sticky_footer.js

/**
 * Assumes your #footer is inside #wrapper
 *
 * Requires jQuery
 */
var stickyFooter = function() {
    var wrapperHeight = $('#wrapper')[0].clientHeight;
    var pageHeight = window.innerHeight;
    if (wrapperHeight < pageHeight) {
        var diffHeight = pageHeight - wrapperHeight;
        $('#footer').css('margin-top', diffHeight+'px');
    }
}

stickyFooter();