There are a couple relatively new tools that give you just about everything you need to build a React app in the browser.
The first is CodeSandbox. It has a couple pre-configured environments for different technologies. The main one is React. CodeSandbox describes itself as such:
an online editor that helps you create web applications, from prototype to deployment.
The second, which just released out of beta, is Glitch which comes with projects like react-starter so that you can jump into a React project that is ready to roll. Glitch describes itself as:
the friendly community where you'll build the app of your dreams
You can quickly build and publish React apps in either of these and share them with the world.