Promise subclass

InstrumentedPromise.js

class InstrumentedPromise extends Promise {
  constructor(...args) {
    super(...args);
    console.log("constructed");
  }
  
  then(...args) {
    console.log("then'd");
    return this.constructor.cast(super(...args));
  }
}

var p = Promise.resolve();
var instrumentedP = InstrumentedPromise.resolve();

p.then(() => instrumentedP).then(() => {
  // By now, "then'd" should have been logged, but under the status quo, it is not.
});

TransformPromise.js

class TransformPromise extends Promise {
  constructor(resolver, transform) {
    super(resolver);
    this._transform = transform;
  }
  
  then(onFulfilled, onRejected) {
    return this.constructor.cast(super(x => onFulfilled(this._transform(x)), onRejected));
  }
}

var p = Promise.resolve(5);
var transformP = new TransformPromise(resolve => resolve(p), x => x + 10);

transformP.then(x => console.log(x)); // logs 15

// should log 15, but logs 5 under status quo.
Promise.resolve().then(() => transformP).then(x => console.log(x));

TypeError: #<InstrumentedPromise> is not a promise

Sad panda :(

$ iojs -v
v1.1.0