Object Method with Es6/Bluebird Promises

Object method with ES6 / Bluebird promises

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);

Calling object methods with promises and the 'this' context

No, you can use the bind method as well:

Promise.resolve().then(test.sayFoo.bind(test));

See also How to access the correct `this` context inside a callback? for the generic problem.

However, Bluebird does offer an extra way to call the method:

Promise.resolve().bind(test).then(test.sayFoo);

How to use ES6 arrow functions with promise binding (bluebird)

If you don't want lexically bound this, there's no need to use arrow functions. If you want dynamically bound this, just don't use arrow functions.

Of course, you could scrap that .bind({}) as a whole and use arrow functions that are bound to the object by putting everything in an object method (or an IIFE in your example):

(function() {
  this; // the value that everything is bound to
  double(2).then(result => {
    this.double = result; // store result here
    return triple(2);
  }).then(result => {
    console.log('double:', this.double);
    console.log('triple:', result);
    console.log('sum:', this.double + result);
  }).catch(err => {
    console.log(err.message);
  });
}.call({}));

However, there are much better ways to access previous results in a promise chain than contextual state, especially if you are using ES6!

Bluebird overwriting constructor

This is a regular Javascript problem with passing a function reference (nothing specific to do with promises).

When you pass this.myself as a function argument, all that gets actually passed is a reference to the myself function and the binding to the this object is lost so when the function is called, the this pointer will be wrong and won't point to your object's instance data. There are numerous ways to work around this. This is a generic Javascript issue with passing callback functions that are methods. There are a number of ways to work around this.

You can use .bind() to keep the this reference property attached like this:

 return this.wait().then(this.myself.bind(this));

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