javascript alert on dropdown change event

Hi and good morning,

I want to display javascript alert or some message like validation after dropdown selection changed.
I have to do some calculations on code behind side and then throw an javascript alert
I am using asp.net with language vb

currently I am doing like on dropdown selected index changed event

Dim strMessage = "alert message"
                    Dim jscript As String
                    jscript = "<script language=JavaScript>alert('" + strMessage + "')<" & "/" & "script>"
                    'RegisterClientScriptBlock("scr", jscript)
                    Page.ClientScript.RegisterClientScriptBlock(Me.GetType, "scr", jscript)

but this will come first and then the page gets loaded.
I want to show like notification or message not like this which will stop remaining execution.

how to do that?

Happy Programming!!
Rohi