Crossfade for list items in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an item is removed from a list using Svelte's crossfade?

Consider a Svelte component that uses crossfade to animate list items when they are removed. What is the visible behavior when an item is deleted?

Svelte

import { crossfade } from 'svelte/transition';
const [send, receive] = crossfade({ duration: 400 });

let items = ['apple', 'banana', 'cherry'];

function removeItem(item) {
  items = items.filter(i => i !== item);
}

A The removed item smoothly fades out and moves to the position of the next item before disappearing.

B The removed item instantly disappears without any animation.

C The removed item fades out but does not move; the other items jump to fill the gap immediately.

D The removed item slides out to the right without fading.

💡 Hint

Think about how crossfade combines fading and movement to create smooth transitions.

Svelte's crossfade transition animates the removed item by fading it out while moving it to the position of the incoming item, creating a smooth visual effect.

📝 Syntax

intermediate

Which code snippet correctly applies crossfade to list items in Svelte?

Choose the correct way to apply crossfade transitions to a list of items in Svelte.

Svelte

import { crossfade } from 'svelte/transition';
const [send, receive] = crossfade({ duration: 300 });

let items = ['one', 'two', 'three'];

A <ul>{#each items as item (item)}<li transition:receive>{item}</li>{/each}</ul>

B <ul>{#each items as item (item)}<li transition:send>{item}</li>{/each}</ul>

C <ul>{#each items as item (item)}<li transition:crossfade>{item}</li>{/each}</ul>

D <ul>{#each items as item (item)}<li in:receive out:send>{item}</li>{/each}</ul>

💡 Hint

Remember that crossfade returns two functions: one for items leaving and one for items entering.

You must use in:receive for items entering and out:send for items leaving to properly apply crossfade transitions.

🔧 Debug

advanced

Why does the crossfade animation not trigger when list items change?

Given this Svelte code, the crossfade animation does not run when items are added or removed. What is the most likely cause?

Svelte

import { crossfade } from 'svelte/transition';
const [send, receive] = crossfade({ duration: 500 });

let items = ['a', 'b', 'c'];

// In markup:
// {#each items as item}<div in:receive out:send>{item}</div>{/each}

A The crossfade transition only works with <li> elements, not <div>.

B The crossfade functions are not imported correctly from 'svelte/animate'.

C The list items lack a unique key, so Svelte cannot track them for animation.

D The duration option must be set to 0 for crossfade to work.

💡 Hint

Think about how Svelte tracks list items for transitions.

Svelte requires a unique key in the each block to track items and apply transitions correctly. Without keys, animations won't trigger.

🧠 Conceptual

advanced

What is the role of the 'send' and 'receive' functions in Svelte's crossfade?

In Svelte's crossfade transition, what do the send and receive functions represent?

A They are event handlers for mouse events during the transition.

B <code>send</code> is used for elements leaving the DOM; <code>receive</code> is for elements entering the DOM.

C <code>send</code> is for elements entering the DOM; <code>receive</code> is for elements leaving the DOM.

D Both <code>send</code> and <code>receive</code> are used only for elements entering the DOM.

💡 Hint

Consider the direction of the animation for items leaving and entering.

send handles the animation of elements leaving the DOM, while receive handles elements entering, enabling smooth crossfade effects.

❓ state_output

expert

What is the final list order and animation behavior after swapping two items with crossfade?

Given a Svelte list ['x', 'y', 'z'] with crossfade applied, if you swap the positions of 'x' and 'z' in the array, what will happen?

Svelte

import { crossfade } from 'svelte/transition';
const [send, receive] = crossfade({ duration: 400 });

let items = ['x', 'y', 'z'];

function swap() {
  items = ['z', 'y', 'x'];
}

// In markup:
// {#each items as item (item)}<div in:receive out:send>{item}</div>{/each}

A The items 'x' and 'z' smoothly crossfade to their new positions without disappearing.

B No animation occurs because the items are the same, only order changed.

C The items 'x' and 'z' fade out and fade back in at their new positions separately.

D The entire list fades out and then fades back in with the new order.

💡 Hint

Think about how crossfade matches elements by their keys during reordering.

Crossfade matches items by their keys and animates their movement smoothly to new positions without fading out completely.