trying to test of checkbox is checked

trying to test of checkbox is checked

from my form:

  <div class="field" id="free_shipping">
   <input type="checkbox" name="free_shipping" id="free_shipping">Free shipping
  </div>

from my js:

  $('#free_shipping').change(function(e)
   {
    if ($('#free_shipping').prop('checked'))
     {
     $("#shipping_cost").hide();
     }
    else
     {
     $("#shipping_cost").show();
     }
  })