How to do validation on mongoose query with graphql

I need to check whether the data exists/already exists or not in the database. I am using graphql and mongoose.

Here's my graphql mutation object:

addBook: {
        type: BookType,
        args: {
            name: { type: new GraphQLNonNull(GraphQLString) },
            genre: { type: new GraphQLNonNull(GraphQLString) },
            authorid: { type: new GraphQLNonNull(GraphQLString) }
        },
        resolve(parent, args){
            let book = new Book({
                name: args.name,
                genre: args.genre,
                authorid: args.authorid
            });
            return book.save();
        }
    },
updateBook: {
        type: BookType,
        args: {
            name: { type: new GraphQLNonNull(GraphQLString) },
            updateName: { type: new GraphQLNonNull(GraphQLString) }
        },
        resolve(parent, args){
            let book = Book.findOneAndUpdate({name: args.name}, {name: args.updateName}, {new: true});
            return book.exec();
        }
    },
deleteBook: {
        type: BookType,
        args: {
            name: { type: new GraphQLNonNull(GraphQLString) }
        },
        resolve(parent, args){
            let book = Book.findOneAndDelete({name: args.name});
            return book.exec();
        }
    }

All of these successfully add, update, and delete in the MongoDB database. But I don't know how to validate it first if the data does exist or not. When I try to run an update on a non-existing book, GraphiQL UI throws null

{
  "data": {
    "updateBook": null
  }
}

How can I check for this? Thank you!