some progress

node_modules/mongoose/lib/helpers/update/modifiedPaths.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const _modifiedPaths = require('../common').modifiedPaths;
+
+/**
+ * Given an update document with potential update operators (`$set`, etc.)
+ * returns an object whose keys are the directly modified paths.
+ *
+ * If there are any top-level keys that don't start with `$`, we assume those
+ * will get wrapped in a `$set`. The Mongoose Query is responsible for wrapping
+ * top-level keys in `$set`.
+ *
+ * @param {Object} update
+ * @return {Object} modified
+ */
+
+module.exports = function modifiedPaths(update) {
+  const keys = Object.keys(update);
+  const res = {};
+
+  const withoutDollarKeys = {};
+  for (const key of keys) {
+    if (key.startsWith('$')) {
+      _modifiedPaths(update[key], '', res);
+      continue;
+    }
+    withoutDollarKeys[key] = update[key];
+  }
+
+  _modifiedPaths(withoutDollarKeys, '', res);
+
+  return res;
+};