React Cosmos - The Fast React Component Sandbox

Why React Cosmos?

React Cosmos is built for developers who find Storybook too heavy or complex for their daily workflow. It strips away the "documentation site" features to focus purely on the developer experience of building components.

Fixture-First Design: Instead of writing "stories," you write "fixtures"—simple files that export component states. This encourages a data-driven approach to UI development.
Blazing Fast: Designed with performance in mind, Cosmos starts up instantly and hot-reloads purely the component tree, making it significantly faster than competitors in large projects.
Universal Support: Works seamlessly with React DOM and React Native, providing a consistent experience across platforms.
Headless Mode: You can run your fixtures in a headless environment (like JSDOM) for automated visual regression testing or snapshot testing.
Props Panel: Automatically generates controls to manipulate component props in real-time, helping you explore edge cases without code changes.

Code Snippet

React Cosmos uses "Fixtures" to capture component states. Here is a simple fixture file illustrating how to define multiple states for a User Card component.

// UserCard.fixture.jsx
import { UserCard } from './UserCard';

export default {
  // A standard state
  'Active User': (
    <UserCard 
      user={{ name: 'Jane Doe', role: 'Admin', status: 'active' }} 
    />
  ),

  // An edge case state
  'Suspended User': (
    <UserCard 
      user={{ name: 'John Smith', role: 'Guest', status: 'suspended' }} 
    />
  ),

  // Simulating a loading state
  'Loading': (
    <UserCard loading={true} />
  )
};

By simply creating this file next to your component, Cosmos automatically detects it and adds it to the sidebar explorer, rendering the component in isolation.

Pros and Cons

No library is perfect; understanding the trade-offs is key to selecting the right tool.

Pros

Performance: Significantly lighter and faster than Storybook. It doesn't try to be a static site generator; it's a dev server.
Simplicity: The mental model is just "Input Data -> Component." There is very little boilerplate or configuration required.
Long-term Maintainability: Because fixtures are just file exports, they are easier to maintain and refactor than complex Storybook DSLs.

Cons

Smaller Ecosystem: Unlike Storybook, which has hundreds of addons for everything from accessibility to Figma integration, Cosmos has a much smaller plugin library.
Documentation Focus: It is less suited for generating public-facing design system documentation for non-developers (designers/PMs). It is a tool for engineers.
Community Size: The community is smaller, meaning fewer third-party tutorials and ready-made recipes for obscure edge cases.

Comparison with Other Sandbox Libraries

The table below outlines the positioning differences between React Cosmos and Storybook:

Library	Design Philosophy	Best For	Pain Points
React Cosmos	Dev Tool First Speed and isolation for the engineer building the UI.	Hardcore Devs Teams who value fast iteration cycles over pretty docs.	Niche Less "polish" for presenting to stakeholders.
Storybook	Documentation First A comprehensive workshop for UI development and documentation.	Design Systems Public component libraries and cross-team collaboration.	Bloat Can become very slow and heavy in large codebases.
Ladle	Vite Native A drop-in Storybook alternative powered by Vite.	Vite Users Those who want Storybook API but faster build times.	Newer Still maturing compared to the other two.

Verdict: When to Adopt

Choose React Cosmos if you are frustrated with the build times and complexity of Storybook and want a tool that gets out of your way. It is the perfect choice for pure component development, enabling you to practice "Visual TDD" without the overhead of a documentation generator.

TL;DR