Conditional classes (class:name) in Svelte - Mini Project: Build & Apply

Conditional classes (class:name) in Svelte

📖 Scenario: You are building a simple to-do list app. Each task can be marked as done or not done. You want to show done tasks with a different style.

🎯 Goal: Create a Svelte component that shows a list of tasks. Use conditional classes with class:done to style tasks that are done.

📋 What You'll Learn

Create a list of tasks with exact names and done status

Create a variable to track the minimum number of done tasks to highlight

Use a for loop to display tasks

Use class:done to conditionally add a CSS class based on task status

💡 Why This Matters

🌍 Real World

Conditional classes are used in real apps to change styles dynamically, like marking completed tasks or highlighting active menu items.

💼 Career

Knowing how to use conditional classes in Svelte is important for building interactive user interfaces that respond to user actions.

1

Create the tasks list

Create a variable called tasks that is an array of objects with these exact entries: { name: 'Buy groceries', done: false }, { name: 'Clean room', done: true }, { name: 'Pay bills', done: false }.

Svelte

// Create the tasks array with the exact objects
// Your code here

Hint

Use let tasks = [ ... ] with objects inside the array.

2

Add a highlight threshold variable

Create a variable called highlightDoneCount and set it to 1 to represent the minimum number of done tasks to highlight.

Svelte

let tasks = [
  { name: 'Buy groceries', done: false },
  { name: 'Clean room', done: true },
  { name: 'Pay bills', done: false }
];
// Create highlightDoneCount variable and set it to 1
// Your code here

Hint

Use let highlightDoneCount = 1; to create the variable.

3

Display tasks with conditional classes

Use a {#each tasks as task} block to display each task's name inside a <li>. Add a conditional class done using class:done={task.done} on the <li>.

Svelte

let tasks = [
  { name: 'Buy groceries', done: false },
  { name: 'Clean room', done: true },
  { name: 'Pay bills', done: false }
];

let highlightDoneCount = 1;


   -->
  // Your code here

Hint

Use {#each tasks as task} and inside it write <li class:done={task.done}>{task.name}</li>.

4

Add CSS for the done class

Add a <style> block with CSS that styles the .done class to have text-decoration: line-through; and color: gray;.

Svelte

let tasks = [
  { name: 'Buy groceries', done: false },
  { name: 'Clean room', done: true },
  { name: 'Pay bills', done: false }
];

let highlightDoneCount = 1;


  {#each tasks as task}
    {task.name}
  {/each}



// Your code here

Hint

Write a <style> block with CSS for the .done class.

Practice

1. What does the Svelte syntax class:active={isActive} do in a component?

easy

A. Always adds the CSS class 'active' regardless of isActive value.

B. Adds the CSS class 'active' only if the variable isActive is true.

C. Removes the CSS class 'active' when isActive is true.

D. Adds the CSS class 'active' only if isActive is false.

Solution

1. Step 1: Understand the syntax class:name={condition}

   This syntax adds the CSS class name only when the condition is true.

2. Step 2: Apply to class:active={isActive}

   The class active is added only if isActive is true; otherwise, it is not added.

3. Final Answer:

   Adds the CSS class 'active' only if the variable isActive is true. -> Option B

4. Quick Check:

   Conditional class = Adds class if true [OK]

Hint: class:name adds class only when condition is true [OK]

Common Mistakes:

• Thinking class:name adds class always
• Confusing true and false conditions
• Assuming it removes class when true

2. Which of the following is the correct syntax to conditionally add a CSS class highlight in Svelte when isHighlighted is true?

easy

A. class:highlight = {isHighlighted}

B. class.highlight={isHighlighted}

C. class:highlight={isHighlighted}

D. class={highlight: isHighlighted}

Solution

1. Step 1: Recall correct Svelte conditional class syntax

   The correct syntax is class:name={condition} without spaces around the equals sign.

2. Step 2: Check each option

   class:highlight={isHighlighted} matches the correct syntax exactly: class:highlight={isHighlighted}. Others have syntax errors or invalid formats.

3. Final Answer:

   class:highlight={isHighlighted} -> Option C

4. Quick Check:

   Correct syntax = class:name={condition} [OK]

Hint: No spaces around = in class:name={condition} [OK]

Common Mistakes:

• Adding spaces around = sign
• Using dot instead of colon
• Trying object syntax inside class attribute

3. Given this Svelte code snippet:

<script>
  let isError = false;
</script>

<div class:error={isError} class:success={!isError}>Status</div>

What CSS classes will the <div> have when isError is false?

medium

A. No classes added

B. Only the class error

C. Both classes error and success

D. Only the class success

Solution

1. Step 1: Evaluate class:error={isError} when isError is false

   Since isError is false, the class error is not added.

2. Step 2: Evaluate class:success={!isError} when isError is false

   The expression !isError is true, so the class success is added.

3. Final Answer:

   Only the class success -> Option D

4. Quick Check:

   Negation adds success class when isError is false [OK]

Hint: Check condition true to add class, false to skip [OK]

Common Mistakes:

• Adding both classes regardless of condition
• Confusing negation operator !
• Assuming false adds class

4. Identify the error in this Svelte code:

<script>
  let isActive = true;
</script>

<button class:active = {isActive}>Click Me</button>

medium

A. There should be no spaces around the equals sign in class:active = {isActive}.

B. The class: directive cannot be used on a <button> element.

C. The class name cannot be active.

D. The variable isActive is not declared.

Solution

1. Step 1: Check variable declaration

   The variable isActive is declared correctly in the script block.

2. Step 2: Check syntax of conditional class

   The syntax class:active = {isActive} has spaces around the equals sign, which is invalid in Svelte. It should be class:active={isActive} without spaces.

3. Final Answer:

   There should be no spaces around the equals sign in class:active = {isActive}. -> Option A

4. Quick Check:

   No spaces around = in class:name={condition} [OK]

Hint: No spaces around = in class:name={condition} [OK]

Common Mistakes:

• Adding spaces around = sign
• Thinking class:name can't be on button
• Missing variable declaration

5. You want to add two conditional classes visible and highlight to a <div> in Svelte. visible should be added when isVisible is true, and highlight only when isHighlighted is also true and isVisible is true.

Which of the following is the correct way to write this?

hard

A. <div class:visible={isVisible} class:highlight={isHighlighted && isVisible}>Content</div>

B. <div class:visible={isVisible} class:highlight={isHighlighted || isVisible}>Content</div>

C. <div class:visible={isVisible && isHighlighted} class:highlight={isVisible}>Content</div>

D. <div class:visible={isVisible} class:highlight={isHighlighted}>Content</div>

Solution

1. Step 1: Understand conditions for each class

   The class visible depends only on isVisible. The class highlight depends on both isHighlighted and isVisible being true.

2. Step 2: Check each option's logic

   <div class:visible={isVisible} class:highlight={isHighlighted && isVisible}>Content</div>

   correctly uses class:visible={isVisible} and class:highlight={isHighlighted && isVisible}, matching the requirements.

   <div class:visible={isVisible} class:highlight={isHighlighted || isVisible}>Content</div>

   uses OR which is incorrect.

   <div class:visible={isVisible && isHighlighted} class:highlight={isVisible}>Content</div>

   swaps conditions incorrectly.

   <div class:visible={isVisible} class:highlight={isHighlighted}>Content</div>

   misses the isVisible condition for highlight.

3. Final Answer:

   <div class:visible={isVisible} class:highlight={isHighlighted && isVisible}>Content</div> -> Option A

4. Quick Check:

   Use && to combine conditions for conditional classes [OK]

Hint: Use && to combine multiple conditions in class:name [OK]

Common Mistakes:

• Using || instead of && for combined conditions
• Swapping conditions between classes
• Omitting one condition in combined class