Next-Generation CSS-in-JS library

Write modular CSS within your JavaScript code with built-in themes and SSR support.

const { classes } = css({
    button : {
        backgroundColor : 'blue',
        color : 'white',
        padding : '10px',
        borderRadius : '5px'
    }
});

const Button = () => <button className={classes.button}>Click me</button>;

Getting Started

Using npm

$ npm install cssfun

import { css } from 'cssfun';

Using ES modules

import { css } from 'https://esm.run/cssfun';

Using <script> tag

<script src="https://cdn.jsdelivr.net/npm/cssfun"></script>

const { css } = CSSFUN;

Create your styles

const { classes } = css({
    button : {
        backgroundColor : 'blue',
        color : 'white',
        padding : '10px',
        borderRadius : '5px'
    }
});

Apply the styles to your components:

const Button = () => <button className={classes.button}>Click me</button>;

Renderers

Renderers are functions that transform style objects into CSS strings.
These are the built-in renderers transformations:

Camelized keys will be transformed to dashed keys

css({
    root : {
        backgroundColor : 'black'
    }
}).toString();

Renders to:

<style id="fun-1">
    .fun-1-root-1 {
        background-color: black;
    }
</style>

Nested selectors will be expanded

• Use & to reference the selector of the parent rule

  css({
      button : {
          backgroundColor : 'white',
          '&:hover' : {
              backgroundColor : 'black'
          },
          '& span' : {
              color : 'blue'
          }
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      .fun-1-button-1 {
          background-color: white;
      }
      .fun-1-button-1:hover {
          background-color: black;
      }
      .fun-1-button-1 span {
          color: blue;
      }
  </style>
  

• Deep nesting

  css({
      button : {
          backgroundColor : 'white',
          '&:active' : {
              backgroundColor : 'black',
              '&:hover' : {
                  backgroundColor : 'blue'
              }
          }
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      .fun-1-button-1 {
          background-color: white;
      }
      .fun-1-button-1:active {
          background-color: black;
      }
      .fun-1-button-1:active:hover {
          background-color: blue;
      }
  </style>
  

Class references will be replaced by the generated class name

• Use $ to reference a local class within the same StyleSheet instance

  css({
      button : {
          backgroundColor : 'white'
      },
      '$button:hover' : {
              backgroundColor : 'black'
          },
      '$button span' : {
          color : 'blue'
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      .fun-1-button-1 {
          background-color: white;
      }
      .fun-1-button-1:hover {
          background-color: black;
      }
      .fun-1-button-1 span {
          color: blue;
      }
  </style>
  

Global selectors will be rendered as global styles

• Global block

  css({
      '@global' : {
          body : {
              backgroundColor : 'black'
          }
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      body {
          background-color : black;
      }
  </style>
  

• Nested global block

  css({
      root : {
          '@global' : {
              a : {
                  color : 'black'
              }
          }
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      .fun-1-root-1 a {
          color : black;
      }
  </style>
  

• Global prefix

  css({
      '@global body' : {
          backgroundColor : 'black'
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      body {
          background-color : black;
      }
  </style>
  

• Nested global prefix

  css({
      root : {
          '@global a' : {
              color : 'black'
          }
      }
  }).toString();
  

  Renders to:

  <style id="fun-1">
      .fun-1-root-1 a {
          color : black;
      }
  </style>
  

When composed, the first renderer receives the styles object, and the final one outputs the resulting CSS string.

Custom Renderers

You can customize the renderers by setting the renderers array on the StyleSheet instance. If passed via options.renderers, they will be automatically added to the instance.

Elements in the renderers array can be either functions or strings that reference methods of the StyleSheet instance. These methods will be bound to the instance before they are invoked.

By default, StyleSheet are rendered using the built-in renderers: ['parseStyles', 'renderStyles'].

Themes

A theme is a StyleSheet that provides access to CSS variables for consistent styling across your application. It supports light, dark, and system color schemes, allowing your components to automatically adapt to changes in the user's system preferences.

The createTheme function accepts a themes object { light, dark }, and an options object, and returns a theme StyleSheet.

Creating a Theme

Create theme StyleSheet.

// Create theme
const theme = createTheme({
    light : {
        color : 'black',
        backgroundColor : 'white',
    },
    dark : {
        color : 'white',
        backgroundColor : 'black',
    },
});

Applying the Theme Class

The generated theme includes a root class, which exposes all the theme's CSS variables to any element that uses this class and its descendants. You can apply this class to the body element to style the entire application, or to the root element of a specific component to apply the theme to just part of your UI.

// Add theme class to the body
document.body.classList.add(theme.classes.root);

Using Theme Variables in Styles

Your theme object is automatically converted into CSS variables. For instance:

{ backgroundLevel1 : 'black' }

This will be converted into the CSS variable --fun-backgroundLevel1.

Similarly, more complex theme structures like:

{
    palette : {
        common : { 
            black : '#000'
        }
    }
}

will be converted into --fun-palette-common-black.

Use these variables in your component styles, even before the theme is applied. Your components will automatically update when the theme or system color scheme changes.

const { classes } = css({
    button : {
        color : 'var(--fun-color)',
        backgroundColor : 'var(--fun-backgroundColor)',
    },
});

const Button = ({ label }) => <button className={classes.button}>{label}</button>;

Server-Side Rendering (SSR)

Easily add your styles to the server-rendered HTML by embedding the StyleSheets as a string within the <head> of your page.

// Creating a theme
const theme = createTheme(themes);

// Express route that renders the app and returns HTML to the browser
app.get('*', (req, res) => {
    // Render the app as an HTML string
    const html = renderToString(<App />);

    // Get all StyleSheets styles as a string of <style> elements
    const styles = StyleSheet.toString();

    // Get the root class name from the theme
    const cls = theme.classes.root;

    // Create the full HTML page template
    const template = `
        <!DOCTYPE html>
        <html lang="en">
            <head>
                <meta charset="UTF-8">
                <meta name="viewport" content="width=device-width, initial-scale=1.0">
                <title>Waving Cat</title>
                ${styles}
            </head>
            <body class="${cls}">
                <div id="root">${html}</div>
                <script src="/bundle.js"></script>
            </body>
        </html>
    `;

    // Send the complete HTML response
    res.send(template);
});

When the app is hydrated on the client side, the styles are preserved and won’t be recreated.

API Documentation

Complete API documentation can be found here.

Examples

The examples folder contains various sample projects demonstrating how to use CSSFUN in different environments and frameworks. Each example is a standalone project that you can run locally to see CSSFUN in action.

Available Examples

• React Example: A basic React application demonstrating the use of CSSFUN for styling React components. Try it.
• Rasti Example: A simple Rasti application illustrating how to apply CSSFUN to style Rasti components. Try it.
• Vanilla JS Example: A straightforward JavaScript example showing how to use CSSFUN for styling HTML components. Try it.
• Rasti with Server-Side Rendering (SSR) Example: A Rasti application with server-side rendering using Express, highlighting how to use CSSFUN for styling in an SSR environment.