Merge pull request Automattic#15250 from Automattic/vkarpov15/Automatticgh-14906

 perf: some more micro optimizations for find() and findOne() re: Automattic#14906

Author: vkarpov15

lib/document.js

@@ -741,15 +741,10 @@ function init(self, obj, doc, opts, prefix) {
   let schemaType;
   let path;
   let i;
-  let index = 0;
   const strict = self.$__.strictMode;
   const docSchema = self.$__schema;
 
-  while (index < len) {
-    _init(index++);
-  }
-
-  function _init(index) {
+  for (let index = 0; index < len; ++index) {
     i = keys[index];
     // avoid prototype pollution
     if (i === '__proto__' || i === 'constructor') {

lib/query.js

@@ -65,6 +65,25 @@ const queryOptionMethods = new Set([
   'wtimeout'
 ]);
 
+// Map from operation name to the name of the function that executes the actual operation against MongoDB.
+// Called a thunk for legacy reasons, "thunk" means function that takes exactly 1 param, a callback.
+// Currently `_countDocuments()`, etc. are async functions that take no params.
+const opToThunk = new Map([
+  ['countDocuments', '_countDocuments'],
+  ['distinct', '__distinct'],
+  ['estimatedDocumentCount', '_estimatedDocumentCount'],
+  ['find', '_find'],
+  ['findOne', '_findOne'],
+  ['findOneAndReplace', '_findOneAndReplace'],
+  ['findOneAndUpdate', '_findOneAndUpdate'],
+  ['replaceOne', '_replaceOne'],
+  ['updateMany', '_updateMany'],
+  ['updateOne', '_updateOne'],
+  ['deleteMany', '_deleteMany'],
+  ['deleteOne', '_deleteOne'],
+  ['findOneAndDelete', '_findOneAndDelete']
+]);
+
 /**
  * Query constructor used for building queries. You do not need
  * to instantiate a `Query` directly. Instead use Model functions like
@@ -2337,18 +2356,17 @@ Query.prototype._find = async function _find() {
   }
 
   const mongooseOptions = this._mongooseOptions;
-  const _this = this;
-  const userProvidedFields = _this._userProvidedFields || {};
+  const userProvidedFields = this._userProvidedFields || {};
 
   applyGlobalMaxTimeMS(this.options, this.model.db.options, this.model.base.options);
   applyGlobalDiskUse(this.options, this.model.db.options, this.model.base.options);
 
   // Separate options to pass down to `completeMany()` in case we need to
   // set a session on the document
-  const completeManyOptions = Object.assign({}, {
+  const completeManyOptions = {
     session: this && this.options && this.options.session || null,
     lean: mongooseOptions.lean || null
-  });
+  };
 
   const options = this._optionsForExec();
 
@@ -2366,7 +2384,7 @@ Query.prototype._find = async function _find() {
   }
 
   if (!mongooseOptions.populate) {
-    const versionKey = _this.schema.options.versionKey;
+    const versionKey = this.schema.options.versionKey;
     if (mongooseOptions.lean && mongooseOptions.lean.versionKey === false && versionKey) {
       docs.forEach((doc) => {
         if (versionKey in doc) {
@@ -2375,17 +2393,17 @@ Query.prototype._find = async function _find() {
       });
     }
     return mongooseOptions.lean ?
-      _completeManyLean(_this.model.schema, docs, null, completeManyOptions) :
-      _this._completeMany(docs, fields, userProvidedFields, completeManyOptions);
+      _completeManyLean(this.model.schema, docs, null, completeManyOptions) :
+      this._completeMany(docs, fields, userProvidedFields, completeManyOptions);
   }
 
-  const pop = helpers.preparePopulationOptionsMQ(_this, mongooseOptions);
+  const pop = helpers.preparePopulationOptionsMQ(this, mongooseOptions);
 
   if (mongooseOptions.lean) {
-    return _this.model.populate(docs, pop);
+    return this.model.populate(docs, pop);
   }
 
-  docs = await _this._completeMany(docs, fields, userProvidedFields, completeManyOptions);
+  docs = await this._completeMany(docs, fields, userProvidedFields, completeManyOptions);
   await this.model.populate(docs, pop);
 
   return docs;
@@ -4397,22 +4415,14 @@ Query.prototype.exec = async function exec(op) {
   if (this.model == null) {
     throw new MongooseError('Query must have an associated model before executing');
   }
-  this._validateOp();
-
-  if (!this.op) {
-    return;
-  }
 
-  if (this.options && this.options.sort) {
-    const keys = Object.keys(this.options.sort);
-    if (keys.includes('')) {
-      throw new Error('Invalid field "" passed to sort()');
-    }
+  const thunk = opToThunk.get(this.op);
+  if (!thunk) {
+    throw new MongooseError('Query has invalid `op`: "' + this.op + '"');
   }
 
-  let thunk = '_' + this.op;
-  if (this.op === 'distinct') {
-    thunk = '__distinct';
+  if (this.options && this.options.sort && typeof this.options.sort === 'object' && this.options.sort.hasOwnProperty('')) {
+    throw new Error('Invalid field "" passed to sort()');
   }
 
   if (this._executionStack != null) {