What is JavaScript ?
Javascript is a scripting language and it was primarily built to add functionality to the websites. HTML is the building block of the websites and css was added to add styles to the HTML. As HTML and CSS are only for the look of the website the interactivity is added by the Javascript.
As for now a days javascript is one of the top languages being used mostly for front end developement. It is also being used now a day as backend development with node.js.