notebook/app/assets/javascripts/content_linking.js

$(document).ready(function () {
  console.log('initializing new content linking js');

  $('.content-field').find('textarea').focus(function (focus_event) {
    var focused_text_field = $(focus_event.target);
    var parent_content_field = focused_text_field.closest('.content-field');

    $('.content-field').removeClass('focused');
    parent_content_field.addClass('focused');
    $('.content-field-link-bar').hide();
    parent_content_field.find('.content-field-link-bar').show();
  });

  $('body').blur(function (focus_event) {
    $('.content-field').removeClass('focused');
  });

  $('.js-content-link-option').click(function (click_event) {
    var selected_option    = $(click_event.target);
    var textarea_to_modify = selected_option
                               .closest('.content-field')
                               .find('textarea');
    var current_textarea_value = textarea_to_modify.val();

    var token_to_insert = [
      '[[',
      selected_option.data('content-type'),
      '-',
      selected_option.data('content-id'),
      ']]'
    ].join('')

    // If the final character of the textarea is a newline or a space
    // character, then we want to just append our token. Otherwise, we want
    // to prepend our token with a space so it is properly separated.
    var final_letter_presently = current_textarea_value.slice(-1);
    if (final_letter_presently != ' ' && final_letter_presently != "\n") {
      // Prepend a space
      token_to_insert = ' ' + token_to_insert;
    }

    // Make the change!
    textarea_to_modify.val(current_textarea_value + token_to_insert);
  });
});