Over-ride the back button of android device through phonegap in javascript -

i making app in html5 , javascript , deploying on android device. after confirmation in application, don't want go on previous page. but, on button of device goes on previous page.

i tried many demos. following 1 of link tried. http://docs.phonegap.com/en/2.8.0/cordova_events_events.md.html#backbutton

i tried displaying alert demo purpose. not work.

please suggestions. thanks.

i tried following. doesn't work. need add external jquery file?

<script type="text/javascript" charset="utf-8">     function onload() {         document.addeventlistener("deviceready", ondeviceready, false);         alert("on load");     }       function ondeviceready() {         document.addeventlistener("backbutton", onbackkeydown, false);         alert("device ready");     }      //document.addeventlistener('backbutton', onbackkeydown, false);      function onbackkeydown(event) {         event.preventdefault();         alert("back pressed");     }     </script> 

the proper way prevent default behavior of button add prevent default method:

document.addeventlistener('backbutton', onbackkeydown, false);  function onbackkeydown(event) {     // handle button     event.preventdefault();     alert('i demo purpose alert thingy'); }