Mongoose model init (#25530)

* Adding init method declaration on Model interface

* Test init method on Model

types/mongoose/index.d.ts

@@ -2714,6 +2714,16 @@ declare module "mongoose" {
     insertMany(doc: any, callback?: (error: any, doc: T) => void): Promise<T>;
     insertMany(doc: any, options?: { ordered?: boolean, rawResult?: boolean }, callback?: (error: any, doc: T) => void): Promise<T>;
 
+    /**
+     * Performs any async initialization of this model against MongoDB.
+     * This function is called automatically, so you don't need to call it.
+     * This function is also idempotent, so you may call it to get back a promise
+     * that will resolve when your indexes are finished building as an alternative
+     * to `MyModel.on('index')`
+     * @param callback optional
+     */
+    init(callback?: (err: any) => void): Promise<T>;
+
     /**
      * Executes a mapReduce command.
      * @param o an object specifying map-reduce options

types/mongoose/mongoose-tests.ts

@@ -1420,6 +1420,7 @@ var MongoModel = mongoose.model('MongoModel', new mongoose.Schema({
     required: true
   }
 }), 'myCollection', true);
+MongoModel.init().then(cb);
 MongoModel.find({}).$where('indexOf("val") !== -1').exec(function (err, docs) {
   docs[0].save();
   docs[0].__v;