Web developers are developing web sites with combination of server-side processing and client-side processing to help the limitation of browser.

Javascript is written within the designing part of a web page, enclosed within the script block. A web page is accessed by thousands or lakhs people. So if all codes are written in server side then the code is executed so many times in the server side that the server may become very slow. As javascript is executed at client side so that does not affect the server. This is the main use of javascript.

Asp.Net is a server side platform which indicates that the codes written in .Net will run in the server. This mainly ensures the security of the code. In .Net web sites Javascript is used mostly to meet the above points.