Add some comments explaining what we do here

Author: lforst

packages/utils/src/object.ts

@@ -220,12 +220,14 @@ export function dropUndefinedKeys<T>(inputValue: T): T {
 
 function _dropUndefinedKeys<T>(inputValue: T, memoizationMap: Map<unknown, unknown>): T {
   if (isPlainObject(inputValue)) {
+    // If this node has already been visited due to a circular reference, return the object it was mapped to in the new object
     const memoVal = memoizationMap.get(inputValue);
     if (memoVal !== undefined) {
       return memoVal as T;
     }
 
     const returnValue: { [key: string]: any } = {};
+    // Store the mapping of this value in case we visit it again, in case of circular data
     memoizationMap.set(inputValue, returnValue);
 
     for (const key of Object.keys(inputValue)) {
@@ -238,12 +240,14 @@ function _dropUndefinedKeys<T>(inputValue: T, memoizationMap: Map<unknown, unkno
   }
 
   if (Array.isArray(inputValue)) {
+    // If this node has already been visited due to a circular reference, return the array it was mapped to in the new object
     const memoVal = memoizationMap.get(inputValue);
     if (memoVal !== undefined) {
       return memoVal as T;
     }
 
     const returnValue: unknown[] = [];
+    // Store the mapping of this value in case we visit it again, in case of circular data
     memoizationMap.set(inputValue, returnValue);
 
     inputValue.forEach((item: unknown) => {