add-to-cart.js

  $(document).on('click','.productId', function(e){
      var qty = 1;
      var id = $(this).data('variant');
  
  addItem=function(qty,id,callback) {
  var params = {quantity:qty,id:id};
 
  $.ajax({
    type: 'POST',
    url: '/cart/add.js',
    dataType: 'json',
    data: params,
    success: function(){
     
          $.getJSON('/cart.js', function(cart) {
      var new_count = cart.item_count ;
      $('.site-header-cart--count ').attr('data-header-cart-count', new_count);          
});   
    },
    error: function(){}
  });
}
addItem(qty,id,function(){
    console.log('done');
    });       
   
});