AutoMapper.js with predicate applying

AutoMapper.js

function DataMapper(schema, items) {
    let _schema = _.clone(schema);
    let transformer = (items) => {
         return _.chain(items)
                        .map((obj) => {
                            return _.mapValues(_schema, (transformation) => {
                                if(!_.isObject(transformation)){
                                    return _.get(obj, transformation);
                                }
                                else if(_.isFunction(transformation)){
                                    let delta = _.get(obj, transformation.path);
                                    return transformation(delta);
                                }
                                else{
                                    let delta = _.get(obj, transformation.path);
                                    return transformation.fn(delta);
                                }
                            });
                        }).value();
    };

    if (items === undefined) {
       return (items) => {
            return transformer(items);
        }
    } else {
        return transformer(items);
    }
}

class Comment {
    constructor(id, message, owner, comment, creationTime){}   
}

class User {
    constructor(id,firstname,lastname,hasAvatar){}
}

let turd = { 
    data : { user : { 
        id : 123,
        firstname : 'john'
    } },
    message: 'some content',
    id:'message-id-1'
}
let schema = { 
    id:'id', 
    message:'message', 
    owner: {
        path: 'data.user',
        fn: (user) => { return _.assignIn(new User(), user); }
    } 
};      
let transformedData = DataMapper(schema, [turd]);
let comment = _.assignIn(new Comment(),transformedData[0]);