Conditional styling patterns in NextJS

Introduction

Conditional styling lets you change how things look based on some condition. It helps make your app more interactive and user-friendly.

Change button color when it is clicked or disabled.

Highlight a menu item when it is active or selected.

Show error styles on a form input when the user enters wrong data.

Make text bold or different color based on user preferences.

Hide or show parts of the page with different styles depending on screen size or user actions.

Syntax

NextJS

const className = condition ? 'class-true' : 'class-false';

// In JSX
<div className={className}>Content</div>

Use JavaScript expressions inside curly braces {} in JSX to apply styles conditionally.

You can combine multiple conditions using template literals or libraries like clsx for cleaner code.

Examples

This example changes the button background and text color based on isActive.

NextJS

const isActive = true;
const className = isActive ? 'bg-blue-500 text-white' : 'bg-gray-200 text-black';

return <button className={className}>Click me</button>;

This example adds extra styles when the button is disabled, making it look faded and unclickable.

NextJS

const isDisabled = false;

return <button className={`btn ${isDisabled ? 'opacity-50 cursor-not-allowed' : ''}`}>Submit</button>;

This example uses the clsx library to add classes based on multiple conditions cleanly.

NextJS

import clsx from 'clsx';

const isError = true;
const isFocused = false;

return <input className={clsx('input', { 'border-red-500': isError, 'border-blue-500': isFocused })} />;

Sample Program

This Next.js component shows a button that toggles between ON and OFF states. The button color changes green when ON and gray when OFF. It also uses accessible attributes and focus styles for keyboard users.

NextJS

import { useState } from 'react';

export default function ToggleButton() {
  const [isOn, setIsOn] = useState(false);

  return (
    <button
      aria-pressed={isOn}
      onClick={() => setIsOn(!isOn)}
      className={`px-4 py-2 rounded transition-colors duration-300 focus:outline-none focus:ring-2 focus:ring-offset-2 ${isOn ? 'bg-green-600 text-white' : 'bg-gray-300 text-black'}`}
    >
      {isOn ? 'ON' : 'OFF'}
    </button>
  );
}

Important Notes

Always use semantic HTML and ARIA attributes like aria-pressed for buttons that toggle states.

Use transition classes or CSS to smoothly animate style changes for better user experience.

Consider accessibility: ensure color contrast is good and keyboard focus styles are visible.

Summary

Conditional styling changes how elements look based on state or conditions.

Use JavaScript expressions inside JSX to apply different classes or styles.

Combine multiple conditions cleanly with template literals or helper libraries like clsx.

Practice

1. In Next.js, which is the simplest way to apply a CSS class conditionally based on a boolean isActive inside a component's JSX?

easy

A. <div className='active' if={isActive}>Content</div>

B. <div className={isActive ? 'active' : ''}>Content</div>

C. <div className={if isActive then 'active'}>Content</div>

D. <div className={isActive && 'active' || 'inactive'}>Content</div>

Solution

1. Step 1: Understand JSX conditional class syntax

   In JSX, you can use JavaScript expressions inside curly braces. The ternary operator condition ? valueIfTrue : valueIfFalse is a common way to apply classes conditionally.

2. Step 2: Evaluate each option's syntax

   <div className={isActive ? 'active' : ''}>Content</div> uses the ternary operator correctly. Options B and C use invalid JSX syntax. <div className={isActive && 'active' || 'inactive'}>Content</div> uses a logical expression that can cause unexpected results.

3. Final Answer:

   <div className={isActive ? 'active' : ''}>Content</div> -> Option B

4. Quick Check:

   Use ternary operator for conditional classes = A [OK]

Hint: Use ternary inside className for simple true/false styling [OK]

Common Mistakes:

• Using invalid JSX syntax like 'if' inside attributes
• Confusing logical AND/OR for conditional classes
• Forgetting to use curly braces for JS expressions

2. Which of the following is the correct syntax to conditionally add multiple CSS classes in Next.js JSX using template literals when isActive and isDisabled are booleans?

easy

A. className={['btn', isActive && 'active', isDisabled && 'disabled'].join(' ')}

B. className='btn' + (isActive ? ' active' : '') + (isDisabled ? ' disabled' : '')

C. className={`btn ${isActive && 'active'} ${isDisabled && 'disabled'}`}

D. className={btn isActive ? 'active' : '' isDisabled ? 'disabled' : ''}

Solution

1. Step 1: Understand template literals with conditional classes

   Template literals use backticks and allow embedding expressions with ${}. Using logical AND returns the class name if true or false (which renders as the string 'false') if false.

2. Step 2: Check each option's syntax

   className={`btn ${isActive && 'active'} ${isDisabled && 'disabled'}`} correctly uses template literals with logical AND. className='btn' + (isActive ? ' active' : '') + (isDisabled ? ' disabled' : '') uses string concatenation but misses curly braces for JSX expression. className={['btn', isActive && 'active', isDisabled && 'disabled'].join(' ')} uses an array join which works but is less common. className={btn isActive ? 'active' : '' isDisabled ? 'disabled' : ''} is invalid JSX syntax.

3. Final Answer:

   className={`btn ${isActive && 'active'} ${isDisabled && 'disabled'}`} -> Option C

4. Quick Check:

   Template literals with ${condition && 'class'} = C [OK]

Hint: Use backticks and ${condition && 'class'} for multiple classes [OK]

Common Mistakes:

• Forgetting curly braces around JSX expressions
• Using invalid syntax mixing strings and expressions
• Not handling false values rendering as 'false' string

3. Given this Next.js component code snippet, what will be the rendered class attribute when isActive is false and isDisabled is true?

function Button({ isActive, isDisabled }) {
  return (
    <button className={`btn ${isActive ? 'active' : ''} ${isDisabled ? 'disabled' : ''}`}>Click</button>
  );
}

medium

A. class="btn disabled"

B. class="btn"

C. class="btn active "

D. class="btn active disabled"

Solution

1. Step 1: Substitute boolean values into the template literal

   Since isActive is false, isActive ? 'active' : '' becomes an empty string. Since isDisabled is true, isDisabled ? 'disabled' : '' becomes 'disabled'.

2. Step 2: Combine the strings with spaces

   The final className string is btn disabled (note the double space between 'btn' and 'disabled' due to empty string). This renders as class="btn disabled", which browsers treat as btn disabled.

3. Final Answer:

   class="btn disabled" -> Option A

4. Quick Check:

   False condition yields empty string, true yields class = D [OK]

Hint: Replace ternary with values, watch spaces in template literals [OK]

Common Mistakes:

• Assuming false condition adds 'active' class
• Ignoring extra spaces in class string
• Confusing true/false in ternary expressions

4. Identify the error in this Next.js JSX snippet for conditional styling and select the fix:

function Alert({ isError }) {
  return <div className={isError ? 'error' : 'success'} else='hidden'>Message</div>;
}

medium

A. Wrap the div in a ternary operator outside JSX

B. Replace else='hidden' with className='hidden'

C. Change to {isError && 'error' || 'success'} and keep else attribute

D. Remove else='hidden' because JSX does not support 'else' attribute

Solution

1. Step 1: Identify invalid JSX attribute usage

   JSX does not support an else attribute on elements. Conditional logic must be inside curly braces.

2. Step 2: Fix by removing invalid attribute

   Removing else='hidden' leaves a valid ternary expression for className. To conditionally add 'hidden', use nested ternary or other logic inside className.

3. Final Answer:

   Remove else='hidden' because JSX does not support 'else' attribute -> Option D

4. Quick Check:

   JSX attributes must be valid props, no 'else' attribute = A [OK]

Hint: JSX attributes can't have 'else'; use ternary inside className [OK]

Common Mistakes:

• Trying to use 'else' as an attribute in JSX
• Not wrapping conditional logic inside curly braces
• Confusing HTML attributes with JSX props

5. You want to apply conditional styling in a Next.js component where the class 'highlight' should be added only if isSelected is true and count is greater than zero. Which of these is the best way to write the className attribute to handle this condition cleanly?

hard

A. className={isSelected && count > 0 ? 'item highlight' : 'item'}

B. className={`item ${isSelected && count > 0 ? 'highlight' : ''}`}

C. className={`item ${isSelected && count > 0 && 'highlight'}`}

D. className={`item ${isSelected ? count > 0 ? 'highlight' : '' : ''}`}

Solution

1. Step 1: Analyze each option's logic and readability

   className={`item ${isSelected && count > 0 ? 'highlight' : ''}`} uses a logical AND combined with ternary inside template literal, which can be confusing and may produce unexpected results. className={`item ${isSelected ? count > 0 ? 'highlight' : '' : ''}`} nests ternaries, which reduces readability. className={`item ${isSelected && count > 0 && 'highlight'}`} uses logical AND but if false returns false which renders as 'false' string in className. className={isSelected && count > 0 ? 'item highlight' : 'item'} uses a clear ternary outside template literal, returning full class strings, which is clean and safe.

2. Step 2: Choose the best practice for conditional classes

   className={isSelected && count > 0 ? 'item highlight' : 'item'} clearly returns either 'item highlight' or 'item' string, avoiding false or empty strings in className, making it the best pattern for this condition.

3. Final Answer:

   className={isSelected && count > 0 ? 'item highlight' : 'item'} -> Option A

4. Quick Check:

   Use full string ternary for multiple conditions = B [OK]

Hint: Use full string ternary to avoid false or empty class names [OK]

Common Mistakes:

• Returning false or empty strings inside template literals
• Nesting ternaries making code hard to read
• Using logical AND that outputs boolean in className