JavaScript also abbreviated as JS is a scripting language that is one of the core technologies of the world wide web alongside HTML and CSS. JavaScript allows you to create content that updates dynamically, control multimedia, animate images among many other things.
Over 93% of websites use JavaScript on the client side for web page behaviour often incorporating third-party libraries. All major web browsers have a dedicated JavaScript engine to execute the code on the user device.
JavaScript is a high-level, often just-in-time compiled language that conforms to the ECMAScript standard.
It has application programming interfaces (APIs) for working with text, dates, regular expressions, standard data structures and the Document Object Model (DOM). 
In the ECMAScript standard, things like input/output (I/O), such as networking, storage or graphics facilities are not included, but web browsers or other runtime systems provide JavaScript APIs for I/O.

In the beginning JavaScript was used only in web browsers but it's now a core component of servers and a variety of applications built with Node JS.
JavaScript is used both for websites, for mobile hybrid applications (e.g React Native), machine learning (e.g Tensorflow.js), backend applications (server side e.g Node JS) can all be built using JavaScript.