slot content

In App.svelte, write the <MessageBox> component with the text Hello, this is a message inside the box! placed between the opening and closing tags. This text should appear inside the box when rendered.

Svelte

import MessageBox from './MessageBox.svelte';

Hint

Put the message text inside the <MessageBox> tags to use the default slot.

Practice

1. What is the purpose of the default slot in a Svelte component?

easy

A. To display content passed from the parent component if no named slot is used

B. To define a CSS style inside the component

C. To handle component state changes

D. To import external JavaScript files

Solution

1. Step 1: Understand what a slot does

   A slot allows a parent component to pass content into a child component.

2. Step 2: Identify the role of the default slot

   The default slot is the unnamed slot that receives content if no specific slot name is given.

3. Final Answer:

   To display content passed from the parent component if no named slot is used -> Option A

4. Quick Check:

   Default slot = main content placeholder [OK]

Hint: Default slot shows passed content if no slot name given [OK]

Common Mistakes:

• Confusing slots with CSS or state
• Thinking default slot is for styles
• Assuming default slot imports scripts

2. Which of the following is the correct syntax to define a default slot in a Svelte component?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall slot syntax in Svelte

   The default slot is defined simply with <slot></slot> without a name attribute.

2. Step 2: Check the options for correct syntax

   Only uses <slot></slot> without a name, which is correct for the default slot.

3. Final Answer:

   <slot></slot> -> Option A

4. Quick Check:

   Default slot = <slot></slot> [OK]

Hint: Default slot uses <slot> without name attribute [OK]

Common Mistakes:

• Adding name="default" attribute incorrectly
• Using custom tags like
• Confusing named slots with default slot syntax

3. Given the following Svelte component code, what will be rendered?

<!-- Child.svelte -->
<slot>No content provided</slot>

<!-- Parent.svelte -->
<Child>
  <p>Hello from parent</p>
</Child>

medium

A. No content provided

B. Hello from parent

C. An empty space

D. Error: slot not found

Solution

1. Step 1: Understand default slot behavior

   The <slot> tag shows content passed from the parent if any is provided.

2. Step 2: Check if parent passes content

   The parent passes <p>Hello from parent</p> inside <Child>, so this content replaces the default slot fallback.

3. Final Answer:

   Hello from parent -> Option B

4. Quick Check:

   Parent content replaces default slot fallback [OK]

Hint: Parent content overrides default slot fallback text [OK]

Common Mistakes:

• Assuming fallback text always shows
• Thinking slot content is ignored
• Expecting an error when slot is used

4. Identify the error in this Svelte component using a default slot:

<slot name>Fallback</slot>

medium

A. The fallback content must be outside the slot tag

B. Slots cannot have fallback content

C. The slot tag should not have a name attribute without a value

D. The slot tag must be self-closing

Solution

1. Step 1: Analyze the slot tag attributes

   The slot tag has a name attribute but no value, which is invalid syntax.

2. Step 2: Recall correct slot syntax

   Named slots require a name with a string value, or omit the name attribute for default slot.

3. Final Answer:

   The slot tag should not have a name attribute without a value -> Option C

4. Quick Check:

   Empty name attribute on slot is invalid [OK]

Hint: Name attribute must have a value or be omitted [OK]

Common Mistakes:

• Leaving name attribute empty
• Thinking fallback content is invalid
• Using self-closing slot tags incorrectly

5. You want a Svelte component to show a message passed from the parent, but if no message is given, it should show "Welcome!" by default. Which code correctly uses the default slot to achieve this?

hard

A.

B. Welcome!

C. {message || 'Welcome!'}

D. Welcome!

Solution

1. Step 1: Understand default slot fallback usage

   The default slot shows fallback content if no content is passed from the parent.

2. Step 2: Identify correct fallback syntax

   Using <slot>Welcome!</slot> sets "Welcome!" as fallback text correctly.

3. Step 3: Check other options for errors

   <slot name="default">Welcome!</slot> incorrectly uses name="default" which is unnecessary; C uses invalid expression inside slot; D uses non-existent fallback attribute.

4. Final Answer:

   <slot>Welcome!</slot> -> Option D

5. Quick Check:

   Default slot fallback = content inside <slot> [OK]

Hint: Put fallback text inside <slot> tags for default content [OK]

Common Mistakes:

• Adding name="default" attribute unnecessarily
• Trying to use JavaScript expressions inside slot tags
• Using invalid fallback attributes