Nothing in this chapter needs a local install to try. A browser sandbox gives you a React 7,897 project in seconds and — more usefully — a URL. When you report a bug or share a snippet with a colleague, a link to a running reproduction saves everyone the "it works here" round trip; maintainers of React libraries routinely close issues that arrive without one.
| Sandbox | Where the code runs | Best for |
|---|---|---|
| StackBlitz 38,726 | In your browser, on WebContainers | Full projects, instant boot |
| CodeSandbox 24,655 | A container in the cloud | Repos, branches, npm 2,036 scripts |
| CodePen | In your browser, no Node | Single-file snippets and demos |
| Replit 3,526 | A container in the cloud | Pairing and teaching |
The interesting one technically is StackBlitz. Its WebContainers (https://webcontainers.io/) runtime brings Node.js 2,131 into the browser tab itself, so npm install and vite dev run on your own machine with no server involved: boot is instant and nothing you paste leaves the browser. CodeSandbox takes the opposite approach and gives you a real Linux container, slower to start but able to run anything Node runs, including an Express 24,430 server from Express.js.
The React documentation links straight to a React project on StackBlitz (https://stackblitz.com/fork/react 38,726 ) and a new CodeSandbox (https://codesandbox.io/s/new 24,655 ); add ?file=src/App.jsx to a shared link to open it on a particular file. For a quick snippet a sandbox beats the import-map page of React in an HTML Page.