cordova back button close app code example
Example: cordova android close app with back button
document.addEventListener("deviceready", onDeviceReady, false);
function onDeviceReady(){
document.addEventListener("backbutton", function(e){
if($.mobile.activePage.is('#homepage')){
e.preventDefault();
navigator.app.exitApp();
}
else {
navigator.app.backHistory();
}
}, false);
}