Use directive syntax in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Svelte component?

Consider this Svelte code using the use: directive. What will be the text content of the <div> after the component mounts?

Svelte

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

<div use:highlight>Hello</div>

A The div text will be 'Hello' with a yellow background.

B The div text will be 'Hello' with no background color change.

C The div will be empty with a yellow background.

D The component will throw a runtime error.

💡 Hint

The use: directive runs the function with the element as argument after mount.

The use:highlight directive calls the highlight function with the div node, which sets its background color to yellow. The text remains 'Hello'.

📝 Syntax

intermediate

Which option correctly uses the use: directive to add a tooltip?

You want to add a tooltip to a button using a use: directive. Which code snippet is correct?

A <script>function tooltip(node) { node.title = 'Click me'; }</script><button use={tooltip}>Press</button>

B <script>function tooltip(node) { node.title = 'Click me'; }</script><button use-tooltip>Press</button>

C <script>function tooltip(node) { node.title = 'Click me'; }</script><button use:tooltip>Press</button>

D <script>function tooltip(node) { node.title = 'Click me'; }</script><button use:tooltip()>Press</button>

💡 Hint

The correct syntax is use:action without parentheses.

The use: directive requires the function name without parentheses or braces. Option C uses use:tooltip correctly.

🔧 Debug

advanced

Why does this use: directive not update when the parameter changes?

Look at this Svelte code. The use:color directive sets the background color. Why does the background not update when color changes?

Svelte

<script>
  export let color = 'red';
  function colorAction(node) {
    node.style.backgroundColor = color;
  }
</script>

<div use:colorAction>Box</div>

<button on:click={() => color = 'blue'}>Change Color</button>

A The button click does not update <code>color</code> because the event handler is missing parentheses.

B The <code>color</code> variable is not reactive because it is not declared with <code>let</code>.

C The <code>use:</code> directive syntax is incorrect and causes the update to fail.

D The action function does not react to changes in <code>color</code> because it runs only once on mount.

💡 Hint

Actions run once when the element is created and do not rerun automatically on variable changes.

The colorAction function runs once when the element mounts. It does not rerun when color changes. To update, the action must handle parameter changes explicitly.

🧠 Conceptual

advanced

What is the purpose of the return value in a use: action function?

In Svelte, a use: action function can return an object with an update and/or destroy method. What is the main purpose of these methods?

A <code>update</code> allows the action to react to parameter changes; <code>destroy</code> cleans up when the element is removed.

B <code>update</code> reruns the entire component; <code>destroy</code> resets all variables.

C <code>update</code> is called only once on mount; <code>destroy</code> is never called.

D <code>update</code> changes the component's props; <code>destroy</code> reloads the page.

💡 Hint

Think about how actions can respond to changes and clean up resources.

The update method lets the action respond when parameters change. The destroy method cleans up event listeners or other resources when the element is removed.

❓ state_output

expert

What is the final background color after clicking the button twice?

Analyze this Svelte component using a use: directive with parameters. What is the background color of the <div> after clicking the button twice?

Svelte

<script>
  import { writable } from 'svelte/store';
  const colors = ['red', 'green', 'blue'];
  let index = 0;
  let currentColor = colors[index];

  function colorAction(node, color) {
    node.style.backgroundColor = color;
    return {
      update(newColor) {
        node.style.backgroundColor = newColor;
      }
    };
  }

  function changeColor() {
    index = (index + 1) % colors.length;
    currentColor = colors[index];
  }
</script>

<div use:colorAction={currentColor}>Color Box</div>
<button on:click={changeColor}>Change Color</button>

A The background color will not change from the initial color.

B The background color will be 'blue'.

C The background color will be 'red'.

D The background color will be 'green'.

💡 Hint

Each click updates currentColor and triggers the action's update method.

Initial color is 'red'. First click changes to 'green', second click changes to 'blue'. After two clicks, the color is 'blue'.