TypeError: $.mobile.activePage is undefined

<script src="js/jquery-1.10.2.min.js" type="text/javascript" language="javascript"></script>
<script src="js/jquery.mobile-1.4.0.min.js" type="text/javascript" language="javascript"></script>
<script src="js/modernizr-2.6.1.min.js" type="text/javascript" language="javascript"></script>
<script src="js/script.js" type="text/javascript" language="javascript"></script>

var header_height  = $.mobile.activePage.children('[data-role="header"]').height(),
            footer_height  = $.mobile.activePage.children('[data-role="footer"]').height(),
            window_height  = $(this).height(),
            content_height = window_height - header_height - footer_height;

            $.mobile.activePage.children('[data-role="content"]').children('.contentMain').height(content_height-130).css('overflow','auto');

            $('#left-menu').height(window_height).css('overflow','auto');

$(function() {
document.addEventListener('deviceready', onDeviceReady, false);
//for testing in Chrome browser uncomment
//onDeviceReady(); });

function onDeviceReady() {
                var header_height  = $.mobile.activePage.children('[data-role="header"]').height(),
                footer_height  = $.mobile.activePage.children('[data-role="footer"]').height(),
                window_height  = $(this).height(),
                content_height = window_height - header_height - footer_height;

                $.mobile.activePage.children('[data-role="content"]').children('.contentMain').height(content_height-130).css('overflow','auto');

                $('#left-menu').height(window_height).css('overflow','auto');
}

$( ".selector" ).pagecontainer( "getActivePage" );