Freeze element once reached to the scrollbar offset

gistfile1.txt

var offset = $("SELECTOR").offset();

if($('SELECTOR').get(0)) {
$(window).scroll(function() {
    if ( $('body').scrollTop() > offset.top){
        $('SELECTOR').addClass('is-fixed');
    } else {
        $('SELECTOR').removeClass('is-fixed');
    }
});
}