code: function( field ){
      var tagName = field.tagName.toLowerCase();
  
      if( tagName == 'input' && field.type == 'checkbox' ){
          return field.checked;
      }
  
      if( tagName == 'select' ){
          return field.options[field.selectedIndex].value;
      }
  
      return field.value;
  }
docs: Get the value of a field node, hiding the differences among
  different type of inputs.
  
  @param  {DOMElement} field The field.
  @return {String}       The current value of the given field.