JavaScript is a text-based programming language used for client- and server-side development. Web developers have used JavaScript since its inception to add simple interactivity to web pages like modals, animated elements, or image slideshows. While that is still one of its primary uses, Javascript has evolved to building complete web applications. SPAs, or single page applications, depend on JavaScript-based frameworks like React, Angular, and Vue to bring a desktop-like application experience to the browser. However, it's important to note that web browsers aren't places that make good use of the programming language. Javascript is also popular for building web services and back-end infrastructure with Node.js. React Native, Xamarin, Ionic, and NativeScript frameworks use JavaScript for developing phone apps as well.