diff --git a/core/misc/details-summarized-content.es6.js b/core/misc/details-summarized-content.es6.js
index 7508f90ddb..de536fb302 100644
--- a/core/misc/details-summarized-content.es6.js
+++ b/core/misc/details-summarized-content.es6.js
@@ -17,18 +17,6 @@
     this.setupSummary();
   }
 
-  $.extend(
-    DetailsSummarizedContent,
-    /** @lends Drupal.DetailsSummarizedContent */ {
-      /**
-       * Holds references to instantiated DetailsSummarizedContent objects.
-       *
-       * @type {Array.<Drupal.DetailsSummarizedContent>}
-       */
-      instances: [],
-    },
-  );
-
   $.extend(
     DetailsSummarizedContent.prototype,
     /** @lends Drupal.DetailsSummarizedContent# */ {
diff --git a/core/misc/details-summarized-content.js b/core/misc/details-summarized-content.js
index 0696f2df5e..2ba84f812e 100644
--- a/core/misc/details-summarized-content.js
+++ b/core/misc/details-summarized-content.js
@@ -11,9 +11,6 @@
     this.setupSummary();
   }
 
-  $.extend(DetailsSummarizedContent, {
-    instances: []
-  });
   $.extend(DetailsSummarizedContent.prototype, {
     setupSummary: function setupSummary() {
       this.$detailsSummarizedContentWrapper = $(Drupal.theme('detailsSummarizedContentWrapper'));