Passing methods that use `this` into promises or promisifying them with bluebird can lead to confusing results. This isn't specific to promises, but rather how the prototype works.

promises-and-this.js

var Promise = require('bluebird');

function Blah() {
	this.foo = 'blah';
}

Blah.prototype.tester = function() {
	console.log(this.foo);
};

var blah = new Blah();
blah.tester(); // 'blah'

var lala = Promise.promisify(blah.tester);
lala(); // undefined

var lala2 = Promise.promisify(blah.tester).bind(blah);
lala2(); // 'blah'

var test = new Promise(function(resolve, reject) {
	return resolve('lol');
});
test.then(blah.tester); // undefined
test.then(blah.tester.bind(blah)); // blah