Component events (createEventDispatcher) in Svelte

Introduction

Component events let parts of your app talk to each other. createEventDispatcher helps a child component send messages to its parent.

You want a button component to tell its parent when it is clicked.

A form component needs to send the entered data back to the main app.

A custom input component should notify the parent when its value changes.

You want to keep components separate but still let them communicate easily.

Syntax

Svelte

import { createEventDispatcher } from 'svelte';

const dispatch = createEventDispatcher();

dispatch('eventName', detailObject);

Always import createEventDispatcher from 'svelte'.

Use dispatch to send an event with a name and optional data.

Examples

Sends a simple event named 'hello' without extra data.

Svelte

import { createEventDispatcher } from 'svelte';

const dispatch = createEventDispatcher();

dispatch('hello');

Sends an event 'greet' with a user object as data.

Svelte

import { createEventDispatcher } from 'svelte';

const dispatch = createEventDispatcher();

const user = { name: 'Anna' };
dispatch('greet', user);

Sample Program

The child component sends a 'message' event with text when the button is clicked. The parent listens for this event and shows the message.

Svelte

<!-- Child.svelte -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();

  function sendMessage() {
    dispatch('message', { text: 'Hello from child!' });
  }
</script>

<button on:click={sendMessage} aria-label="Send message to parent">Send Message</button>


<!-- Parent.svelte -->
<script>
  import Child from './Child.svelte';

  let received = '';

  function handleMessage(event) {
    received = event.detail.text;
  }
</script>

<main>
  <Child on:message={handleMessage} />
  <p>Received message: {received}</p>
</main>

Important Notes

Event names are case-sensitive and usually lowercase.

Use event.detail in the parent to get data sent by the child.

Always add accessible labels like aria-label for buttons.

Summary

createEventDispatcher lets child components send events to parents.

Use dispatch('eventName', data) to send messages.

Parents listen with on:eventName and get data from event.detail.

Practice

1. What is the main purpose of createEventDispatcher in a Svelte component?

easy

A. To manage component state internally

B. To create a new component dynamically

C. To send custom events from a child component to its parent

D. To style components with CSS variables

Solution

1. Step 1: Understand the role of createEventDispatcher

   This function is used to create a dispatcher that sends events from child to parent components.

2. Step 2: Identify the correct purpose

   Among the options, only sending custom events matches the purpose of createEventDispatcher.

3. Final Answer:

   To send custom events from a child component to its parent -> Option C

4. Quick Check:

   createEventDispatcher sends events to parent [OK]

Hint: Remember: dispatch sends events upward to parent [OK]

Common Mistakes:

• Confusing event dispatch with state management
• Thinking it creates components
• Assuming it styles components

2. Which of the following is the correct way to import and use createEventDispatcher in a Svelte component?

easy

A. import { createEventDispatcher } from 'svelte'; const dispatch = createEventDispatcher();

B. import createEventDispatcher from 'svelte'; const dispatch = createEventDispatcher();

C. import { createEventDispatcher } from 'svelte/internal'; const dispatch = createEventDispatcher();

D. import { dispatchEvent } from 'svelte'; const dispatch = dispatchEvent();

Solution

1. Step 1: Check the import source

   The correct import is from 'svelte' using named import syntax.

2. Step 2: Confirm usage

   Calling createEventDispatcher() returns the dispatch function to send events.

3. Final Answer:

   import { createEventDispatcher } from 'svelte'; const dispatch = createEventDispatcher(); -> Option A

4. Quick Check:

   Correct import and call syntax = import { createEventDispatcher } from 'svelte'; const dispatch = createEventDispatcher(); [OK]

Hint: Always import from 'svelte' with curly braces [OK]

Common Mistakes:

• Importing from 'svelte/internal' which is private
• Using default import instead of named import
• Confusing dispatchEvent with createEventDispatcher

3. Given this Svelte child component code:

import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher();

function sendData() {
  dispatch('update', { value: 42 });
}

What will the parent component receive when listening with on:update?

medium

A. An event with event.detail containing { value: 42 }

B. An event with event.value equal to 42

C. A string 'update' without data

D. No event is sent because dispatch is not called

Solution

1. Step 1: Understand dispatch call

   The dispatch function sends an event named 'update' with data inside an object as the second argument.

2. Step 2: Recall event data access

   The parent receives the event and accesses data via event.detail, which holds the object { value: 42 }.

3. Final Answer:

   An event with event.detail containing { value: 42 } -> Option A

4. Quick Check:

   Data sent in event.detail = An event with event.detail containing { value: 42 } [OK]

Hint: Event data is always inside event.detail [OK]

Common Mistakes:

• Expecting event.value instead of event.detail
• Thinking dispatch sends only event name string
• Assuming dispatch is not called

4. Identify the error in this Svelte child component code:

import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher;

dispatch('click');

medium

A. dispatch cannot send events without a second argument

B. dispatch should be imported from 'svelte/internal'

C. dispatch must be called inside a function

D. Missing parentheses when calling createEventDispatcher

Solution

1. Step 1: Check createEventDispatcher usage

   The code assigns dispatch to createEventDispatcher without calling it, missing parentheses.

2. Step 2: Understand consequences

   Without calling createEventDispatcher(), dispatch is not a function, so dispatch('click') causes an error.

3. Final Answer:

   Missing parentheses when calling createEventDispatcher -> Option D

4. Quick Check:

   createEventDispatcher() must be called [OK]

Hint: Always call createEventDispatcher() with () [OK]

Common Mistakes:

• Forgetting parentheses after createEventDispatcher
• Importing from wrong module
• Thinking second argument is mandatory

5. You want a child Svelte component to send two different events, save and cancel, each with different data. Which is the best way to implement this using createEventDispatcher?

hard

A. Create two dispatchers by calling createEventDispatcher() twice, one for each event

B. Create one dispatcher and call dispatch('save', data) or dispatch('cancel', data) as needed

C. Use dispatch('save_cancel', {saveData, cancelData}) to send both events at once

D. Send events by calling dispatchEvent directly on the DOM element

Solution

1. Step 1: Understand dispatcher usage

   One dispatcher handles all custom events; you specify event names when calling dispatch.

2. Step 2: Evaluate options

   Calling dispatch with different event names ('save' or 'cancel') is the correct pattern.

3. Final Answer:

   Create one dispatcher and call dispatch('save', data) or dispatch('cancel', data) as needed -> Option B

4. Quick Check:

   Single dispatcher handles multiple events [OK]

Hint: One dispatcher can send many event types by name [OK]

Common Mistakes:

• Creating multiple dispatchers unnecessarily
• Sending combined event instead of separate
• Using DOM dispatchEvent instead of Svelte dispatch