CSS-in-JS patterns with Svelte - Mini Project: Build & Apply

CSS-in-JS Patterns with Svelte

📖 Scenario: You are building a simple Svelte component that shows a colored box with text. You want to style the box using CSS-in-JS patterns, meaning the styles are written inside the component file using JavaScript variables and applied dynamically.

🎯 Goal: Create a Svelte component that uses CSS-in-JS patterns to style a box with dynamic colors and padding. The styles should be defined as JavaScript variables and applied inline or via style bindings.

📋 What You'll Learn

Create a Svelte component with a div element representing a box

Define JavaScript variables for backgroundColor and padding

Apply these style variables to the div using Svelte's style binding

Add a text inside the box that says 'Styled Box'

💡 Why This Matters

🌍 Real World

CSS-in-JS patterns help keep styles close to components, making it easier to manage dynamic styling in modern web apps.

💼 Career

Understanding CSS-in-JS with Svelte is useful for frontend developers building interactive, component-based user interfaces.

1

Set up the box element

Create a Svelte component with a div element that contains the text Styled Box.

Svelte

  Styled Box

Hint

Start by writing a simple <div> with the text inside it.

2

Define style variables

Add two JavaScript variables inside a <script> tag: backgroundColor set to "lightblue" and padding set to "1rem".

Svelte

  // Define backgroundColor and padding variables here



  Styled Box

Hint

Use let to declare variables inside the <script> tag.

3

Apply styles using CSS-in-JS pattern

Use Svelte's style binding to apply the backgroundColor and padding variables as inline styles on the div. Use the syntax style="background-color: {backgroundColor}; padding: {padding};".

Svelte

  let backgroundColor = "lightblue";
  let padding = "1rem";



  Styled Box
  

Hint

Use curly braces {} inside the style attribute to insert JavaScript variables.

4

Make the styles reactive

Add a button below the div that changes backgroundColor to "lightcoral" when clicked. Use Svelte's on:click event binding with a function that sets backgroundColor = "lightcoral".

Svelte

  let backgroundColor = "lightblue";
  let padding = "1rem";

  // Add function to change backgroundColor



  Styled Box



  Change Color

Hint

Define a function inside <script> and bind it to the button's on:click event.

Practice

1. What is the main advantage of using CSS-in-JS patterns in Svelte components?

easy

A. It forces all styles to be global across the app.

B. It requires importing external CSS files for every component.

C. It disables scoped styles by default.

D. Styles stay close to the component's markup and logic, improving maintainability.

Solution

1. Step 1: Understand CSS-in-JS purpose in Svelte

   CSS-in-JS means writing styles inside the component file, close to markup and logic.

2. Step 2: Identify the benefit of this approach

   This closeness improves maintainability by keeping related code together and avoiding global style conflicts.

3. Final Answer:

   Styles stay close to the component's markup and logic, improving maintainability. -> Option D

4. Quick Check:

   CSS-in-JS = Styles close to component [OK]

Hint: Remember CSS-in-JS keeps styles inside components [OK]

Common Mistakes:

• Thinking CSS-in-JS requires external CSS files
• Assuming styles become global automatically
• Confusing scoped styles with global styles

2. Which of the following is the correct way to write scoped CSS inside a Svelte component?

easy

A.

B. h1 { color: red; }

C.

D.

Solution

1. Step 1: Identify correct tag for scoped styles in Svelte

   Svelte uses the <style> tag inside components for scoped CSS by default.

2. Step 2: Eliminate incorrect tags

   <style global> makes styles global, <css> and <script> are invalid for CSS.

3. Final Answer:

   <style> h1 { color: red; } </style> -> Option C

4. Quick Check:

   Scoped CSS uses <style> tag [OK]

Hint: Use <style> for scoped CSS, not

Common Mistakes:

• Using
• Writing CSS inside <script> tags
• Using non-existent <css> tags

3. Consider this Svelte component snippet:

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

<div style="color: {color};">Hello</div>

What color will the text "Hello" appear as when rendered?

medium

A. Blue

B. Green

C. Black

D. Red

Solution

1. Step 1: Check the variable value

   The variable 'color' is set to 'blue' in the script.

2. Step 2: Understand inline style binding

   The div's style attribute uses {color}, so it applies 'color: blue;'.

3. Final Answer:

   Blue -> Option A

4. Quick Check:

   Inline style uses variable 'color' = blue [OK]

Hint: Inline style uses JS variable value directly [OK]

Common Mistakes:

• Assuming default black color instead of variable
• Confusing CSS property with variable name
• Forgetting to use curly braces for JS in style

4. Identify the error in this Svelte component CSS-in-JS snippet:

<script>
  let bg = 'yellow';
</script>

<div style="background-color: bg;">Box</div>

medium

A. Variable 'bg' should be declared with const, not let.

B. Missing curly braces around 'bg' in style attribute.

C. CSS property 'background-color' is invalid in Svelte.

D. Style attribute must be written inside <style> tag.

Solution

1. Step 1: Check how JS variables are used in inline styles

   JS variables must be wrapped in curly braces inside style attributes to interpolate values.

2. Step 2: Identify the missing curly braces

   The style uses "background-color: bg;" but should be "background-color: {bg};" to apply the variable.

3. Final Answer:

   Missing curly braces around 'bg' in style attribute. -> Option B

4. Quick Check:

   Use {variable} inside style attribute [OK]

Hint: Wrap JS variables in {} inside style attributes [OK]

Common Mistakes:

• Forgetting curly braces for JS in style
• Thinking CSS properties are invalid in Svelte
• Believing style must be only in <style> tag

5. You want to create a Svelte component that changes text color dynamically based on a prop isError. Which approach correctly applies CSS-in-JS to achieve this?

hard

A. <script>
export let isError = false;
</script>

<p style="color: {isError ? 'red' : 'black'}">Message</p>

B. <script>
export let isError = false;
</script>

<p style="color: isError ? 'red' : 'black';">Message</p>

C. <script>
export let isError = false;
</script>

<style>
.error { color: red; }
.normal { color: black; }
</style>

<p class="normal">Message</p>

D. <script>
export let isError = false;
</script>

<style>
p { color: red; }
</style>

<p>Message</p>

Solution

1. Step 1: Understand dynamic inline style syntax in Svelte

   To use JS expressions inside style attributes, wrap them in curly braces.

2. Step 2: Check each option's style usage

   <script>
     export let isError = false;
   </script>
   
   <style>
     .error { color: red; }
     .normal { color: black; }
   </style>
   
   <p class="normal">Message</p>

   uses CSS classes but statically applies class='normal', ignoring the isError prop.

   <script>
     export let isError = false;
   </script>
   
   <p style="color: isError ? 'red' : 'black';">Message</p>

   misses curly braces around JS expression.

   <script>
     export let isError = false;
   </script>
   
   <p style="color: {isError ? 'red' : 'black'}">Message</p>

   correctly uses curly braces with a JS ternary expression inside style attribute.

   <script>
     export let isError = false;
   </script>
   
   <style>
     p { color: red; }
   </style>
   
   <p>Message</p>

   applies static red color always.

3. Final Answer:

   <p style="color: {isError ? 'red' : 'black'}">Message</p> -> Option A

4. Quick Check:

   Dynamic style needs {JS expression} inside style attribute [OK]

Hint: Use {condition ? 'red' : 'black'} inside style attribute [OK]

Common Mistakes:

• Forgetting curly braces for JS in style attribute
• Using static classes instead of dynamic styles
• Applying styles without condition logic