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React Cosmos

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Become a Sponsor to invest in the future of React Cosmos

A tool for ambitious UI developers.

  • Laser focus. Isolate the components you're working on and iterate quickly. Works for both UI primitives and entire app sections. Running your whole app on every change is slowing you down!
  • Component library. From blank states to normal states to edge cases, define component states to come back to. Your component library helps you stay organized and makes a great foundation of test cases.
  • Long term design. Keeping your components decoupled leads to reusable code, a robust architecture, and saves you from having to rewrite your UI every two years.

馃懇鈥嶐煔 Live demo

[Cosmos Next](next.png)

Keep in mind this is a static export. The dev environment, which requires a local server running, has more functionality.

Table of contents

The [example package](example) is a useful complement to this guide.

Getting started

1. Install React Cosmos

# Using npm
npm i --D react-cosmos@next
# or Yarn
yarn add --dev react-cosmos@next

2. Create package.json scripts

"scripts": {
+  "cosmos": "cosmos",
+  "cosmos:export": "cosmos-export"

3. Start React Cosmos

# Using npm
npm run cosmos
# or Yarn
yarn cosmos

馃殌 localhost:5000

You may also run npx react-cosmos@next in your project without installing any deps.

4. Create your first fixture

Choose a simple component to get started.

<!-- prettier-ignore -->

// Hello.jsx
import React from 'react';

export function Hello({ greeting, name }) {
  return <h1>{greeting}, {name}!</h1>;

Create a fixture file in a __fixtures__ directory. You can customize this convention later.

Fixture files contain a default export, which can be a React Component or any React Node.

// __fixtures__/hello.jsx
import React from 'react';
import { Hello } from '../Hello';

export default <Hello greeting="Aloha" name="Alexa" />;

The hello fixture will show up in your React Cosmos UI and will render when you select it.

5. Build amazing user interfaces

You've taken the first step towards designing reusable components. You can now prototype, test and interate on components in isolation. Use this to your advantage!

Something wrong? Don't hesitate to create a GitHub issue (be helpful and include details) and to join us on Slack.


The only hard requirements are React 16.8 and Node 6 (or newer).

React Cosmos works best with webpack. It takes extra effort to make it work with other bundlers, but it's not as scary as it might seem. Don鈥檛 be afraid to ask for support.

Browserify and Parcel examples are available for Cosmos Classic. Props to whoever adapts them to Cosmos Next!


No config is required to start. If you have custom needs or would like to convert a Cosmos Classic config, here's what you need to know.

The Cosmos Next config is a JSON file, so it can only host serializable values. This design decision is meant to discourage complex configuration, make it easy to embed config options into the UI, and enable visual config management in the future.

By default, Cosmos reads cosmos.config.json from your root directory. You can pass a --config CLI arg for a custom config path.

Most Cosmos Classic config options are still supported in the new JSON format. Let me know if you need old config options that no longer work.

Available options

The best way to learn about the available options in the Cosmos config is to use [config.schema.json](packages/react-cosmos/config.schema.json).

The schema is human readable, but you can also enhance your config with autocomplete in code editors like VS Code.

  "$schema": "http://json.schemastore.org/cosmos-config"
  // your options...

And if you use VS Code you can map the Cosmos config schema globally by extending your user settings.

"json.schemas": [
    "fileMatch": ["cosmos.config.json"],
    "url": "http://json.schemastore.org/cosmos-config"


How you compile your code is 100% your business. React Cosmos jumps through hoops to compile your code using your existing build pipeline, but it doesn't have opinions nor does it install dependencies your setup might require.

Unless you use a framework like Create React App or Next.js, install build dependencies yourself. This include stuff like Babel, TypeScript, webpack loaders, etc. Cosmos uses build dependencies already installed in your project.


Configuring webpack is the least romantic aspect of the Cosmos setup. Luckily, you only do it once. Depending on your setup, one of the following options will work for you.

Default webpack config

In many cases Cosmos manages to get webpack working without human intervention. Try running Cosmos as is first.

Custom webpack config

Probably the most common scenario. Most of us end up with a hairy webpack config sooner or later. Use the webpack.configPath setting to point to an existing webpack config.

You can also point to a module inside a dependency, like in the following Create React App example.

  "watchDirs": ["src"],
  "webpack": {
    "configPath": "react-scripts/config/webpack.config"

Both watchDirs and webpack.configPath options are recommended for a seamless integration with Create React App.

Webpack config override

Overriding the webpack config gives you complete control. Use the webpack.overridePath setting to point to a module that customizes the webpack config used by Cosmos.

  "webpack": {
    "overridePath": "./webpack.override.js"

The override function receives a base webpack config 鈥 the default one generated by Cosmos or a custom one loaded from webpack.configPath. Extend the input config and return the result.

// webpack.override.js
module.exports = (webpackConfig, env) => {
  return { ...webpackConfig /* do your thing */ };


Fixture files contain a default export, which can be a React Component or any React Node.

React must be imported in every fixture file.

The file paths of your fixture files (relative to your project root) are used to create a tree view explorer in the React Cosmos UI.

Node fixtures

Think of Node fixtures as the return value of a function component, or the first argument to React.render.

// __fixtures__/disabled.js
export default <Button disabled>Click me</Button>;

Component fixtures

Component fixtures are just function components with no props. They enable using Hooks inside fixtures, which is powerful for simulating state with stateless components.

// CounterButton.fixture.js
export default () => {
  const [count, setCount] = React.useState(0);
  return <CounterButton count={count} increment={() => setCount(count + 1)} />;

Multi fixture files

A fixture file can also export multiple fixtures if the default export is an object.

// buttons.fixture.js
export default {
  primary: <PrimaryButton>Click me</PrimaryButton>,
  primaryDisabled: <PrimaryButton disabled>Click me</PrimaryButton>,
  secondary: <SecondaryButton>Click me</SecondaryButton>,
  secondaryDisabled: <SecondaryButton disabled>Click me</SecondaryButton>

The object property names will show up as fixture names in the Cosmos UI.

See this comment for the reasoning behind this solution (vs named exports).

How to create fixture files

Two options:

  1. End fixture file names with .fixture.{js,jsx,ts,tsx}
  2. Put fixture files inside __fixtures__


  1. blankState.fixture.js
  2. __fixtures__/blankState.js

File name conventions can be configured using the fixturesDir and fixtureFileSuffix options.

IMPORTANT: Fixture files must be placed in the src directory when using Create React App, in order for Cosmos to bundle in the exact same environment as Create React App's.


Wrapping components inside fixtures is easy, but can become repetitive. Decorators can be used to apply one or more component wrappers to a group of fixtures automatically.

A cosmos.decorator file looks like this:

// cosmos.decorator.js
export default ({ children }) => <Provider store={store}>{children}</Provider>;

A decorator only applies to fixture files contained in the decorator's directory. Decorators can be composed, in the order of their position in the file system hierarchy (from outer to inner).

Redux state mock

Check out react-cosmos-redux to see what a Cosmos Next decorator looks like.

Declarative mocks

Coming up with dummy prop values is all that's required to create fixtures for many components. In other cases, however, components have special needs.

Some components need to be wrapped in certain contexts, like a Router provider. Other components fire fetch requests willy-nilly. All these implicit dependencies are component inputs and understanding them goes a long way.

The react-mock project provides ways for mocking implicit component dependencies and helps you create fixtures for stubborn components.

UI-controlled values

The props panel allows you to manipulate component props visually by default. But you can also get a custom values panel with minimal work.

// CounterButton.fixture.js
import { useValue } from 'react-cosmos/fixture';

export default () => {
  const [count, setCount] = useValue('count', { defaultValue: 0 });
  return <CounterButton count={count} increment={() => setCount(count + 1)} />;

Heads up: useValue (and Cosmos in general) works great with TypeScript.

UI plugins

A main feature of the Cosmos Next redesign is the brand-new UI plugin architecture. While the new UI is created 100% from plugins, the plugin API is not yet documented nor made accessible. It will take a few big steps to get there, but this is the future.

Custom responsive viewports

responsivePreview is a plugin included by default, and you can customize it through the Cosmos config.

  "ui": {
    "responsivePreview": {
      "devices": [
        { "label": "iPhone 5", "width": 320, "height": 568 },
        { "label": "iPhone 6", "width": 375, "height": 667 },
        { "label": "iPhone 6 Plus", "width": 414, "height": 736 },
        { "label": "Medium", "width": 1024, "height": 768 },
        { "label": "Large", "width": 1440, "height": 900 },
        { "label": "1080p", "width": 1920, "height": 1080 }

Static export

Run cosmos-export and get a nice component library that you can deploy to any static hosting service. The exported version won't have all the Cosmos features available in development (like opening the selected fixture in your code editor), but allows anybody with access to the static export URL to browse fixtures and play with component inputs.

Use http-server or any static file server to load the export locally.

React Native

npm run cosmos-native

Cosmos Next works great with React Native. Put the following inside App.js to get started.

import React, { Component } from 'react';
import { NativeFixtureLoader } from 'react-cosmos/native';
// You generate cosmos.userdeps.js when you start the Cosmos server
import { rendererConfig, fixtures, decorators } from './cosmos.userdeps';

export default class App extends Component {
  render() {
    return (

Once your fixtures are loading properly, you'll probably want to split your App entry point to load Cosmos in development and your root component in production. Something like this:

module.exports = global.__DEV__
  ? require('./App.cosmos')
  : require('./App.main');

IMPORTANT: React Native blacklists __fixtures__ dirs by default. Unless you configure Cosmos to use a different directory pattern, you need to override getBlacklistRE in the React Native CLI config.

React Native for Web

Run cosmos --external-userdeps instead of cosmos-native and Cosmos will mirror your fixtures on both DOM and Native renderers.

Server-side APIs

Do NOT use these APIs in your fixture files, or any of your client code, as they require access to the file system and may bundle unwanted Node code in your client build.


Fetching a Cosmos config can be done in a number of ways, depending on whether or not you have a config file and, in case you do, if you prefer to specify the path manually or to rely on automatic detection.

Detect existing config based on cwd

detectCosmosConfig uses the same config detection strategy as the cosmos command.

import { detectCosmosConfig } from 'react-cosmos';

const cosmosConfig = detectCosmosConfig();

Read existing config at exact path

getCosmosConfigAtPath is best when you don't want to care where you run a script from.

import { getCosmosConfigAtPath } from 'react-cosmos';

const cosmosConfig = getCosmosConfigAtPath(require.resolve('./cosmos.config'));

Create default config

The minimum requirement to create a config is a root directory.

import { createCosmosConfig } from 'react-cosmos';

const cosmosConfig = createCosmosConfig(__dirname);

Create custom config

You can also customize your config programatically, without the need for an external config file.

import { createCosmosConfig } from 'react-cosmos';

const cosmosConfig = createCosmosConfig(__dirname, {
  // Options... (TypeScript is your friend)

Fixture URLs

A list with one Playground URL for each fixture, optionally in full-screen mode. A common use case for getFixtureUrls is to create visual snapshots for each fixture, and potentially to diff them between deploys.

import { getFixtureUrls } from 'react-cosmos';

const fixtureUrls = await getFixtureUrls({ cosmosConfig, fullScreen: true });

// localhost:5000/?fixtureId=%7B%22path%22%3A%22\_\_fixtures\_\_%2FHello%20World.ts%22%2C%22name%22%3Anull%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22\_\_fixtures\_\_%2FProps%20Playground.tsx%22%2C%22name%22%3Anull%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22\_\_fixtures\_\_%2FState%20Playground.tsx%22%2C%22name%22%3Anull%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22Counter%2Findex.fixture.tsx%22%2C%22name%22%3A%22default%22%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22Counter%2Findex.fixture.tsx%22%2C%22name%22%3A%22small%20number%22%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22Counter%2Findex.fixture.tsx%22%2C%22name%22%3A%22large%20number%22%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22CounterButton%2Findex.fixture.tsx%22%2C%22name%22%3Anull%7D&fullScreen=true
// localhost:5000/?fixtureId=%7B%22path%22%3A%22WelcomeMessage%2Findex.fixture.tsx%22%2C%22name%22%3Anull%7D&fullScreen=true
// ...

Fixture elements (rendered anywhere)

A list of fixture elements to render by hand. A common use case for getFixtures is to run snapshot tests in alternative environments like jsdom.

import { getFixtures } from 'react-cosmos';

const fixtures = await getFixtures({ cosmosConfig });

fixtures.forEach(({ fixtureId, getElement }) => {
  const renderer = create(getElement());

Check out the full example for more details on how to use the getFixtures API.


Using Next.js?

"Failed to execute postMessage..."?

"localhost:3001/__get-internal-source..." 404s?

Where's my old Cosmos?

Cosmos Classic packages have been moved to a dedicated repo, which means we can continue to maintain Cosmos Classic or even run it alongside Cosmos Next in the same project (during the migration period).

That said, it's ideal for all Cosmos users to use the latest version. Please let me know if you need help upgrading.

Why Cosmos?

Many other component-oriented tools have emerged since Cosmos was conceived. Storybook and React Styleguidist are good examples, but you can find an exhaustive list here. Choose the tool that matches your needs the most.

React Cosmos is a dev tool first, made to address all components, big and small, not just the stateless UI bits. It aims to boost productivity and push UI developers into a Pit of Success, creating robust components that survive the test of time.

You can also create a living style guide using React Cosmos, but it's a secondary goal and you might get more value from alternatives if this is your chief concern.


Hi, this is Ovidiu, the core maintainer of React Cosmos. I spend ridiculous amounts of time perfecting this project because I love building user interfaces and making useful stuff.

React Cosmos is licensed as MIT and will always be free. If you want to support me, however, become a Sponsor and ensure this journey continues indefinitely!

Special thanks to @maxsalven and @xavxyz for the long conversations and recurring interest in this project, as well as @catalinmiron, @flaviusone, @NiGhTTraX, @ovidiubute, @RadValentin, @tkjone, and all the other contributors. You're a big reason why React Cosmos is still alive 鉂わ笍.

Shout-out to Kreativa Studio for offering the Cosmos illustration for free!

For feedback create a GitHub issue or join us on Slack.

*Note that all licence references and agreements mentioned in the cosmos-js README section above are relevant to that project's source code only.