CSS Modules in NextJS

Introduction

CSS Modules help keep styles organized and avoid conflicts by making CSS local to components.

When you want to style a React component without affecting other parts of the app.

When multiple developers work on the same project and need to avoid style clashes.

When you want to write CSS that is scoped automatically to a component.

When you want to use CSS with Next.js in a simple and modular way.

Syntax

NextJS

import styles from './Component.module.css';

function Component() {
  return <div className={styles.className}>Hello</div>;
}

CSS files must be named with .module.css to use CSS Modules in Next.js.

Imported styles are objects where keys are class names and values are unique generated class names.

Examples

This example shows a button styled with a CSS Module. The button class is local to this component.

NextJS

/* Button.module.css */
.button {
  background-color: blue;
  color: white;
  padding: 1rem;
}

// Button.jsx
import styles from './Button.module.css';

export default function Button() {
  return <button className={styles.button}>Click me</button>;
}

This example shows a header with a local title style that won't affect other headers.

NextJS

/* Header.module.css */
.title {
  font-size: 2rem;
  color: green;
}

// Header.jsx
import styles from './Header.module.css';

export default function Header() {
  return <h1 className={styles.title}>Welcome</h1>;
}

Sample Program

This component shows a box with a border and padding. The styles come from a CSS Module, so they only apply here.

NextJS

/* styles.module.css */
.container {
  border: 2px solid black;
  padding: 1rem;
  max-width: 300px;
  margin: 1rem auto;
  text-align: center;
}

/* Component.jsx */
import styles from './styles.module.css';

export default function Box() {
  return (
    <div className={styles.container}>
      <p>This box is styled with CSS Modules.</p>
    </div>
  );
}

Important Notes

CSS Modules generate unique class names to avoid style conflicts.

You can combine CSS Modules with global styles if needed.

Use descriptive class names to keep your styles clear and maintainable.

Summary

CSS Modules scope CSS to components automatically.

They prevent style conflicts in large projects.

Next.js supports CSS Modules by naming files .module.css.

Practice

1. What is the main benefit of using CSS Modules in a Next.js project?

easy

A. They enable global CSS styles to override component styles.

B. They automatically minify CSS files for faster loading.

C. They scope CSS styles locally to components, avoiding conflicts.

D. They allow CSS to be written inside JavaScript files only.

Solution

1. Step 1: Understand CSS Modules purpose

   CSS Modules scope CSS locally to components, so styles don't leak or conflict.

2. Step 2: Compare options with this purpose

   Only They scope CSS styles locally to components, avoiding conflicts. describes this key benefit. Others describe unrelated features.

3. Final Answer:

   They scope CSS styles locally to components, avoiding conflicts. -> Option C

4. Quick Check:

   CSS Modules = Local scoped styles [OK]

Hint: Remember: CSS Modules isolate styles per component [OK]

Common Mistakes:

• Thinking CSS Modules minify CSS automatically
• Confusing CSS Modules with CSS-in-JS
• Believing CSS Modules create global styles

2. Which of the following is the correct way to import a CSS Module file named Button.module.css in a Next.js component?

easy

A. import styles from './Button.css';

B. import styles from './Button.module.css';

C. import './Button.module.css';

D. import styles from './Button.cssmodule';

Solution

1. Step 1: Identify correct file extension for CSS Modules

   Next.js requires CSS Modules files to end with .module.css.

2. Step 2: Check import syntax

   Importing with import styles from './Button.module.css'; correctly imports the module as an object.

3. Final Answer:

   import styles from './Button.module.css'; -> Option B

4. Quick Check:

   Import CSS Modules with .module.css and assign to variable [OK]

Hint: Always use .module.css extension and import with 'from' [OK]

Common Mistakes:

• Using wrong file extension like .css or .cssmodule
• Importing without assigning to a variable
• Forgetting the .module part in filename

3. Given the CSS Module styles.module.css with content:

.title { color: red; }

And the React component:

import styles from './styles.module.css';

export default function Header() {
  return <h1 className={styles.title}>Hello</h1>
}

What color will the <h1> text be when rendered?

medium

A. Red

B. Black (default)

C. Blue

D. Green

Solution

1. Step 1: Check CSS Module style for .title

   The CSS sets .title color to red.

2. Step 2: Confirm component uses styles.title as className

   The component applies className={styles.title}, so the red color applies.

3. Final Answer:

   Red -> Option A

4. Quick Check:

   CSS Module class applied = red text [OK]

Hint: ClassName from styles object applies CSS Module styles [OK]

Common Mistakes:

• Assuming default black color without checking CSS
• Confusing CSS Modules with global CSS
• Not using styles.title but a string instead

4. What is wrong with this Next.js component using CSS Modules?

import styles from './Card.module.css';

export default function Card() {
  return <div className="styles.card">Content</div>
}

medium

A. The component must be a class component to use CSS Modules.

B. The CSS file should be imported with a different name.

C. The CSS Module file must be named Card.css, not Card.module.css.

D. The className should use curly braces, not quotes, to apply styles.card.

Solution

1. Step 1: Identify how className is assigned

   The code uses className="styles.card", which is a string literal, not the variable.

2. Step 2: Correct syntax for applying CSS Module class

   It should be className={styles.card} to use the imported styles object.

3. Final Answer:

   The className should use curly braces, not quotes, to apply styles.card. -> Option D

4. Quick Check:

   Use curly braces to apply CSS Module classes [OK]

Hint: Use {styles.className}, not "styles.className" for CSS Modules [OK]

Common Mistakes:

• Using quotes instead of curly braces for className
• Renaming CSS Module file incorrectly
• Thinking CSS Modules require class components

5. You want to style a Next.js component with two CSS Module classes: primary and large from Button.module.css. Which is the correct way to apply both classes to a <button> element?

hard

A. Click

B. Click

C. Click

D. Click

Solution

1. Step 1: Understand how to combine multiple CSS Module classes

   Use template literals or string concatenation to combine classes as strings.

2. Step 2: Evaluate each option

   Click uses template literals correctly to combine classes with a space.
   Click uses a string literal, so classes won't apply.
   Click concatenates without space, merging class names incorrectly.
   Click passes an array, which React does not accept as className.

3. Final Answer:

   <button className={`${styles.primary} ${styles.large}`}>Click</button> -> Option A

4. Quick Check:

   Combine CSS Module classes with template literals [OK]

Hint: Use backticks and ${} to combine multiple CSS Module classes [OK]

Common Mistakes:

• Using quotes instead of variables for className
• Concatenating without spaces between classes
• Passing arrays directly to className