Dynamic inline styles in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the rendered background color?

Consider this Svelte component that sets a dynamic background color based on a variable. What background color will the

have when rendered?

Svelte

<script>
  let color = 'blue';
</script>

<div style="background-color: {color}; width: 100px; height: 100px;"></div>

A The div will have a transparent background color.

B The div will have a red background color.

C The div will have a blue background color.

D The div will have no background color because the style syntax is invalid.

💡 Hint

Look at how the variable 'color' is used inside the style attribute with curly braces.

In Svelte, you can use curly braces inside attribute values to insert JavaScript expressions. Here, {color} inserts the string 'blue', so the div's background color becomes blue.

📝 Syntax

intermediate

Which option correctly applies a dynamic font size?

You want to set the font size dynamically in a Svelte component using a variable 'size' that holds a number representing pixels. Which option correctly applies this style?

Svelte

let size = 24;

A <p style="font-size: size + 'px';">Text</p>

B <p style="font-size: {size}px;">Text</p>

C <p style="font-size: '{size}px';">Text</p>

D <p style="font-size: {size};">Text</p>

💡 Hint

Remember to use curly braces to insert variables and include units like 'px' as part of the string.

Option B correctly uses curly braces to insert the variable 'size' and appends 'px' outside the braces. This results in a valid CSS value like '24px'. Other options either treat the variable as a string or omit units.

❓ state_output

advanced

What is the background color after clicking the button twice?

This Svelte component toggles the background color between 'red' and 'green' each time the button is clicked. What is the background color after two clicks?

Svelte

<script>
  let isRed = true;
  function toggle() {
    isRed = !isRed;
  }
</script>

<div style="background-color: {isRed ? 'red' : 'green'}; width: 100px; height: 100px;"></div>
<button on:click={toggle}>Toggle Color</button>

A The background color is red.

B The background color is green.

C The background color is blue.

D The background color does not change.

💡 Hint

Each click flips the boolean 'isRed'. Think about the initial value and how it changes after two toggles.

Initially, isRed is true, so background is red. After one click, isRed becomes false (green). After the second click, isRed flips back to true (red). So after two clicks, background is red again.

🔧 Debug

advanced

Why does this dynamic style not update on variable change?

This Svelte component tries to update the text color dynamically, but the color does not change when the button is clicked. What is the cause?

Svelte

<script>
  let color = 'black';
  function changeColor() {
    color = 'blue';
  }
</script>

<p style="color: {color};">Hello</p>
<button on:click={changeColor}>Change Color</button>

A The variable 'color' is not declared with let.

B Svelte does not support dynamic inline styles.

C The on:click event is not correctly attached to the button.

D The style uses quotes around 'color', so it is treated as a string, not a variable.

💡 Hint

Check how the variable is used inside the style attribute.

Putting 'color' in quotes inside the style attribute makes it a literal string, so the style is always 'color' instead of the variable's value. Removing the quotes and using {color} fixes the issue.

🧠 Conceptual

expert

How does Svelte optimize dynamic inline styles?

Svelte compiles components to efficient JavaScript. How does it handle dynamic inline styles to minimize DOM updates?

A Svelte updates only the changed style properties on the element instead of replacing the entire style attribute.

B Svelte re-renders the entire component whenever any style changes.

C Svelte uses inline CSS classes instead of style attributes for dynamic styles.

D Svelte does not optimize dynamic styles and updates the whole style attribute every time.

💡 Hint

Think about how frameworks avoid unnecessary DOM changes for better performance.

Svelte generates code that updates only the specific style properties that changed, avoiding full attribute replacements. This reduces DOM operations and improves performance.