超过多少字加省略号的一次处理方法

//当便利店介绍超过四行时，隐去四行，加省略号
            if($('#shop_dialog').height()>82){
                $('#shop_dialog').css({overflow:'hidden',height:'80px'});
                var a=parseFloat($('#shop_dialog').css('font-size'));
                var b=$('#shop_dialog').width();
                var c=$.trim($('#shop_dialog').text()).substring(0,parseInt(b/a)*4-2);
                c=c+'…'
                $('#shop_dialog').text(c);
            }