If you have ever juggled complex, sprawling CSS files or struggled to keep styles organized in large projects, then you will love CSS in JS. That's because, with this approach, you can write CSS directly inside your JavaScript, which makes the styles modular and flexible, making them perfectly fit into component-based frameworks like React.

We are going to dive into two of the most popular CSS-in-JS libraries, Styled-Components and Emotion, and give you some practical tips on how to effectively use them.

Let's dive in, how this tool can change your styling without sacrificing clean, maintainable, scalable code.

Why CSS-in-JS?
One reason why CSS in JS becomes popular is that it solves one of the biggest challenges in modern web development, especially for teams building big applications. By moving styles inside JavaScript, we achieve the following:

Scoped Styles: No more class name collisions. Styles scoped to each component.

Dynamic Styling: With JavaScript logic, change styles concerning props, theme or application state.

Component-Based Development: CSS-in-JS libraries naturally go with component-based frameworks such as React, Vue and Angular. That keeps your styles encapsulated and modular.

Styled-Components: The Basics
Styled-Components is, probably, one of the most popular libraries in the CSS-in-JS ecosystem, which boasts simplicity and is easy to use with React.

Quick Start
Install Styled-Components:
$ npm install styled-components
Then import it in your component:
$ import styled from 'styled-components';

Styled component:

const Button = styled.button`
background-color: #6200ea;
color: white;
padding: 10px 20px;
border-radius: 5px;
border: none;
cursor: pointer;

&:hover {
background-color: #3700b3;
}
Use the styled component in your JSX:

Click Me

Key Features:
Themization: With Styled-Components, you are able to create themes for consistent styling across your application.

Dynamic Styling: One might want to change how things are styled dynamically. This would go hand in hand with props and create very flexible and reusable components.

Emotion: Flexibility and Performance
Another powerful CSS-in-JS library is Emotion, known for flexibility and performance. It provides two major ways of styling: for inline styles, the css prop, and styled, for reusable components.

Quick Start
Install Emotion:

npm install @emotion/react @emotion/styled
Import and use:

import styled from '@emotion/styled';

const Card = styled.div`
padding: 20px;
box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
border-radius: 8px;
background-color: white;

For dynamic inline styles:

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

const dynamicStyle = css`
color: blue;
font-size: 16px;

Hello, Emotion!

Key Features:
Inline and Reusable Styles: More control, more flexibility with styling methods through Emotion's css and styled APIs.

Server-Side Rendering: Emotion is highly optimized for SSR and hence finds a perfect use case in applications needing great performance on SEO.

Best Practices for CSS-in-JS
There are a few best practices to effectively use CSS-in-JS, which help in maintaining performance and keeping your styles clean:

Avoid Too Many Inline Dynamic Styles: Inline styles are great for one-time minor adjustments. However, too many will create bottlenecks in performance. Instead, apply styled components for reusable styles and limit inline styling to whenever that's really necessary.

Leverage Theming: Use theming to enforce a consistent structure and look in your application. Libraries like Styled-Components and Emotion allow one to define a global theme. This will make applying branding colors, fonts, and other styling criteria across your components so much easier.

import { ThemeProvider } from 'styled-components';

const theme = {
primaryColor: '#6200ea',
fontSize: '16px',
};

Prop-Driven Styles: Dynamic props allow you to change styles based on component state or even incoming data. In both Styled-Components and Emotion, props provide a straightforward way for your components to meet the needs of different use scenarios without rewriting styles.

const Button = styled.button`
background-color: ${(props) => (props.primary ? '#6200ea' : '#eee')};
color: ${(props) => (props.primary ? 'white' : 'black')};

Primary Button
Secondary Button

Code Splitting: Be aware that CSS-in-JS libraries can result in bigger bundles if you're not careful. Make use of techniques such as code-splitting and only load the styles that are needed for each part of your app to see decent performance.

Styled-Components vs Emotion
Which one to use? Both are amazing libraries, and each fills a different need. A quick comparison:

styled-components: Best for React applications when theming and ease of use are required. It's highly adopted and has very good documentation, thus being friendly to beginners.

Emotion: More flexible and performance-optimized. Emotion will fit best if you need SSR support or seek a library that would grant you both inline and reusable styling options.

CSS-in-JS represents a powerful way to do modern web styling, especially if you're a developer with a component-driven framework. With libraries such as Styled-Components and Emotion, one can keep styles modular, dynamic, and consistent throughout your application.

By staying close to best practices-scoped inline styles, theme utilization, and prop-driven styles-you are able to get the most from CSS-in-JS to clean up and scale your code.

Ready to change how you work in CSS? Well, try out CSS-in-JS for yourself!

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