react.dev/content/docs/hello-world.md

id	title	permalink	prev	next	redirect_from
hello-world	Hello World	docs/hello-world.html	cdn-links.html	introducing-jsx.html	[docs/ docs/index.html docs/getting-started.html docs/getting-started-ko-KR.html docs/getting-started-zh-CN.html]

The easiest way to get started with React is to use this Hello World example code on CodePen. You don't need to install anything; you can just open it in another tab and follow along as we go through examples. If you'd rather use a local development environment, check out the Installation section.

The smallest React example looks like this:

ReactDOM.render(
  <h1>Hello, world!</h1>,
  document.getElementById('root')
);

It renders a header saying "Hello, world!" on the page.

The next few sections will gradually introduce you to using React. We will examine the building blocks of React apps: elements and components. Once you master them, you can create complex apps from small reusable pieces.

A Note on JavaScript

React is a JavaScript library, and so it assumes you have a basic understanding of the JavaScript language. If you don't feel very confident, we recommend refreshing your JavaScript knowledge so you can follow along more easily.

We also use some of the ES6 syntax in the examples. We try to use it sparingly because it's still relatively new, but we encourage you to get familiar with arrow functions, classes, template literals, let, and const statements. You can use the Babel REPL to check what ES6 code compiles to.