Unhandled promise rejection: TypeError: Cannot read property 'username' of undefined

Question

I am pretty new to javascript and currently I am using MEAN stack for developing an application. I use the following code to find the username, friends' id and username of a particular user:

router.get('/init', function(req, res) {

    var db = req.db;
    var userList = db.get('userList');

    //get username & friends
    userList.findOne({ "_id" : ObjectId("584d576ef0000ef5a94706f5") },{ fields: { "username": true, "_id": false, "friends":true } }, function(err, currUser){

        userList.find({ "username": { $in: currUser.friends} }, { fields: {"username": true, "_id": true}}).toArray(function(err, friendList){

            // Generate response

            var response = {

                "username": currUser.username,
                "friends": friendList
            };

            res.json(response);

        });

    });
}

And it always return the error:

    (node:6044) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): TypeError: Cannot read property 'friends' of undefined

Any help in this problem is greatly appreciated!

Edit: I changed the code as follow to get the err log:

router.get('/init', function(req, res) {

    var db = req.db;
    var userList = db.get('userList');

        //var username, friends;
        userList.findOne({ "_id" : ObjectId("584d576ef0000ef5a94706f5") },{ fields: { "username": true, "_id": false, "friends":true } }, function(err, currUser){

            console.log(currUser);
            if(err) console.log("findOne: "+err);

            userList.find({ "username": { $in: currUser.friends} }, { fields: {"username": true, "_id": true}}).toArray(function(err, friendList){

                // Generate response

                if(err) console.log("find: "+err);

                var response = {

                    "username": currUser.username,
                    "friends": friendList
                };

                res.json(response);

            });

        });


});

And the console log is:

{ username: 'Eddie', friends: [ 'Ken', 'Alice', 'Bill' ] }
undefined
findOne: TypeError: userList.find(...).toArray is not a function
(node:8888) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): TypeError: Cannot read property 'friends' of undefined

Answer 1

Just replace toArray with then.

userList.find({ "username": { $in: currUser.friends} }, { fields: {"username": true, "_id": true}}).toArray(function(err, friendList){

with

userList.find({ "username": { $in: currUser.friends} }, { fields: {"username": true, "_id": true}}).then(function(err, friendList){