CSS-in-JS considerations in NextJS - Performance & Optimization

Performance: CSS-in-JS considerations

MEDIUM IMPACT

This affects page load speed and rendering performance by how styles are generated and applied in the browser.

Applying styles in a Next.js app using CSS-in-JS

NextJS

import { css } from '@emotion/react';

const buttonStyle = css`
  color: red;
  font-size: 1rem;
`;

export default function Page() {
  return <button css={buttonStyle}>Click me</button>;
}

Using Emotion with server-side rendering extracts styles at build time, reducing runtime style generation and improving load speed.

📈 Performance GainReduces blocking time by 50-100ms; smaller runtime JS bundle

Applying styles in a Next.js app using CSS-in-JS

NextJS

import styled from 'styled-components';

const Button = styled.button`
  color: red;
  font-size: 1rem;
`;

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

Styles are generated at runtime on the client, causing a flash of unstyled content and delaying Largest Contentful Paint.

📉 Performance CostBlocks rendering for 100-200ms on slow devices; adds ~10-20kb to JS bundle

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Runtime CSS-in-JS (styled-components)	Low	1 per style injection	Medium due to delayed styles	[X] Bad
Static CSS extraction (Emotion SSR)	Low	Single reflow	Low paint cost	[OK] Good

Rendering Pipeline

CSS-in-JS styles are generated either at build time or runtime. Runtime generation delays style calculation and layout, causing slower paint.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation due to runtime style injection

Core Web Vital Affected

LCP

This affects page load speed and rendering performance by how styles are generated and applied in the browser.

Optimization Tips

1Avoid generating CSS styles at runtime to reduce blocking time.

2Use server-side rendering or static extraction for CSS-in-JS styles.

3Monitor Largest Contentful Paint to detect style injection delays.

Performance Quiz - 3 Questions

What is a main performance drawback of runtime CSS-in-JS in Next.js?

A It delays style calculation causing slower Largest Contentful Paint

B It increases server CPU usage during build

C It reduces JavaScript bundle size

D It improves caching of styles

DevTools: Performance

How to check: Record page load and look for long tasks related to style recalculation and layout shifts caused by style injection.

What to look for: Look for delayed Largest Contentful Paint and style recalculation events indicating runtime CSS generation.

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