check if for loop with mysql call inside is finished nodejs

Question

so I have an array from another function that passes res which is a list looking like this:

[ RowDataPacket { UserID: 26 }, RowDataPacker { UserID: 4 } ]

it stores user id's, what I want is a function that finds the user id's username, and stores them in another array. This is what I have:

function getThem(res, params) {
        var promises = res.map(function (item) { // return array of promises
            // return the promise:
                    for (i = 0; i < Object.keys(res).length; i++) {
                        console.log("user: ", res[i].UserId);
                        getUsernameFromId(res[users.length].UserId).then(function() {
                            console.log("username: ", res[0].username);
                            users.push(res[0].username); 
                        });
                    }
                }, function (err) {
                    console.error(err);
                });
        Promise.all(promises).then(function () {
            console.log("users: ", users);
            //do something with the finalized list of albums here
        });
    }

output in console:

user: 26
user: 4
user: 26
user: 4
users: []
username: undefined
username: undefined
username: undefined
username: undefined

so how can I wait for the for loop to complete the mysql call? Maybe there is another way of doing this?

edit: don't mind the undefined usernames, it's easy to fix later. Just tell me how I can have those undefined inside an array

see where you have a commen // return the promise: ... well, you aren't — Jaromanda X
– Jaromanda X, Commented Nov 5, 2017 at 3:37
where can I return it to? The example I got this from they were returning it into another function and .then(function() { // for loop here}) — Zack Mitkin
– Zack Mitkin, Commented Nov 5, 2017 at 3:41
you're performing res.map and within that loop, you're performing a loop on Object.keys(res) - that looks all kinds of wrong - and anyway, [ RowDataPacket { UserID: 26 }, RowDataPacker { UserID: 4 } ] isn't valid javascript anything — Jaromanda X
– Jaromanda X, Commented Nov 5, 2017 at 3:59

Jaromanda X · Accepted Answer · 2017-11-05 04:06:29Z

Assuming (have to assume, because your code seems to use res like a majick object that has everything you need before you do anything with it) the actual res looks like

[ { UserID: 26 }, { UserID: 4 } ]

and getUsernameFromId returns an object with a username property, like

{ username: 'blah', ...otherproperties }

getThem can be simply

function getThem(res, params) {
    return Promise.all(res.map(({UserID}) => getUsernameFromId(UserId).then(({username}) => username)))
    .then(users => {
        console.log("users: ", users);
        //do something with the finalized list of albums here
    });
}

or in "old school" javascript

function getThem(res, params) {
    return Promise.all(res.map(function (_ref) {
        var UserID = _ref.UserID;
        return getUsernameFromId(UserId).then(function (_ref2) {
            var username = _ref2.username;
            return username;
        });
    })).then(function (users) {
        console.log("users: ", users);
        //do something with the finalized list of albums here
    });
}

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