Component events (createEventDispatcher) in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the child component dispatches an event?

Consider a Svelte child component that uses createEventDispatcher to send a 'notify' event with a message. The parent listens to this event. What will the parent display after the child dispatches?

Svelte

/* Child.svelte */
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();
  function send() {
    dispatch('notify', { message: 'Hello from child' });
  }
</script>
<button on:click={send}>Send</button>

<!-- Parent.svelte -->
<script>
  import Child from './Child.svelte';
  let msg = '';
  function handleNotify(event) {
    msg = event.detail.message;
  }
</script>
<Child on:notify={handleNotify} />
<p>{msg}</p>

A <p>Hello from child</p>

B <p>undefined</p>

C <p></p>

D Runtime error: event.detail is undefined

💡 Hint

Remember that dispatch sends an event with a detail property containing the data.

The child dispatches a 'notify' event with { message: 'Hello from child' }. The parent handler receives the event and sets msg to event.detail.message, so the paragraph shows 'Hello from child'.

📝 Syntax

intermediate

Which option correctly dispatches a custom event with data in Svelte?

Choose the correct syntax to dispatch a 'change' event with a payload { value: 42 } using createEventDispatcher.

Svelte

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

// Which line correctly dispatches the event?

A dispatch('change', [value: 42]);

B dispatch('change', value: 42);

C dispatch('change', value = 42);

D dispatch('change', { value: 42 });

💡 Hint

The second argument to dispatch must be an object with the data.

Option D correctly passes an object with the property value. Other options have invalid syntax or wrong argument types.

🔧 Debug

advanced

Why does the parent not receive the dispatched event?

A child component dispatches a 'submit' event, but the parent never reacts. What is the likely cause?

Svelte

<!-- Child.svelte -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();
  function submit() {
    dispatch('submit', { data: 123 });
  }
</script>
<button on:click={submit}>Submit</button>

<!-- Parent.svelte -->
<script>
  import Child from './Child.svelte';
  let received = false;
  function onSubmit() {
    received = true;
  }
</script>
<Child on:submit={onSubmit} />
<p>{received ? 'Event received' : 'Waiting...'}</p>

A The child dispatches 'submit' but the parent component is missing the import of Child.

B The parent uses 'on:submit' but the child dispatches 'submit' event correctly; issue is elsewhere.

C The child dispatches 'submit' but the parent listens to 'onsubmit' (wrong event name).

D The parent event handler 'onSubmit' does not accept the event parameter, causing failure.

💡 Hint

Check if the parent component properly imports and uses the child component.

If the parent does not import or include the child component correctly, the event will never be dispatched to the parent. The event name and handler are correct, so the missing import is the likely cause.

❓ state_output

advanced

What is the value of count after dispatching an event?

In this Svelte parent component, the child dispatches an 'increment' event. The parent updates count accordingly. What is the final value of count after clicking the button once?

Svelte

<!-- Child.svelte -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();
  function inc() {
    dispatch('increment', { amount: 5 });
  }
</script>
<button on:click={inc}>Add 5</button>

<!-- Parent.svelte -->
<script>
  import Child from './Child.svelte';
  let count = 0;
  function handleIncrement(event) {
    count += event.detail.amount;
  }
</script>
<Child on:increment={handleIncrement} />
<p>{count}</p>

A NaN

B 0

C 5

D undefined

💡 Hint

The event detail contains the amount to add to count.

The child dispatches 'increment' with { amount: 5 }. The parent adds this to count, so after one click, count is 5.

🧠 Conceptual

expert

Why use createEventDispatcher instead of props for child-to-parent communication?

In Svelte, what is the main advantage of using createEventDispatcher for child-to-parent communication compared to passing callback functions as props?

A Props cannot pass functions in Svelte, so events are the only way.

B Events allow decoupling so the child does not need to know about the parent's implementation details.

C Using events is faster because props cause unnecessary re-renders.

D Events automatically update the parent's state without handlers.

💡 Hint

Think about how events help keep components independent.

Using createEventDispatcher lets the child send messages without knowing what the parent does with them. This keeps components loosely connected and easier to maintain. Props with callbacks require the child to know about the parent's functions.

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