Action with event dispatching in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the event dispatcher from Svelte.

Svelte

import { [1] } from 'svelte';

Drag options to blanks, or click blank then click option'

AcreateEventDispatcher

BuseEvent

CdispatchEvent

DeventDispatcher

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong import name like 'dispatchEvent' which is a DOM method.

Trying to import something not exported by Svelte.

The createEventDispatcher function is imported from Svelte to create a dispatcher for custom events.

2fill in blank

medium

Complete the code to create the event dispatcher inside the action function.

Svelte

function myAction(node) {
  const dispatch = [1]();
  // action logic
}

Drag options to blanks, or click blank then click option'

AdispatchEvent

BcreateEventDispatcher

CcreateEventDispatcher()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses twice like 'createEventDispatcher()()'.

Using 'dispatchEvent' which is not the Svelte dispatcher.

You call createEventDispatcher (without parentheses here) to get the dispatcher function inside the action.

3fill in blank

hard

Fix the error in dispatching a custom event named 'custom' with detail data.

Svelte

dispatch('[1]', { detail: { message: 'Hello' } });

Drag options to blanks, or click blank then click option'

Aevent

Bdispatch

Ccustom

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of the event name string.

Passing the detail object as the first argument.

The event name should be a string that matches the custom event you want to dispatch, here 'custom'.

4fill in blank

hard

Fill both blanks to dispatch an event named 'update' with a payload containing count.

Svelte

dispatch([1], { [2]: count });

Drag options to blanks, or click blank then click option'

A'update'

Bdetail

Ccount

D'count'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the payload directly without wrapping in 'detail'.

Using count as the event name.

The event name is 'update' as a string, and the payload must be inside a detail object.

5fill in blank

hard

Fill all three blanks to create an action that dispatches 'change' event with newValue when the node is clicked.

Svelte

function changeAction(node) {
  const dispatch = [1]();
  node.addEventListener('click', () => {
    dispatch([2], { [3]: newValue });
  });
  return {
    destroy() {
      node.removeEventListener('click', () => {});
    }
  };
}

Drag options to blanks, or click blank then click option'

AcreateEventDispatcher

B'change'

Cdetail

DdispatchEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'dispatchEvent' instead of 'createEventDispatcher'.

Not wrapping the payload in 'detail'.

Forgetting to remove the event listener properly.

First, create the dispatcher with createEventDispatcher. Then dispatch the 'change' event with the payload inside detail.

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