Provide a list of commonly-used tags to populate a text field.

gistfile1.js

/*
 * Provide links to commonly-used tags that can be clicked by a user to
 * populate a text field with comma-separated tags (ensuring that no
 * duplicates are entered).
 *
 * This assumes that the text field has an ID of `tag_string` and the links
 * have the class `tag`, e.g.
 *
 *   <input type="text" name="tag_string" id="tag_string" />
 *   <ul>
 *     <li><a href="#" class="tag">blue</a></li>
 *     <li><a href="#" class="tag">red</a></li>
 *   </ul>
 */

$('a.tag').click(function() {
  var current_tag_string = $('#tag_string').val();
  var current_tags = current_tag_string.split(/\s*,\s*/);
  var new_tag = $(this).text();

  if (-1 == $.inArray(new_tag, current_tags)) {
    if ('' == current_tag_string) {
      $('#tag_string').val(new_tag);
    } else {
      $('#tag_string').val(current_tag_string + ', ' + new_tag);
    }
  }
  return false;
});