Named actions in Svelte

Introduction

Named actions let you add reusable behavior to HTML elements in Svelte. They help keep your code clean and easy to understand.

You want to add a common effect like focusing an input when it appears.

You need to handle events or animations on elements in multiple places.

You want to separate behavior logic from your component markup.

You want to reuse the same interaction on different elements without repeating code.

Syntax

Svelte

function actionName(node, parameters) {
  // setup code here

  return {
    update(newParameters) {
      // handle parameter changes
    },
    destroy() {
      // cleanup code here
    }
  };
}

<!-- Use in markup -->
<div use:actionName={parameters}></div>

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

The function can return an object with update and destroy to handle changes and cleanup.

Examples

This action focuses an input element when it appears.

Svelte

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

<!-- Usage -->
<input use:focus>

This action highlights text with a background color that can change.

Svelte

function highlight(node, color = 'yellow') {
  node.style.backgroundColor = color;

  return {
    update(newColor) {
      node.style.backgroundColor = newColor;
    },
    destroy() {
      node.style.backgroundColor = '';
    }
  };
}

<!-- Usage -->
<p use:highlight={'lightblue'}>Text with highlight</p>

Sample Program

This named action adds a tooltip that appears when you hover over the button. It cleans up the tooltip when the mouse leaves or the element is removed.

Svelte

<script>
  function tooltip(node, text) {
    let div;

    function mouseOver() {
      div = document.createElement('div');
      div.textContent = text;
      div.style.position = 'absolute';
      div.style.background = 'black';
      div.style.color = 'white';
      div.style.padding = '0.25rem 0.5rem';
      div.style.borderRadius = '0.25rem';
      div.style.top = `${node.getBoundingClientRect().bottom + window.scrollY + 5}px`;
      div.style.left = `${node.getBoundingClientRect().left + window.scrollX}px`;
      document.body.appendChild(div);
    }

    function mouseOut() {
      if (div) {
        div.remove();
        div = null;
      }
    }

    node.addEventListener('mouseover', mouseOver);
    node.addEventListener('mouseout', mouseOut);

    return {
      update(newText) {
        text = newText;
      },
      destroy() {
        node.removeEventListener('mouseover', mouseOver);
        node.removeEventListener('mouseout', mouseOut);
        if (div) div.remove();
      }
    };
  }
</script>

<button use:tooltip={'Click me to do something'}>Hover me</button>

Important Notes

Always clean up event listeners or DOM changes in the destroy method to avoid memory leaks.

You can update the action's behavior by using the update method when parameters change.

Named actions help keep your components tidy by moving behavior outside the markup.

Summary

Named actions add reusable behavior to HTML elements in Svelte.

They receive the element and optional parameters, and can clean up after themselves.

Use them to keep your code organized and avoid repeating logic.

Practice

1. What is the main purpose of named actions in Svelte?

easy

A. To style elements with CSS

B. To define new components

C. To add reusable behavior to HTML elements

D. To manage application state

Solution

1. Step 1: Understand what named actions do

   Named actions in Svelte are used to add behavior to HTML elements that can be reused across components.

2. Step 2: Compare with other options

   Defining components, styling, and state management are handled differently in Svelte, not by named actions.

3. Final Answer:

   To add reusable behavior to HTML elements -> Option C

4. Quick Check:

   Named actions = reusable element behavior [OK]

Hint: Named actions add behavior to elements, not styles or state [OK]

Common Mistakes:

• Confusing actions with components
• Thinking actions handle styling
• Assuming actions manage state

2. Which of the following is the correct syntax to define a named action in Svelte?

easy

A. function myAction(node) { /* code */ }

B. const myAction = () => { /* code */ }

C. let myAction = node => { /* code */ }

D. action myAction(node) { /* code */ }

Solution

1. Step 1: Recall named action syntax

   Named actions are functions that receive the element node as the first argument, typically declared as function myAction(node) {}.

2. Step 2: Check other options

   Arrow functions can work but must return an object for cleanup; action myAction(node) { /* code */ } uses invalid syntax.

3. Final Answer:

   function myAction(node) { /* code */ } -> Option A

4. Quick Check:

   Named action = function with node parameter [OK]

Hint: Named actions are functions with node parameter, not keywords [OK]

Common Mistakes:

• Using invalid 'action' keyword
• Defining without node parameter
• Using arrow function without return object

3. Given this named action and usage:

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

<div use:highlight>Hello</div>

What happens when the component is removed from the page?

medium

A. The background color resets to default

B. The element is removed from the DOM immediately

C. An error occurs because destroy is missing

D. The background color stays yellow

Solution

1. Step 1: Understand the action's destroy method

   The destroy method resets the background color to empty string when the element is removed.

2. Step 2: Effect on component removal

   When the component unmounts, destroy runs, so the background color resets to default.

3. Final Answer:

   The background color resets to default -> Option A

4. Quick Check:

   destroy resets styles on removal [OK]

Hint: Destroy cleans up styles when element is removed [OK]

Common Mistakes:

• Assuming styles persist after removal
• Thinking destroy is optional
• Confusing destroy with element removal

4. What is wrong with this named action code?

function focus(node) {
  node.focus();
  return {
    destroy() {
      node.blur();
    }
  }
}

<input use:focus>

medium

A. Calling node.blur() in destroy may cause errors if node is removed

B. The action should not call node.focus() immediately

C. The action does not return an object

D. The action must be async to work properly

Solution

1. Step 1: Analyze the focus call timing

   Calling node.focus() immediately may not work reliably because the element might not be fully rendered, visible, or ready (e.g., during CSS transitions or animations).

2. Step 2: Best practice for focus actions

   In Svelte, it's often necessary to delay the focus call using tick(), requestAnimationFrame, or setTimeout(..., 0) to ensure DOM readiness.

3. Final Answer:

   The action should not call node.focus() immediately -> Option B

4. Quick Check:

   Delay focus with tick() or RAF [OK]

Hint: Delay node.focus() with tick() for reliability [OK]

Common Mistakes:

• Calling focus immediately without delay
• Ignoring DOM/transition timing issues
• Assuming focus always succeeds synchronously

5. You want to create a named action that changes an element's text color based on a parameter and updates it when the parameter changes. Which approach is correct?

hard

A. Use a reactive statement inside the action function to watch parameters

B. Set the color once in the action and ignore parameter changes

C. Recreate the action every time parameters change instead of updating

D. Return an update function from the action that sets the color when parameters change

Solution

1. Step 1: Understand parameter updates in named actions

   Named actions can return an update function to handle parameter changes dynamically.

2. Step 2: Evaluate options for handling parameter changes

   Setting color once ignores updates; reactive statements inside action function don't track parameters; recreating action is inefficient.

3. Final Answer:

   Return an update function from the action that sets the color when parameters change -> Option D

4. Quick Check:

   Use update() to handle parameter changes [OK]

Hint: Use update() function to react to parameter changes [OK]

Common Mistakes:

• Ignoring parameter updates
• Misusing reactive statements inside actions
• Recreating actions unnecessarily