New source found from dndbeyond.com

ddb_main/node_modules/@emotion/weak-memoize/dist/emotion-weak-memoize.esm.js

@@ -1,9 +1,9 @@
 var weakMemoize = function weakMemoize(func) {
-  // $FlowFixMe flow doesn't include all non-primitive types as allowed for weakmaps
   var cache = new WeakMap();
   return function (arg) {
     if (cache.has(arg)) {
-      // $FlowFixMe
+      // Use non-null assertion because we just checked that the cache `has` it
+      // This allows us to remove `undefined` from the return value
       return cache.get(arg);
     }