// Original DOM element:
// <div id="item" data-tags="popcorn,chickens"></div>

// SET (for eg: based on an ajax response):
// The first call (with .attr) updates the DOM element only:
$item.attr('data-tags',response.tag); // "popcorn,chickens"
$item.attr('data-tags'); // "popcorn,chickens,dandelions"
// <div id="item" data-tag="popcorn,chickens,dandelions"></div>
$item.data('tags',response.tag);      // [popcorn,chickens]

// This call references the $.data object stored by jQuery independently of the actual data-attr:
$item.data('tags',response.tag);      // [popcorn,chickens,dandelions]
// (A 'get' call to $item.attr('data-tags') would still yield "popcorn,chickens" if we
// had not set it using .attr above). So need to do both.