var tabs = $('.tabs');
var selector = $('.tabs').find('a').length;
var activeItem = tabs.find('.active');
var activeWidth = activeItem.innerWidth();
$(".selector").css({
"left": activeItem.position.left + "px",
"width": activeWidth + "px"
});
$(".tabs").on("click","a",function(e){
e.preventDefault();
$('.tabs a').removeClass("active");
$(this).addClass('active');
var activeWidth = $(this).innerWidth();
var itemPos = $(this).position();
$(".selector").css({
"left":itemPos.left + "px",
"width": activeWidth + "px"
});
});