Default actions in Svelte

Introduction

Default actions let you add behavior to HTML elements easily in Svelte. They help you run code when elements are created or removed.

You want to focus an input automatically when it appears.

You need to add a tooltip or popup to an element.

You want to listen to events or modify elements without extra code in your component.

You want to clean up event listeners or timers when an element is removed.

Syntax

Svelte

function actionName(node) {
  // code to run when element is created
  return {
    destroy() {
      // code to run when element is removed
    }
  };
}

<!-- Use in markup -->
<div use:actionName></div>

The node parameter is the HTML element the action is applied to.

The destroy function cleans up when the element is removed.

Examples

This action sets focus on an input when it appears.

Svelte

function focus(node) {
  node.focus();
  return {
    destroy() {
      // no cleanup needed here
    }
  };
}

<input use:focus>

This action logs a message when the button is clicked and cleans up the listener when removed.

Svelte

function logClick(node) {
  const handleClick = () => console.log('Clicked!');
  node.addEventListener('click', handleClick);
  return {
    destroy() {
      node.removeEventListener('click', handleClick);
    }
  };
}

<button use:logClick>Click me</button>

Sample Program

This example uses a default action to highlight a paragraph by changing its background color. When the paragraph is removed, the highlight is cleared.

Svelte

<script>
  function highlight(node) {
    node.style.backgroundColor = 'yellow';
    return {
      destroy() {
        node.style.backgroundColor = '';
      }
    };
  }
</script>

<p use:highlight>This text is highlighted when shown.</p>

Important Notes

Actions run when the element is added to the page.

Always clean up in destroy to avoid memory leaks.

You can pass parameters to actions for more control.

Summary

Default actions add behavior to elements easily.

They run code on element creation and cleanup on removal.

Use them to focus, add events, or modify elements simply.

Practice

1. What is the main purpose of a default action in Svelte?

easy

A. To add behavior to an element when it is created and clean up when removed

B. To style an element with CSS automatically

C. To create a new component dynamically

D. To fetch data from an API when the page loads

Solution

1. Step 1: Understand what default actions do

   Default actions in Svelte run code when an element is created and can clean up when it is removed.

2. Step 2: Match purpose with options

   Only To add behavior to an element when it is created and clean up when removed describes adding behavior on creation and cleanup on removal, which matches default actions.

3. Final Answer:

   To add behavior to an element when it is created and clean up when removed -> Option A

4. Quick Check:

   Default actions add and clean behavior = B [OK]

Hint: Default actions run on element mount and cleanup on unmount [OK]

Common Mistakes:

• Confusing default actions with styling or data fetching
• Thinking default actions create components
• Assuming default actions only run once globally

2. Which of the following is the correct syntax to apply a default action named focus to an input element in Svelte?

easy

A. <input on:focus />

B. <input action:focus />

C. <input bind:focus />

D. <input use:focus />

Solution

1. Step 1: Recall Svelte default action syntax

   Default actions are applied using use:actionName on elements.

2. Step 2: Compare options with correct syntax

   <input use:focus /> uses use:focus, which is the correct way to apply the action.

3. Final Answer:

   <input use:focus /> -> Option D

4. Quick Check:

   Default action syntax = use:actionName [OK]

Hint: Use use: prefix to apply default actions [OK]

Common Mistakes:

• Using action: or on: instead of use:
• Confusing event handlers with actions
• Trying to bind actions like variables

3. Consider this Svelte code snippet:

function highlight(node) {
  node.style.backgroundColor = 'yellow';
  return {
    destroy() {
      node.style.backgroundColor = '';
    }
  };
}

<div use:highlight>Hello</div>

What will be the background color of the <div> when it is rendered?

medium

A. Yellow

B. No background color (default)

C. Red

D. Transparent

Solution

1. Step 1: Analyze the highlight action code

   The action sets backgroundColor to 'yellow' on the node when applied.

2. Step 2: Understand the effect on the <div>

   The <div> uses use:highlight, so its background color becomes yellow on render.

3. Final Answer:

   Yellow -> Option A

4. Quick Check:

   Action sets background to yellow = A [OK]

Hint: Default action runs on mount, sets style immediately [OK]

Common Mistakes:

• Thinking destroy runs immediately and clears color
• Confusing background color with text color
• Assuming no style changes without CSS

4. Given this default action code:

function logClick(node) {
  node.addEventListener('click', () => console.log('Clicked!'));
}

What is the main problem with this action?

medium

A. It should return a promise to work correctly

B. It uses an arrow function which is not allowed in actions

C. It does not remove the event listener when the element is removed

D. It must use use:logClick() with parentheses

Solution

1. Step 1: Check event listener management

   The action adds a click listener but does not remove it on cleanup.

2. Step 2: Understand Svelte action cleanup

   Actions should return an object with a destroy method to remove listeners and avoid memory leaks.

3. Final Answer:

   It does not remove the event listener when the element is removed -> Option C

4. Quick Check:

   Missing cleanup causes leaks = A [OK]

Hint: Always return destroy() to clean event listeners [OK]

Common Mistakes:

• Ignoring cleanup leads to memory leaks
• Thinking arrow functions are disallowed
• Assuming parentheses are required on use

5. You want to create a default action autoFocus that focuses an input element when it appears and logs "Focused" once. Which code correctly implements this behavior?

hard

A. function autoFocus(node) {
node.focus();
return {
update() {
console.log('Focused');
}
};
}

B. function autoFocus(node) {
node.focus();
console.log('Focused');
return {
destroy() {}
};
}

C. function autoFocus(node) {
setTimeout(() => node.focus(), 0);
console.log('Focused');
}

D. function autoFocus(node) {
node.addEventListener('focus', () => console.log('Focused'));
node.focus();
return {
destroy() {
node.removeEventListener('focus', () => console.log('Focused'));
}
};
}

Solution

1. Step 1: Focus element and log once on mount

   Calling node.focus() and logging immediately runs once when the element appears.

2. Step 2: Provide cleanup method

   Returning an object with an empty destroy method is valid and ensures cleanup structure.

3. Final Answer:

   Function that focuses and logs once, with destroy method -> Option B

4. Quick Check:

   Focus and log once on mount = D [OK]

Hint: Focus and log immediately, return destroy() for cleanup [OK]

Common Mistakes:

• Adding unnecessary event listeners for logging
• Missing return object with destroy method
• Logging multiple times on updates