CSS-in-JS is an approach to styling in which the CSS for a piece of user interface is written in JavaScript and attached directly to the component it styles, rather than living in a separate global stylesheet. The documentation for one of the foundational libraries in this space describes the goal as removing “the mapping between components and styles,” so that “when you’re defining your styles, you’re actually creating a normal React component, that has your styles attached to it.” Styles become a property of the component itself instead of a rule that lives somewhere else and is connected by a class name.
The problem this approach targets is the global nature of plain CSS. In ordinary CSS, every class name and selector lives in one shared namespace, so a rule written for one part of an application can unintentionally affect another, and removing a component does not make it obvious which styles are now dead. CSS-in-JS solves this by generating unique class names automatically and tying each component’s styles to that component’s lifecycle. The same library’s docs highlight that this “eliminates class name bugs,” makes dead-style removal obvious because styles are bound to specific components, and supports “automatic critical CSS” by injecting only the styles for components actually rendered on a page.
A defining benefit is dynamic styling. Because the styles live in JavaScript, they can read a component’s props and application state directly, computing colors, sizes, or whole rule sets based on data without manually juggling conditional class names. This made it natural to express themeable, state-driven interfaces, which is one reason CSS-in-JS rose alongside component-based JavaScript frameworks where each piece of the UI is already a self-contained unit.
Those benefits came with a debated cost. Many CSS-in-JS libraries do their work at runtime in the browser: they parse style definitions, generate class names, and inject style rules into the document as components render. That runtime overhead can add to bundle size and slow rendering compared with a static stylesheet produced ahead of time. The trade-off between developer ergonomics and runtime performance became one of the central arguments in front-end styling, and it pushed later tools toward extracting styles at build time to recover the scoping benefits without the runtime price.
The category as a whole was popularized by libraries that turned the idea into practical tools, most prominently the tagged-template approach used by styled-components and similar libraries such as Emotion. CSS-in-JS sits among a family of solutions to the same scoping problem, alongside build-time approaches like CSS Modules, and the choice between them remains a recurring decision in how teams structure styling.