The Transformer API is used to create different views of the same domain object. If you want to add or hide information to be returned to the client, the way to go is to use a Transfomer. For instance:

public class BasicObjectTransformer extends Transformer<Person> {

    public Map<String, Object> upperCase(Person person) {
        Map<String, Object> map = asMap(person);
        map.put("name", person.getName().toUpperCase());
        return map;

Now, to transform a given person, let’s say with id 123, you can:

curl -X GET http://localhost:8080/api/people/123?t=upperCase

The Javascript equivalent would be:

yawp('/people/123').transform('upperCase').first( function(person) {} );

Note: All transformers can be applied for collections queries or fetching single objects.

Finally, you can also create more sophisticated transformers using the presenter pattern, like this:

public class UserTransformer extends Transformer<User> {

   public static class UserView {
        private String name;
        private String company;
        private int birthYear;

        public UserView(User user) {
   = user.getName();
   = user.getCompany();
            this.birthYear = Calendar.getInstance().get(Calendar.YEAR) - user.getAge();

    public UserView withYear(User user) {
        return new UserView(user);