CSS-in-JS considerations in NextJS

Introduction

CSS-in-JS lets you write CSS styles directly inside your JavaScript code. This helps keep styles close to the components they belong to, making your code easier to manage.

When you want to style React components in Next.js without separate CSS files.

When you need dynamic styles that change based on component state or props.

When you want to avoid global CSS conflicts by scoping styles to components.

When you want to use JavaScript variables or logic inside your styles.

When you want to improve developer experience by co-locating styles and components.

Syntax

NextJS

import styled from 'styled-components';

const Button = styled.button`
  background-color: blue;
  color: white;
  padding: 1rem;
  border-radius: 0.5rem;
`;

export default function MyComponent() {
  return <Button>Click me</Button>;
}

Use backticks `` to write CSS inside JavaScript template literals.

Styled components create real React components with styles attached.

Examples

Simple styled component for a red heading.

NextJS

const Title = styled.h1`
  font-size: 2rem;
  color: red;
`;

Styled div using Flexbox to center content.

NextJS

const Container = styled.div`
  display: flex;
  justify-content: center;
  padding: 2rem;
`;

Dynamic styles using props to change background color.

NextJS

const Button = styled.button`
  background-color: ${props => props.primary ? 'blue' : 'gray'};
  color: white;
  padding: 1rem;
`;

Sample Program

This Next.js component shows two buttons styled with CSS-in-JS. The first button uses a prop to get a blue background. The second button uses the default gray background.

NextJS

import styled from 'styled-components';

const Button = styled.button`
  background-color: ${props => props.primary ? 'blue' : 'gray'};
  color: white;
  padding: 1rem 2rem;
  border: none;
  border-radius: 0.5rem;
  cursor: pointer;
  font-size: 1rem;
`;

export default function App() {
  return (
    <>
      <Button primary>Primary Button</Button>
      <Button>Default Button</Button>
    </>
  );
}

Important Notes

CSS-in-JS can increase your JavaScript bundle size, so use it wisely.

Server-side rendering in Next.js works well with styled-components but requires setup.

Always test styles on different screen sizes for responsiveness.

Summary

CSS-in-JS keeps styles close to components for easier management.

It allows dynamic styling using JavaScript logic and props.

Next.js supports CSS-in-JS well, but remember to handle server-side rendering properly.

Practice

1. What is a key benefit of using CSS-in-JS in Next.js projects?

easy

A. CSS files load faster than inline styles.

B. It requires no JavaScript knowledge to write styles.

C. It disables server-side rendering automatically.

D. Styles are scoped to components, avoiding global conflicts.

Solution

1. Step 1: Understand CSS-in-JS scope

   CSS-in-JS keeps styles inside components, so styles don't leak globally.

2. Step 2: Compare options

   Options A, B, and D are incorrect because CSS-in-JS requires JavaScript, does not guarantee faster loading than CSS files, and does not disable server-side rendering.

3. Final Answer:

   Styles are scoped to components, avoiding global conflicts. -> Option D

4. Quick Check:

   CSS-in-JS scope = C [OK]

Hint: Remember CSS-in-JS scopes styles to components [OK]

Common Mistakes:

• Thinking CSS-in-JS disables server-side rendering
• Assuming CSS-in-JS needs no JavaScript knowledge
• Believing CSS-in-JS always loads styles faster than CSS files

2. Which of the following is the correct way to define a styled component using the styled-components library in Next.js?

easy

A. const Button = styled.button`background: blue; color: white;`;

B. const Button = styled('button') { background: blue; color: white; };

C. const Button = styled.button({ background: 'blue', color: 'white' });

D. const Button = styled.button = 'background: blue; color: white;';

Solution

1. Step 1: Recall styled-components syntax

   The correct syntax uses tagged template literals: styled.element`styles`.

2. Step 2: Check each option

   const Button = styled.button`background: blue; color: white;`; matches the correct syntax. Options A, B, and C have syntax errors or wrong usage.

3. Final Answer:

   const Button = styled.button`background: blue; color: white;`; -> Option A

4. Quick Check:

   styled-components syntax = D [OK]

Hint: Use backticks with styled.element for styles [OK]

Common Mistakes:

• Using parentheses instead of backticks
• Trying to assign styles as strings without template literals
• Confusing object syntax with styled-components tagged templates

3. Given this Next.js component using styled-jsx:

export default function Box() {
  return (
    <>
      
Hello
      
    </>
  );
}

What color will the text "Hello" appear in the browser?

medium

A. Blue

B. Red

C. Black (default)

D. Green

Solution

1. Step 1: Understand styled-jsx behavior

   styled-jsx applies scoped CSS styles to the component's elements.

2. Step 2: Check the style rule

   The style sets .box color to red, so the div with className "box" will have red text.

3. Final Answer:

   Red -> Option B

4. Quick Check:

   styled-jsx scoped style color = A [OK]

Hint: styled-jsx styles apply only inside component [OK]

Common Mistakes:

• Assuming styles apply globally
• Ignoring className matching
• Confusing default browser text color

4. You wrote this Next.js component using styled-components:

import styled from 'styled-components';

const Title = styled.h1`color: blue;`;

export default function Page() {
  return Hello World;
}

But the text color is not blue in the browser. What is the most likely cause?

medium

A. You forgot to install the styled-components package.

B. You did not wrap your app with ThemeProvider.

C. You did not add the babel-plugin-styled-components for SSR support.

D. You forgot to import React in the component.

Solution

1. Step 1: Identify styled-components SSR issue

   Without babel-plugin-styled-components, styles may not render correctly on server-side in Next.js.

2. Step 2: Check other options

   Options A and D cause errors, not silent style failure. You did not wrap your app with ThemeProvider. is optional for theming, not required for basic styles.

3. Final Answer:

   You did not add the babel-plugin-styled-components for SSR support. -> Option C

4. Quick Check:

   SSR plugin missing = B [OK]

Hint: Add babel-plugin-styled-components for SSR styles [OK]

Common Mistakes:

• Assuming ThemeProvider is required for basic styles
• Forgetting SSR plugin causes silent style issues
• Thinking missing React import causes style failure

5. You want to create a Next.js component that changes its background color based on a prop using styled-components. Which code snippet correctly implements this dynamic styling?

hard

A. const Box = styled.div`background: ${props => props.bgColor};`;

export default function ColoredBox() {
return Hi;
}

B. const Box = styled.div`background: props.bgColor;`;

export default function ColoredBox() {
return Hi;
}

C. const Box = styled.div({ background: props.bgColor });

export default function ColoredBox() {
return Hi;
}

D. const Box = styled.div`background: bgColor;`;

export default function ColoredBox() {
return Hi;
}

Solution

1. Step 1: Understand dynamic props in styled-components

   Use a function inside template literals to access props: ${props => props.propName}.

2. Step 2: Check each option

   const Box = styled.div`background: ${props => props.bgColor};`; export default function ColoredBox() { return Hi; } correctly uses the function syntax. Options B, C, and D misuse props or syntax.

3. Final Answer:

   const Box = styled.div`background: ${props => props.bgColor};`; export default function ColoredBox() { return Hi; } -> Option A

4. Quick Check:

   Dynamic props in styled-components = A [OK]

Hint: Use ${props => props.value} for dynamic styles [OK]

Common Mistakes:

• Writing props.bgColor directly inside template literal
• Using object syntax without function for props
• Forgetting to use arrow function for dynamic values