perf(document): add a couple of small perf improvements re: #10400

Author: vkarpov15

lib/document.js

@@ -747,7 +747,7 @@ Document.prototype.$__init = function(doc, opts) {
 
   const hasIncludedChildren = this.$__.exclude === false && this.$__.fields ?
     $__hasIncludedChildren(this.$__.fields) :
-    {};
+    null;
   $__applyDefaults(this, this.$__.fields, this.$__.exclude, hasIncludedChildren, false, this.$__.skipDefaults);
 
   return this;

lib/schema/documentarray.js

@@ -365,6 +365,9 @@ DocumentArrayPath.prototype.getDefault = function(scope) {
   return ret;
 };
 
+const _toObjectOptions = Object.freeze({ transform: false, virtuals: false });
+const initDocumentOptions = Object.freeze({ skipId: true, willInit: true });
+
 /**
  * Casts contents
  *
@@ -384,7 +387,7 @@ DocumentArrayPath.prototype.cast = function(value, doc, init, prev, options) {
 
   let selected;
   let subdoc;
-  const _opts = { transform: false, virtuals: false };
+
   options = options || {};
 
   if (!Array.isArray(value)) {
@@ -414,9 +417,7 @@ DocumentArrayPath.prototype.cast = function(value, doc, init, prev, options) {
   }
 
   const rawArray = utils.isMongooseDocumentArray(value) ? value.__array : value;
-
   const len = rawArray.length;
-  const initDocumentOptions = { skipId: true, willInit: true };
 
   for (let i = 0; i < len; ++i) {
     if (!rawArray[i]) {
@@ -464,7 +465,7 @@ DocumentArrayPath.prototype.cast = function(value, doc, init, prev, options) {
           subdoc = prev.id(rawArray[i]._id);
         }
 
-        if (prev && subdoc && utils.deepEqual(subdoc.toObject(_opts), rawArray[i])) {
+        if (prev && subdoc && utils.deepEqual(subdoc.toObject(_toObjectOptions), rawArray[i])) {
           // handle resetting doc with existing id and same data
           subdoc.set(rawArray[i]);
           // if set() is hooked it will have no return value