CSSinJS

CSS-in-JS is a styling approach in web development in which CSS is authored inside JavaScript. Styles are defined as objects or template literals and applied to components by generating unique class names or by inlining styles. This approach enables dynamic styling based on component props, themes, or state, and typically scopes styles to a component to prevent leakage and cascade issues. Many implementations provide features such as theming, vendor prefixing, and automatic style rehydration for server rendering.

Origins and approaches: CSS-in-JS emerged to address the scalability challenges of large CSS codebases in component-based frameworks. Early libraries include JSS; modern, widely used tools include styled-components and Emotion, which rely on template literals, and other options like Radium, Aphrodite, and JSS themselves. Styles can be computed at runtime or extracted at build time for server-side rendering. Thematic systems often use a ThemeProvider to supply tokens across components.

Benefits: improved encapsulation and predictable styling, dynamic styling via props, easier maintenance in component-driven apps, avoidance of global scope conflicts, built-in theming, and often automatic vendor prefixing. For server-side rendering, CSS-in-JS can extract critical CSS to improve performance.

Criticisms and trade-offs: potential runtime overhead and larger bundle sizes, complexity, difficulties with debugging or tooling, and a departure from traditional CSS workflows. Some teams prefer CSS Modules or plain CSS when performance and simplicity are priorities. The choice often depends on project size, team expertise, and the needs for theming and dynamic styles.

Adoption and ecosystem: CSS-in-JS is widely used in React ecosystems, with numerous libraries supporting component styling and theming. While most implementations target JavaScript-based UI frameworks, ideas have spread to other ecosystems, sometimes through framework-agnostic APIs.