Nodejs Promise for custom api

Question

I am new to nodejs and using promise and actually this is my first real app with nodejs.

So i have been reading all day and i am a bit confused.

So this is my module :

function User() {
    var self   = this;
    self.users = {};

    self.start = function (user, botId) {
        return new Promise(function () {
           return get(user).then(function (data) {
                debug(data);
                if (data.botId.indexOf(botId) === false) {
                    return Repo.UserBotModel.addUser(user.id, botId).then(function () {
                        data.botId.push(botId);
                        return data;
                    });
                } else
                    return data;
            });
        });
    };

    self.getDisplayName = function (user) {
        if (user.real_name)
            return user.real_name;
        if (user.last_name)
            return user.firstname + ' ' + user.last_name;
        return user.first_name;
    };
    /**
     * check if user exist in our database/memory cache and return it,
     * otherwise insert in the database and cache it in memory and the return it
     * @param user
     */
    function get(user) {

        return new Promise(function () {

            //check if user is loaded in our memory cache
            if (self.users.hasOwnProperty(user.id))
                return self.users[user.id];
            else {
                //get from database if exist
                return Repo.UserModel.get(user.id).then(function (rows) {
                    if (rows && rows.length) {
                        //user exist cache it and resolve
                        var data = rows[0];
                        if (data.botId && data.botId.length)
                            data.botId = data.botId.split(',');
                        else
                            data.botId = [];

                        self.users[user.id] = data;
                        //------------------------------ code execution reaches here
                        return data;
                    }
                    else {
                        //user dose not exist lets insert it
                        return Repo.UserModel.insert(user).then(function (result) {
                            return get(user);
                        });
                    }
                });
            }
        });
    }
}

I call the start method witch calls the private get method the call reaches return data;(marked with comment) but then function dose not gets executed in the start method ???

So what am i doing wrong?

UPDATE : Sorry I forgot to mention that I am using bluebird and not the native promise if that makes a difference?

`getDisplayName` should be a method of `user`, or a static function. — Bergi, Apr 19 '16 at 15:12
You should cache the promise itself in `users`, not the `data` value - see [here](http://stackoverflow.com/a/31820876/1048572) for an example — Bergi, Apr 19 '16 at 15:15

score 3 · Answer 1 · edited May 23 '17 at 11:45

3

You cannot return from the Promise constructor - you have to call resolve (expected to happen asynchronously). You're not supposed to use the Promise constructor at all here. You can just omit it, and it should work.

edited May 23 '17 at 11:45

Community

1
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answered Apr 19 '16 at 15:08

Bergi

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score 1 · Accepted Answer · answered Apr 19 '16 at 15:34

The methods from your Repo.UserModel already return promises, so you do not have to create new ones using new Promise.
You can read the values inside those promises using then.
then also provides a way to transform promises. If you return a value in a function passed to then, then will return a new promise that wraps the value you returned. If this value is a promise, it will be awaited.
To convert a value to a promise, you can use Promise.resolve.

Knowing that, you can simplify get like so:

function get(user) {
    if (...) {
        return Promise.resolve(...)
    } else {
        return Repo.UserModel.get(...).then(function(rows) {
            ...
            return ...
        })
    }
}

This version of getwill always return a promise that you can use like so:

get(...).then(function(resultOfGet) {
    // process resultOfGet
})

tnx i did not notice that `Promise.resolve` can be used with literal values — Exlord, Apr 20 '16 at 07:02

Nodejs Promise for custom api

2 Answers2