What is cssinjs?

CSS-in-JS is a technique where you write CSS styles directly inside JavaScript code instead of in separate .css files. The styles are usually defined as JavaScript objects or template strings and are applied to components at runtime, often using a library that injects the generated CSS into the page.

Let's break it down

  • JavaScript file: You create a .js or .tsx file and import a styling helper (e.g., styled, css, makeStyles).
  • Style definition: Write styles as an object ({ color: 'red', marginTop: 10 }) or a template literal (\color: red; margin-top: 10px;“).
  • Component binding: Attach the style to a component, either by wrapping the component (styled.button) or by passing a class name generated by the helper.
  • Runtime injection: When the component renders, the library turns the JavaScript style into real CSS, inserts it into a <style> tag, and gives the element a unique class name.

Why does it matter?

  • Scoped styles: Each component gets its own unique class, so styles don’t leak or clash with others.
  • Dynamic theming: You can change colors, sizes, or any CSS value based on props, state, or user preferences without writing separate CSS files.
  • Better developer experience: All the logic (JS) and styling live together, making it easier to see how a component looks and behaves in one place.
  • Tree‑shaking: Unused styles are not included in the final bundle, potentially reducing file size.

Where is it used?

  • Popular React libraries like styled‑components, Emotion, and Material‑UI’s makeStyles.
  • Vue’s vue-styled-components and CSS Modules (which compile to a similar concept).
  • Some Svelte and Angular projects also adopt CSS‑in‑JS via community plugins.
  • Large web apps (e.g., Facebook, Airbnb, Shopify) use it to keep their UI codebase maintainable.

Good things about it

  • Guarantees style isolation, preventing accidental overrides.
  • Enables conditional styling based on component props or application state.
  • Improves code readability for developers who prefer staying in JavaScript/TypeScript.
  • Works well with modern build tools; styles are generated only when needed.
  • Supports server‑side rendering, so the initial HTML can include the correct styles.

Not-so-good things

  • Adds a runtime cost: the library must parse and inject CSS while the app loads.
  • Can increase bundle size if many small style objects are generated.
  • Debugging may be harder because the generated class names are often hashed and not human‑readable.
  • Developers accustomed to traditional CSS may need to learn new syntax and patterns.
  • Tooling (like linting or IDE autocomplete) may be less mature compared to plain CSS.