React
React (also known as React.js or ReactJS) is a JavaScript library for building user interfaces. It is maintained by Facebook and a community of individual developers and companies. It is also the basis of Gutenberg, the new WordPress editor.
Because React is written in JavaScript, and themes written to use the Classic Editor use PHP, themes using the Classic Editor cannot automatically migrate to Gutenberg. They must be tested or rewritten to be fully compatible with the new WordPress editor.