A React application is primarily JavaScript. If you have experience working with JavaScript, you know that the browser is perfectly capable of parsing and executing JavaScript on its own.

In most basic websites, we link to the JavaScript needed for the page in a <script> tag, and the browser downloads and runs it.

We'll be doing something similar with our React application (with considerable complications; more on that in Chapter 2, Getting Started with Webpack).

However, JavaScript is no longer confined to the browser. More and more applications are using JavaScript on the backend as well, with JavaScript running in its own environment.

Long story short, JavaScript is now everywhere, and the driving force behind this proliferation is Node.js, a JavaScript runtime library, which lets you run JavaScript outside of a browser environment.

Okay, this is exciting, but why does this matter for our React project?

Node also introduced the idea of packages to JavaScript. Packages are essentially third-party libraries of code that you can install to your application and then import and use where and when you need them. You can use packages even if your application is not a Node application.

React is one such package. Webpack, mentioned earlier, is another one. In short, in order to build a complex web application, we will inevitably rely on a lot of other people's code, so we need packages, and we need Node's package manager (shorthand npm) to install them.

We’ll also use npm to start up our application and do some basic tasks, but its primary purpose is to manage packages.