Object method with ES6 / Bluebird promises

Solution 1:

Why this ends up being undefined in this case?

Because you're passing a function, not a method-bound-to-an-instance. This problem is not even promise-specific, see How to access the correct `this` context inside a callback? for the generic solution:

….then(this.method2.bind(this))… // ES5 .bind() Function method

….then((r) => this.method2(r))… // ES6 arrow function

However, Bluebird does offer an other way to call the function as a method:

this.service.serviceMethod(args)
    .bind(this)
    .then(this.method2)
    .catch(onRejected);

Solution 2:

I should add that this is a generic Javascript issue and can also be solved using plain javascript features. For example, you could also do this:

User.prototype.method1 = function () {
    .............
    this.service.serviceMethod(args)
        .then(this.method2.bind(this))
        .catch(onRejected);
};

This uses Function.prototype.bind() which is built into Javascript and present on every function. This creates a function stub (which is what is passed to .then() and that stub will automatically reattach the desired this value before calling method2().