some progress

node_modules/mongoose/lib/helpers/projection/hasIncludedChildren.js

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+'use strict';
+
+/**
+ * Creates an object that precomputes whether a given path has child fields in
+ * the projection.
+ *
+ * #### Example:
+ *
+ *     const res = hasIncludedChildren({ 'a.b.c': 0 });
+ *     res.a; // 1
+ *     res['a.b']; // 1
+ *     res['a.b.c']; // 1
+ *     res['a.c']; // undefined
+ *
+ * @param {Object} fields
+ * @api private
+ */
+
+module.exports = function hasIncludedChildren(fields) {
+  const hasIncludedChildren = {};
+  const keys = Object.keys(fields);
+
+  for (const key of keys) {
+    if (key.indexOf('.') === -1) {
+      hasIncludedChildren[key] = 1;
+      continue;
+    }
+    const parts = key.split('.');
+    let c = parts[0];
+
+    for (let i = 0; i < parts.length; ++i) {
+      hasIncludedChildren[c] = 1;
+      if (i + 1 < parts.length) {
+        c = c + '.' + parts[i + 1];
+      }
+    }
+  }
+
+  return hasIncludedChildren;
+};