What is the importance of javascript in the web?

JavaScript is important part of any web application, it useful for validating input in a web browser without needing to go to the server. It useful for mainulating HTML DOM, CSS when  user is interacting with a webpage. It is very useful to send asynchronous requests to server without refreshing the page, which is very much useful for chat applications, games and any other application which needes instant feedback without reloading the page.