learn.js

// pre hook
var emailAuthor = function( done ){
	console.log( '\nemailing author!\n' );
	done();
};
BlogPost.pre( 'save', function( next ){
	//next( new Error('something went wrong') );
	next();
});
BlogPost.pre( 'save', true, function( next, done ){
  //                  ^^^^ // pass `true` for async middleware
	next();
	emailAuthor( done ); // <-- `done` must come after `next`
});