how to get dropdownlist selected value and text in javascript

var IndexValue = $get('<%=ddlGraphperiod.ClientID %>').selectedIndex;
var SelectedText = $get('<%=ddlGraphperiod.ClientID %>').options[IndexValue].text;
var SelectedVal = $get('<%=ddlGraphperiod.ClientID %>').options[IndexValue].value;

How to call Server Side function using ajax in asp.net

Using Web method we can call server side method from java script.

Use following link for more details.

"http://69.10.233.10/KB/ajax/ajax_server_side_call.aspx?display=Print"

Navigation in Page using JS

Use "history.go(-1);return false;" to go back in browser based application.
"history.go(1);return false;" to go forward in browser based application.