refactor: remove unnecessary _this and add comment explaining opToThunk

lib/query.js

@@ -65,6 +65,9 @@ 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'],
@@ -2353,8 +2356,7 @@ 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);
@@ -2391,17 +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;