CSS-in-JS considerations in NextJS - Deep Dive

Overview - CSS-in-JS considerations

What is it?

CSS-in-JS is a way to write CSS styles directly inside JavaScript code. Instead of separate CSS files, styles live with the components they style. This approach helps keep styles scoped and easier to manage in modern web apps. It is popular in frameworks like Next.js for building user interfaces.

Why it matters

Without CSS-in-JS, styles can become hard to track and maintain, especially in large apps where many components share similar names or styles. CSS-in-JS solves this by keeping styles close to the code they affect, reducing bugs and improving developer experience. It also enables dynamic styling based on component state or props, making apps more interactive and responsive.

Where it fits

Before learning CSS-in-JS, you should understand basic CSS and JavaScript, plus how React components work. After mastering CSS-in-JS, you can explore advanced styling techniques like theming, server-side rendering of styles, and performance optimization in Next.js apps.

Mental Model

Core Idea

CSS-in-JS means writing styles as JavaScript objects or strings inside components, so styles and logic live together and can change dynamically.

Think of it like...

It's like having your recipe and ingredients together in one cookbook page instead of separate books, so you can cook faster and adjust flavors on the spot.

Component
  ├─ Styles (JS objects or template strings)
  └─ Logic (JS code)

Styles and logic combined → Scoped, dynamic styling

Flow:
[Component code] → [CSS-in-JS styles] → [Rendered styled element]

Build-Up - 6 Steps

1

Foundation

What is CSS-in-JS?

Concept: Introducing the idea of writing CSS styles inside JavaScript code.

Traditionally, CSS lives in separate files. CSS-in-JS puts styles inside JavaScript files, often inside components. This means styles are written as JavaScript objects or template strings and applied directly to elements.

Result

You get styles that are scoped to components and easier to manage alongside component code.

Understanding that styles can be code helps you see how styling can be dynamic and scoped, not just static files.

2

Foundation

How CSS-in-JS scopes styles

Concept: Explaining how CSS-in-JS prevents style conflicts by scoping styles to components.

CSS-in-JS libraries generate unique class names or inline styles so that styles only apply to the component they belong to. This avoids global CSS conflicts where styles accidentally affect other parts of the app.

Result

Styles are isolated, so you don't worry about one component's styles breaking another's.

Knowing styles are scoped by default reduces debugging time and makes components more reusable.

3

Intermediate

Dynamic styling with props

🤔Before reading on: do you think styles can change based on component data or user actions? Commit to your answer.

Concept: Using JavaScript logic to change styles dynamically based on component props or state.

In CSS-in-JS, you can write functions that return different styles depending on input. For example, a button can have a different background color if it is disabled or active, controlled by props.

Result

Components become more interactive and responsive to user input or app state.

Understanding dynamic styles unlocks powerful UI behaviors that static CSS can't easily achieve.

4

Intermediate

Server-side rendering of styles

🤔Before reading on: do you think CSS-in-JS styles appear immediately on page load or after JavaScript runs? Commit to your answer.

Concept: How CSS-in-JS works with Next.js to render styles on the server for faster page loads and better SEO.

Next.js can extract CSS-in-JS styles during server rendering and send them with the HTML. This means users see styled pages immediately without waiting for JavaScript to load.

Result

Pages load faster and look correct from the start, improving user experience and search engine ranking.

Knowing server-side rendering of styles is key to building fast, accessible web apps with CSS-in-JS.

5

Advanced

Performance considerations in CSS-in-JS

🤔Before reading on: do you think CSS-in-JS always improves performance or can it sometimes slow things down? Commit to your answer.

Concept: Understanding when CSS-in-JS can impact app performance and how to optimize it.

Generating styles at runtime can add overhead, especially if styles are recalculated often or many components use CSS-in-JS. Techniques like caching styles, extracting static styles, and minimizing re-renders help keep apps fast.

Result

You can build apps that use CSS-in-JS without sacrificing speed or responsiveness.

Knowing performance tradeoffs helps you choose the right CSS-in-JS approach and avoid common slowdowns.

6

Expert

Internals of CSS-in-JS libraries

🤔Before reading on: do you think CSS-in-JS libraries inject styles as inline styles or generate CSS classes? Commit to your answer.

Concept: How CSS-in-JS libraries generate, inject, and manage styles behind the scenes.

Most CSS-in-JS libraries create unique class names and inject CSS rules into the document head. They track which styles are used to avoid duplicates and remove unused styles. Some use inline styles for simple cases, but class generation is common for performance and specificity.

Result

You understand how styles stay scoped, avoid conflicts, and update efficiently.

Understanding the internal style injection mechanism explains why CSS-in-JS can be both powerful and complex.

Under the Hood

CSS-in-JS libraries parse JavaScript style definitions and generate unique class names or inline styles. They inject these styles into the document's