Regarding Calling a Code Behind in C# using JavaScript

Question

hi dear

i want to know how can i call a code behind method written using c# with the help of javascript.

e.g. i have two dropdownlist one is populated with the names of states on pageload and i want to populate another with the city name on selection of the first dropdownlist. i want to use javascript for this to avoid page postback .

waiting for a healthy reply

thanks & regards
vishal sharma