How the Validator Works

JS validation

var mariaItem = function (req, res, next) {
        Item.findOne({ _id: "5099803df3f4948bd2f98391"}, function (err, result){
             req.contents = {};
             req.contents.result = result;
            next();
        });
    };
    
var mariaView = function (req, res, next){
    async.waterfall([
        function(callback){
            Item.findOne({ _id: "5099803df3f4948bd2f98391"}, function (err, result){
                req.contents = {};
                req.contents.result = result;
                callback();
            });
        },
 
        function(callback){
            User.findOne({ _id: req.contents.result.seller.sellerId}, function (err, result) {
                req.contents.seller = result;
                callback();
            });
        },
        function(callback){
            Group.find({ _id: {$in: req.contents.result.groups}}, function (err, result){
                req.contents.groups = result; 
                callback();   
            }); 
        },
        function(callback){
            var newComment = {};
                newComment.commenterId = req.user._id;
                newComment.commenterName = req.user.username;
                newComment.body = req.body.comments;
                Item.findOneAndUpdate({_id: "5099803df3f4948bd2f98391"}, {$addToSet: {comments: newComment} }, function (err, result) {
                callback();
                });
            }
    ], function(err, results){
             res.json(req.contents);             
      
            });
};     
// +++++++++++++++ VALIDATION ++++++++++++++++++++ 
var validateMaria = function (req, res, next){
    var Validator = function(seller){
        this.seller = seller;
    };

    Validator.prototype.onlySeller = function() {
        if (req.params._id == this.seller.sellerId) {
            next();
        } else{
            res.json("You are not the seller!");
        }
    };

    var thing = new Validator(req.contents.result.seller);
    thing.onlySeller();
};

app.get('/maria/item', //when you hit this route it goes to mariaItem and gets the info and puts it in req.contents.results 
        mariaItem,
        validateMaria,
        //then it goes to the validateMaria function which creates a Validator object and a prototype of that object called
        //onlySeller. then it creates a new Validator object called thing that takes the contentsof the findOne query from mariaItem.
        //and then we call thing with its prototype (thing.onlySeller) which checks to see if you are the seller and gives an error if not!!
        mariaView
        );