Danilo Oliveira Danilo Oliveira - 9 months ago 57
Javascript Question

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

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...)

user = item.username;
adapter.getNotifications(user, function(notificationList){

Elegant Tentative Way (not working...)

var getNotifications = Promise.promisify(adapter.getNotifications);
adapter.getUsers().then(function(users) {
var dbUser = "sigalei/" + item.value.name;
return getNotifications(dbUser);

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)


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


Answer Source

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;
        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, …).