How to ensure correct "this" with Promise.promisify?

Question

I am trying to refactory my nodejs server using promises with Bluebird library, but I am stuck in a simple problem.

After to get the users from my db, I want to list all notification class associated with this user:

Bad Way (working...)

adapter.getUsers(function(users){
    users.rows.forEach(function(item){
        user = item.username;
        adapter.getNotifications(user, function(notificationList){
            console.log(notificationList);
        })
    });
});

Elegant Tentative Way (not working...)

var getNotifications = Promise.promisify(adapter.getNotifications);
adapter.getUsers().then(function(users) {
    users.rows.forEach(function(item){
        var dbUser = "sigalei/" + item.value.name;
        console.log(dbUser);
        return getNotifications(dbUser);
    });
}).then(function(result){
    console.log(result);
    console.log("NOTIFICATIONLIST");
});

However when I execute this code I get this error inside my getNotification method:

Unhandled rejection TypeError: Cannot read property 'nano' of undefined at Adapter.getNotifications (/Users/DaniloOliveira/Workspace/sigalei-api/api/tools/couchdb-adapter.js:387:30) at tryCatcher (/Users/DaniloOliveira/Workspace/sigalei-api/node_modules/bluebird/js/main/util.js:26:23)

EDIT

After the user2864740`s precious comments, I noticed that the error is related with some scope problem. So, why after to use promisify method, the method dont getNotifications recognize the "this" env variable?

var Adapter = module.exports = function(config) {
    this.nano = require('nano')({
        url: url,
        request_defaults: config.request_defaults
    });
};

Adapter.prototype.getNotifications = function(userDb, done) {

    var that = this;
    console.log(that);
    var userDbInstance = that.nano.use(userDb);
    userDbInstance.view('_notificacao', 'lista',
      {start_key: "[false]", end_key: "[false,{}]"},
      function(err, body) {
        if(err){ done(err); }
        done(body);
    });

};

Hey, the error is generated inside the getNotifications method that is being "promisify". — Danilo Oliveira, Sep 10 '15 at 19:25
@user2864740 Again, I edited the question, I think that the problem is related with scope variables... — Danilo Oliveira, Sep 10 '15 at 19:39

score 7 · Accepted Answer · edited May 23 '17 at 11:45

This is just the very common problem of calling "unbound" methods.
You can pass the context as an option to Promise.promisify to have it bound:

var getNotifications = Promise.promisify(adapter.getNotifications, {context: adapter});

Alternatively, you'd need to .bind() the method, or call the new getNotifications function on the adapter (using .call()). You might also consider using Promise.promisifyAll(adapater) and then just calling adapter.getNotificationsAsync(…).

Notice that this still doesn't work. You cannot simply create promises in a loop - you need to await them explicitly and return a promise from the then callback, otherwise just the undefined value you returned will be passed to the next callback immediately.

adapter.getUsers().then(function(users) {
    return Promise.all(users.rows.map(function(item){
        var dbUser = "sigalei/" + item.value.name;
        console.log(dbUser);
        return getNotifications(dbUser);
    }));
}).then(function(results) {
    for (var i=0; i<results.length; i++)
        console.log("result:", results[i]);
});

Instead of Promise.all(users.rows.map(…)), in Bluebird you can also use Promise.map(users.rows, …).

You are correct, thank you for your detailed explanation for this question and the "unbound" methods. — Danilo Oliveira, Sep 10 '15 at 23:21
In Bluebird 3.4.6 context is bound like this: `Promise.promisify(adapter.getNotifications, {context: adapter});` — Sam, Oct 24 '16 at 22:09

score 2 · Answer 2 · answered Sep 10 '15 at 20:02

What about simply

var getNotifications = Promise.promisify(adapter.getNotifications.bind(adapter));

or possibly

var getNotifications = Promise.promisify(function () {
    return adapter.getNotifications.apply(adapter, arguments);
});

?

I'm not sure I understand your problem well, but this should make sure this is bound and not undefined when you do return getNotifications(dbUser);

How to ensure correct "this" with Promise.promisify?

EDIT

2 Answers2

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