diff -u b/core/misc/jquery.cookie.shim.es6.js b/core/misc/jquery.cookie.shim.es6.js
--- b/core/misc/jquery.cookie.shim.es6.js
+++ b/core/misc/jquery.cookie.shim.es6.js
@@ -48,7 +48,9 @@
     try {
       value = decodeURIComponent(value.replace(/\+/g, ' '));
       return parseJson ? JSON.parse(value) : value;
-    } catch (e) {}
+    } catch (e) {
+      // Exceptions on JSON parsing should be ignored.
+    }
   };
 
   /**
@@ -170,7 +172,20 @@
         ),
     });
 
-    return key === undefined ? cookiesShim.get() : cookiesShim.get(key);
+    return (() => {
+      if (key === undefined) {
+        const results = cookiesShim.get();
+        Object.keys(results).forEach(resultKey => {
+          if (results[resultKey] === undefined) {
+            delete results[resultKey];
+          }
+        });
+
+        return results;
+      }
+
+      return cookiesShim.get(key);
+    })();
   };
 
   /**
diff -u b/core/misc/jquery.cookie.shim.js b/core/misc/jquery.cookie.shim.js
--- b/core/misc/jquery.cookie.shim.js
+++ b/core/misc/jquery.cookie.shim.js
@@ -69,7 +69,19 @@
         return reader(cookieValue, cookieName, userProvidedConverter, $.cookie.raw, $.cookie.json);
       }
     });
-    return key === undefined ? cookiesShim.get() : cookiesShim.get(key);
+    return function () {
+      if (key === undefined) {
+        var results = cookiesShim.get();
+        Object.keys(results).forEach(function (resultKey) {
+          if (results[resultKey] === undefined) {
+            delete results[resultKey];
+          }
+        });
+        return results;
+      }
+
+      return cookiesShim.get(key);
+    }();
   };
 
   $.cookie.defaults = _objectSpread({
reverted:
--- b/core/tests/Drupal/Nightwatch/Tests/jsCookieTest.js
+++ a/core/tests/Drupal/Nightwatch/Tests/jsCookieTest.js
@@ -222,7 +222,6 @@
                   {
                     js_cookie_test_first: 'red panda',
                     js_cookie_test_second: 'second red panda',
-                    js_cookie_test_third: undefined,
                   },
                   '$.cookie() returns object containing all cookies',
                 );