React

Statistically speaking, if you see a website today, it's written in React. (This one isn't.)

React is a front-end framework for making more modular pages. It's good for making interactive dynamic sites. When I first did web development, I thought React was the only thing in the world. I was wrong. But it's not a bad place to start.

React uses JSX, but that's not the only special thing about it. The rest has to do with how it handles state and reactivity (hence the name): how a dynamic website changes.

React is owned by Facebook, but it's open-source (here).