Default actions in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a default action returns false?

In Svelte, if a default action returns false, what is the effect on the event's default behavior?

Svelte

<script>
  function preventDefaultAction(node) {
    node.addEventListener('click', event => {
      event.preventDefault();
    });
    return false;
  }
</script>

<button use:preventDefaultAction on:click={() => alert('Clicked!')}>Click me</button>

A The code causes a runtime error because returning false is invalid.

B The button's default click behavior is not prevented, and the alert shows.

C The button's default click behavior is prevented, and the alert does not show.

D The button's default click behavior is prevented, but the alert still shows.

💡 Hint

Think about what returning false from a default action means in Svelte.

In Svelte, returning false from a default action does not automatically prevent the event's default behavior. You must explicitly call event.preventDefault() inside the event listener. Returning false alone has no effect.

📝 Syntax

intermediate

Which syntax correctly defines a default action in Svelte?

Choose the correct way to define and use a default action named highlight that adds a yellow background to an element.

A

<script>
  export default function highlight(node) {
    node.style.backgroundColor = 'yellow';
  }
</script>

<p use:highlight>This is highlighted</p>

B

<script>
  function highlight(node) {
    node.style.backgroundColor = 'yellow';
  }
</script>

<p use:highlight>This is highlighted</p>

C

<script>
  export function highlight(node) {
    node.style.backgroundColor = 'yellow';
  }
</script>

<p use:highlight>This is highlighted</p>

D

<script>
  export const highlight = (node) => {
    node.style.backgroundColor = 'yellow';
  };
</script>

<p use:highlight>This is highlighted</p>

💡 Hint

Default actions must be exported functions.

Default actions in Svelte must be exported functions so they can be used with use:. Option C correctly exports the function. Option B is not exported, so it won't work. Option A exports a default function, which is not how default actions are imported. Option D exports a constant function, which is valid syntax and recognized as an action.

🔧 Debug

advanced

Why does this default action not clean up event listeners?

Examine the default action below. Why might it cause a memory leak or unexpected behavior after the component is destroyed?

Svelte

export function trackClicks(node) {
  function onClick() {
    console.log('Clicked');
  }
  node.addEventListener('click', onClick);
}

A Because the event listener is added inside the function instead of outside.

B Because the action does not return a destroy function to remove the event listener.

C Because the function is not exported as default.

D Because the event listener callback is not an arrow function.

💡 Hint

Think about how Svelte cleans up actions when elements are removed.

Svelte expects default actions to return an object with a destroy method that cleans up any side effects like event listeners. Without this, the event listener remains attached even after the element is removed, causing memory leaks or unexpected behavior.

❓ state_output

advanced

What is the output when a default action updates a reactive variable?

Consider this Svelte component using a default action that increments a count on click. What is displayed after clicking the button twice?

Svelte

<script>
  import { onMount } from 'svelte';
  let count = 0;

  export function increment(node) {
    function onClick() {
      count += 1;
    }
    node.addEventListener('click', onClick);
    return {
      destroy() {
        node.removeEventListener('click', onClick);
      }
    };
  }
</script>

<button use:increment>Clicked {count} times</button>

A Clicked 0 times

B Clicked 1 times

C Clicked NaN times

D Clicked 2 times

💡 Hint

Consider if the default action can update component variables directly.

The default action runs outside Svelte's reactivity system. Updating count inside the action does not trigger reactive updates because count is not in scope inside the action function. The displayed count remains 0.

🧠 Conceptual

expert

Why use default actions instead of inline event handlers in Svelte?

Which reason best explains why default actions are preferred over inline event handlers for certain behaviors in Svelte?

A Default actions run faster than inline event handlers because they are compiled differently.

B Inline event handlers cannot access component variables, but default actions can.

C Default actions can encapsulate setup and cleanup logic tied to the element lifecycle, which inline handlers cannot.

D Default actions automatically prevent default browser behavior, inline handlers do not.

💡 Hint

Think about lifecycle and cleanup responsibilities.

Default actions allow you to run code when an element is created and clean up when it is destroyed. Inline event handlers only respond to events but do not manage setup or teardown. This makes default actions ideal for behaviors that need lifecycle management.