React

server/node_modules/mongoose/lib/helpers/query/isOperator.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const specialKeys = new Set([
+  '$ref',
+  '$id',
+  '$db'
+]);
+
+module.exports = function isOperator(path) {
+  return (
+    path[0] === '$' &&
+    !specialKeys.has(path)
+  );
+};