CSS-in-JS considerations in NextJS - Mini Project: Build & Apply

CSS-in-JS Styling in Next.js

📖 Scenario: You are building a simple Next.js app that shows a greeting message. You want to style this message using CSS-in-JS to keep styles close to the component and make your app easier to maintain.

🎯 Goal: Create a Next.js functional component that uses CSS-in-JS styling with the styled-jsx approach. You will define styles inside the component and apply them to the greeting text.

📋 What You'll Learn

Create a functional component named Greeting that returns a <div> with the text 'Hello, Next.js!'

Add a CSS-in-JS block using <style jsx> inside the component

Style the <div> with a blue text color and a font size of 2rem

Export the Greeting component as default

💡 Why This Matters

🌍 Real World

CSS-in-JS is popular in React and Next.js apps to keep styles close to components, making maintenance easier and avoiding global CSS conflicts.

💼 Career

Understanding CSS-in-JS is important for frontend developers working with modern React frameworks like Next.js, as many companies use this pattern for scalable styling.

1

Create the Greeting component with text

Create a functional component called Greeting that returns a <div> containing the exact text Hello, Next.js!.

NextJS

// Your code here

Hint

Use a function named Greeting that returns JSX with a <div> and the text inside.

2

Add a CSS-in-JS style block

Inside the Greeting component, add a <style jsx> block after the <div> that will hold your CSS styles.

NextJS

export default function Greeting() {
  return 
    Hello, Next.js!
    {/* Add a  block here */}
  ;
}

Hint

Use <style jsx>{``}</style> to add CSS inside the component.

3

Style the div with blue text and 2rem font size

Inside the <style jsx> block, write CSS to style the <div> so that its text color is blue and its font size is 2rem.

NextJS

export default function Greeting() {
  return 
    Hello, Next.js!
    {`
      /* Style the div here */
    `}
  ;
}

Hint

Write CSS inside the template literal to style the div selector.

4

Wrap the return in a fragment and export default

Ensure the Greeting component's return value is wrapped in a React fragment <></> and that the component is exported as default.

NextJS

export default function Greeting() {
  return (
    
      Hello, Next.js!
      {`
        div {
          color: blue;
          font-size: 2rem;
        }
      `}
    
  );
}
// Make sure the component is exported as default

Hint

Wrap the JSX in () and use <></> around the elements. Confirm the export default is present.

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