How to remove products from cart using AJAX in shopify

I want to remove cart products if cart contains more than 20 products then it should be removed latest more than 20 products for example if I add 21 products then last 1 product will be removed from cart if there any way to achieve this type of functionality

  jQuery.getJSON('/cart.js', function (data) {
        var count_new = data.items.id;  
        console.log(count_new);
  });

I have tried this code but its not working