Action with event dispatching in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Svelte action?

A Svelte action is a reusable function that can be applied to HTML elements to add behavior or side effects when the element is created or updated.

intermediate

How do you dispatch a custom event from a Svelte action?

You create a new CustomEvent and use the element's dispatchEvent method to send it, allowing parent components to listen and react.

intermediate

Why use event dispatching inside a Svelte action?

Event dispatching lets the action communicate with the component or other parts of the app, sending information like user interactions or state changes.

advanced

Show the basic structure of a Svelte action that dispatches a 'custom' event.

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

beginner

How do you listen to a custom event dispatched by an action in a Svelte component?

Use the on:event syntax on the element where the action is applied, for example: <div use:myAction on:custom={handler}>.

What parameter does a Svelte action function receive?

A The component's props

B The component instance

C The event object

D The HTML element node it is applied to

A Svelte action receives the HTML element node it is attached to as its first parameter.

How do you remove event listeners in a Svelte action?

A By returning a destroy function from the action

B By calling removeEventListener outside the action

C By using the onDestroy lifecycle function

D By setting the event listener to null

Returning a destroy function from the action allows cleanup like removing event listeners when the element is removed.

Which method sends a custom event from an element in Svelte?

A dispatchEvent(new CustomEvent('eventName'))

B emit('eventName')

C trigger('eventName')

D sendEvent('eventName')

The standard way is to call dispatchEvent with a new CustomEvent on the element.

How do you apply an action named 'myAction' to a div in Svelte?

A <div on:myAction></div>

B <div action={myAction}></div>

C <div use:myAction></div>

D <div bind:myAction></div>

The correct syntax to apply an action is use:actionName on the element.

What is the purpose of dispatching events from actions?

A To style the element

B To communicate from the action to the component

C To fetch data from the server

D To change the component's props directly

Dispatching events allows the action to send messages or data back to the component or other listeners.

Explain how to create a Svelte action that dispatches a custom event when the user clicks the element.

Think about how to listen to clicks and send events from the element.

Describe how a Svelte component listens to and handles a custom event dispatched by an action.

Remember how events bubble and how Svelte binds event listeners.

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