Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.

This was a pleasant surprise:

There are links to React examples on CodePen sprinkled all over the revamped docs. The first sentence of the documentation is:

The easiest way to get started with React is to use this Hello World example code on CodePen.

Some other pages include embedded Pens, like “Thinking in React” page that walks through the creation of an interactive component in a React-y way.

A massive screenshot, for posterity ;)