app/assets/javascripts/tinymce.js
// Import TinyMCE
import 'tinymce/tinymce';
// A theme is also required
import 'tinymce/themes/silver/theme';
// Icons are also required
import 'tinymce/icons/default';
// Any plugins you want to use has to be imported
import 'tinymce/plugins/code';
import 'tinymce/plugins/link';
import 'tinymce/plugins/lists';