JavaScript (JS) is a high-level, interpreted programming language, used as a scripting language in web pages to generate dynamic content. Along with HTML and CSS, JavaScript is a part of the core technologies powering the World Wide Web and the Internet we know today.

Though it was intended to be used as a client-side scripting language to generate dynamic web pages, JS has different variants that can handle applications such as the front-end of a website/web application (Vue.js, React.js, etc) and server-side scripting a.k.a the back-end of a website/web application (Node.js, etc).

Some of its variants are also used in creating desktop and mobile native apps (Proton, Electron, etc)

The file extension of JavaScript is .js

The ".js" extension in the names of some frameworks like
React.js is supposed to mean that they are related to
JavaScript and not that they are individual files
written in JavaScript.