What is JS?

Javascript called JS in short name. It is a scripting language used to enhance HTML pages and is commonly found embedded in HTML code. JavaScript is an interpreted language. Thus, it doesn't need to be compiled. JavaScript renders web pages in an interactive and dynamic fashion.

Wikipedia Says:

JavaScript often abbreviated as JS, is a high-level, interpreted programming language.
It is a language which is also characterized as dynamic, weakly typed, prototype-based and multi-paradigm. Alongside HTML and CSS, JavaScript is one of the three core technologies of the World Wide Web. JavaScript enables interactive web pages and thus is an essential part of web applications.
The vast majority of websites use it, and all major web browsers have a dedicated JavaScript engine to execute it.

 JavaScript used to better performance of website. The extension of  JavaScript is .js 

*** Example:

  <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.3.1.min.js"></script>

     <script type="text/javascript">  
        $(document).ready(function()
        {
            alert("This is js.");
        });
    </script>