title: event is not defined javascript firefox (json)
body: <h2>can any body tell me why my code error on firefox?</h2>
  
  <p><strong>HTML:</strong></p>
  
  <pre><code>Size : &lt;select class="size" id="size&lt;?php echo $dat_women-&gt;id; ?&gt;" name="size" onchange="change_size(&lt;?php echo $dat_women-&gt;id; ?&gt;)"&gt;
  </code></pre>
  
  <p><strong>JS:</strong></p>
  
  <pre><code>function change_size(id_product){
      var size  = document.getElementById(event.target.id).value;
      $.ajax({
          dataType:"json",
          url: site_url + "main/add_cart/" + id_product + "/" + size,
          success:function(data){ 
              window.location.assign("cart");
          },
          error: function(data){
              alert('Anda belum memilih ukuran');
          }
      });
  };
  </code></pre>
  
  <p><strong>Error:</strong></p>
  
  <blockquote>
    <p>event is not defined</p>
  </blockquote>