add interface for mongoose document optional properties

mongoose/mongoose.d.ts

@@ -715,7 +715,7 @@ declare module "mongoose" {
    * section document.js
    * http://mongoosejs.com/docs/api.html#document-js
    */
-  class MongooseDocument {
+  class MongooseDocument implements MongooseDocumentOptionals {
     /** Checks if a path is set to its default. */
     $isDefault(path?: string): boolean;
 
@@ -870,8 +870,6 @@ declare module "mongoose" {
 
     /** Hash containing current validation errors. */
     errors: Object;
-    /** The string version of this documents _id. */
-    id?: string;
     /** This documents _id. */
     _id: any;
     /** Boolean flag specifying if the document is new. */
@@ -880,6 +878,11 @@ declare module "mongoose" {
     schema: Schema;
   }
 
+  interface MongooseDocumentOptionals {
+    /** The string version of this documents _id. */
+    id?: string;
+  }
+
   interface DocumentToObjectOptions {
     /** apply all getters (path and virtual getters) */
     getters?: boolean;