Action with event dispatching in Svelte - Performance & Optimization

Performance: Action with event dispatching

MEDIUM IMPACT

This affects interaction responsiveness and event handling efficiency in the browser.

Handling user interaction with a custom action that dispatches events

Svelte

function myAction(node) {
  const handleClick = () => {
    node.dispatchEvent(new CustomEvent('myEvent', { detail: { clicked: true } }));
  };
  node.addEventListener('click', handleClick);
  return {
    destroy() {
      node.removeEventListener('click', handleClick);
    }
  };
}

Reuses the same function reference for adding and removing event listeners, preventing memory leaks.

📈 Performance GainReduces memory usage and avoids unnecessary event listener buildup.

Handling user interaction with a custom action that dispatches events

Svelte

function myAction(node) {
  node.addEventListener('click', () => {
    node.dispatchEvent(new CustomEvent('myEvent', { detail: { clicked: true } }));
  });
  return {
    destroy() {
      node.removeEventListener('click', () => {});
    }
  };
}

Creates a new anonymous function for event listener removal, causing the listener not to be removed and potential memory leaks.

📉 Performance CostLeads to increased memory usage and potential slowdowns on long-lived pages.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Anonymous event listener in action	No extra DOM nodes	0 reflows	Minimal paint	[X] Bad
Named event listener with proper cleanup	No extra DOM nodes	0 reflows	Minimal paint	[OK] Good

Rendering Pipeline

Event dispatching in actions triggers the browser's event system without forcing layout recalculations unless DOM changes occur.

→Event Handling

→Composite

⚠️ BottleneckEvent Handling when many listeners or inefficient dispatching occurs

Core Web Vital Affected

INP

This affects interaction responsiveness and event handling efficiency in the browser.

Optimization Tips

1Always use named functions for event listeners to enable proper cleanup.

2Avoid DOM mutations during event dispatch to prevent layout thrashing.

3Keep event handlers lightweight to maintain fast interaction responsiveness.

Performance Quiz - 3 Questions

Why should you use a named function for event listeners in Svelte actions?

A To reduce the size of the JavaScript bundle.

B To ensure the event listener can be properly removed, preventing memory leaks.

C Because anonymous functions run slower in the browser.

D To make the code look cleaner.

DevTools: Performance

How to check: Record a performance profile while interacting with the element using the action. Look for event handler execution time and memory usage.

What to look for: Low event handler execution time and no increasing memory usage over repeated interactions indicate good performance.

Practice

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

easy

A. To create new components

B. To add reusable behavior to HTML elements

C. To style elements with CSS

D. To manage global state

Solution

1. Step 1: Understand what actions do

   Actions in Svelte are functions that add behavior to elements, like event listeners or animations.

2. Step 2: Compare options

   Only To add reusable behavior to HTML elements correctly describes adding reusable behavior to elements, while others describe unrelated tasks.

3. Final Answer:

   To add reusable behavior to HTML elements -> Option B

4. Quick Check:

   Actions = reusable element behavior [OK]

Hint: Actions add behavior to elements, not create components [OK]

Common Mistakes:

• Confusing actions with components
• Thinking actions manage styles
• Assuming actions handle global state

2. Which of the following is the correct way to dispatch a custom event named notify from a Svelte action?

easy

A. new Event('notify').dispatch(node)

B. dispatch('notify')

C. dispatchEvent('notify')

D. node.dispatchEvent(new CustomEvent('notify'))

Solution

1. Step 1: Recall how to dispatch events in Svelte actions

   Custom events are dispatched using node.dispatchEvent(new CustomEvent('eventName')).

2. Step 2: Check each option

   node.dispatchEvent(new CustomEvent('notify')) matches the correct syntax. Others are either missing new CustomEvent or use invalid syntax.

3. Final Answer:

   node.dispatchEvent(new CustomEvent('notify')) -> Option D

4. Quick Check:

   Dispatch custom event = node.dispatchEvent(new CustomEvent()) [OK]

Hint: Use node.dispatchEvent with new CustomEvent('name') [OK]

Common Mistakes:

• Omitting 'new CustomEvent'
• Calling dispatchEvent without node
• Using incorrect method names

3. Given this Svelte action code, what will be logged when the button is clicked?

function clickAction(node) {
  const handleClick = () => {
    node.dispatchEvent(new CustomEvent('customClick', { detail: 'clicked' }));
  };
  node.addEventListener('click', handleClick);
  return {
    destroy() {
      node.removeEventListener('click', handleClick);
    }
  };
}

<button use:clickAction on:customClick={e => console.log(e.detail)}>Click me</button>

medium

A. 'clicked' will be logged to the console

B. Nothing will be logged

C. An error will occur because 'customClick' is invalid

D. 'click' will be logged instead of 'clicked'

Solution

1. Step 1: Understand the action's event dispatch

   The action dispatches a 'customClick' event with detail 'clicked' when the button is clicked.

2. Step 2: Check the event listener on the button

   The button listens for 'customClick' and logs the event's detail, which is 'clicked'.

3. Final Answer:

   'clicked' will be logged to the console -> Option A

4. Quick Check:

   Custom event detail logged = 'clicked' [OK]

Hint: Custom event detail is logged on event listener [OK]

Common Mistakes:

• Expecting default 'click' event detail
• Thinking event name must be 'click'
• Ignoring event detail property

4. Identify the error in this Svelte action code that dispatches a custom event:

function myAction(node) {
  const onClick = () => {
    node.dispatchEvent(CustomEvent('myEvent'));
  };
  node.addEventListener('click', onClick);
  return {
    destroy() {
      node.removeEventListener('click', onClick);
    }
  };
}

medium

A. destroy method is missing

B. Event listener is added to the wrong node

C. Missing 'new' keyword before CustomEvent

D. Custom event name must be lowercase

Solution

1. Step 1: Check how CustomEvent is created

   The code calls CustomEvent('myEvent') without the new keyword, which is required to create an event instance.

2. Step 2: Verify other parts

   Event listener is correctly added and removed; destroy method exists; event names can be uppercase.

3. Final Answer:

   Missing 'new' keyword before CustomEvent -> Option C

4. Quick Check:

   CustomEvent requires 'new' keyword [OK]

Hint: Always use 'new CustomEvent' to create events [OK]

Common Mistakes:

• Forgetting 'new' before CustomEvent
• Removing event listener incorrectly
• Assuming event names must be lowercase

5. You want to create a Svelte action that dispatches a custom event resize whenever the window is resized. Which of the following is the best way to implement this action?

hard

A. Add a window 'resize' listener inside the action, dispatch 'resize' event from the node, and clean up in destroy

B. Add a 'resize' listener to the node element directly and dispatch event from node

C. Dispatch 'resize' event once when the action is initialized

D. Use a setInterval to check window size and dispatch event repeatedly

Solution

1. Step 1: Understand event source and dispatch

   Window resize events come from the window object, so the listener must be added to window, not the node.

2. Step 2: Implement dispatch and cleanup

   The action should dispatch a custom 'resize' event from the node when window resizes, and remove the listener in destroy to avoid leaks.

3. Final Answer:

   Add a window 'resize' listener inside the action, dispatch 'resize' event from the node, and clean up in destroy -> Option A

4. Quick Check:

   Listen on window, dispatch from node, cleanup in destroy [OK]

Hint: Listen on window, dispatch from node, cleanup in destroy [OK]

Common Mistakes:

• Adding listener to node instead of window
• Not cleaning up event listener
• Dispatching event only once