refactor: remove unnecessary _populate() re: code review comments

lib/model.js

@@ -4245,51 +4245,34 @@ Model.populate = async function populate(docs, paths) {
   if (typeof paths === 'function' || typeof arguments[2] === 'function') {
     throw new MongooseError('Model.populate() no longer accepts a callback');
   }
-  const _this = this;
   // normalized paths
   paths = utils.populate(paths);
 
-  await _populate(_this, docs, paths);
-  return docs;
-};
-
-/**
- * Populate helper
- *
- * @param {Model} model the model to use
- * @param {Document|Array} docs Either a single document or array of documents to populate.
- * @param {Object} paths
- * @param {never} cache Unused
- * @param {Function} [callback] Optional callback, executed upon completion. Receives `err` and the `doc(s)`.
- * @return {Function}
- * @api private
- */
-
-async function _populate(model, docs, paths) {
   if (paths.length === 0) {
-    return;
+    return docs;
   }
+
   // each path has its own query options and must be executed separately
   const promises = [];
   for (const path of paths) {
-    promises.push(populate(model, docs, path));
+    promises.push(_populatePath(this, docs, path));
   }
-
   await Promise.all(promises);
-}
+
+  return docs;
+};
 
 /*!
- * Populates `docs`
+ * Populates `docs` for a single `populateOptions` instance.
  */
 const excludeIdReg = /\s?-_id\s?/;
 const excludeIdRegGlobal = /\s?-_id\s?/g;
 
-async function populate(model, docs, options) {
-  const populateOptions = options;
-  if (options.strictPopulate == null) {
-    if (options._localModel != null && options._localModel.schema._userProvidedOptions.strictPopulate != null) {
-      populateOptions.strictPopulate = options._localModel.schema._userProvidedOptions.strictPopulate;
-    } else if (options._localModel != null && model.base.options.strictPopulate != null) {
+async function _populatePath(model, docs, populateOptions) {
+  if (populateOptions.strictPopulate == null) {
+    if (populateOptions._localModel != null && populateOptions._localModel.schema._userProvidedOptions.strictPopulate != null) {
+      populateOptions.strictPopulate = populateOptions._localModel.schema._userProvidedOptions.strictPopulate;
+    } else if (populateOptions._localModel != null && model.base.options.strictPopulate != null) {
       populateOptions.strictPopulate = model.base.options.strictPopulate;
     } else if (model.base.options.strictPopulate != null) {
       populateOptions.strictPopulate = model.base.options.strictPopulate;