how to use javascript code in between C# Code?

How to use return value of javacript alert function when we use in C# by RegisterStartupScript. This is my code: string scriptString = ""; Page.RegisterStartupScript("ScriptString ", scriptString ); I want to use return value of that alert i.e i user click yes iwant to do something. if No some other thing can i use that return value:if yes how to use that?Is ther modifications in the to scriptString use the return value?