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core/modules/comment/js/comment-by-viewer.js

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+/**
+ * @file
+ * Attaches behaviors for the Comment module's "by-viewer" class.
+ */
+
+(function ($, Drupal, drupalSettings) {
+  /**
+   * Add 'by-viewer' class to comments written by the current user.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.commentByViewer = {
+    attach(context) {
+      const currentUserID = parseInt(drupalSettings.user.uid, 10);
+      $('[data-comment-user-id]')
+        .filter(function () {
+          return (
+            parseInt(this.getAttribute('data-comment-user-id'), 10) ===
+            currentUserID
+          );
+        })
+        .addClass('by-viewer');
+    },
+  };
+})(jQuery, Drupal, drupalSettings);