some progress

node_modules/mongoose/lib/helpers/document/handleSpreadDoc.js

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+'use strict';
+
+const utils = require('../../utils');
+
+const keysToSkip = new Set(['__index', '__parentArray', '_doc']);
+
+/**
+ * Using spread operator on a Mongoose document gives you a
+ * POJO that has a tendency to cause infinite recursion. So
+ * we use this function on `set()` to prevent that.
+ */
+
+module.exports = function handleSpreadDoc(v, includeExtraKeys) {
+  if (utils.isPOJO(v) && v.$__ != null && v._doc != null) {
+    if (includeExtraKeys) {
+      const extraKeys = {};
+      for (const key of Object.keys(v)) {
+        if (typeof key === 'symbol') {
+          continue;
+        }
+        if (key[0] === '$') {
+          continue;
+        }
+        if (keysToSkip.has(key)) {
+          continue;
+        }
+        extraKeys[key] = v[key];
+      }
+      return { ...v._doc, ...extraKeys };
+    }
+    return v._doc;
+  }
+
+  return v;
+};