let Observable = Rx.Observable;

let resultA, resultB, resultC;

​

function addAsync(num1, num2) {

    // use ES6 fetch API, which return a promise

    const promise = Promise.resolve(num1 + num2);

    return Observable.fromPromise(promise);

}

​

addAsync(1,2)

  .do(x => resultA = x)

  .flatMap(x => addAsync(x, 3))

  .do(x => resultB = x)

  .flatMap(x => addAsync(x, 4))

  .do(x => resultC = x)

  .subscribe(x => {

    console.log('total: ' + x)

    console.log(resultA, resultB, resultC)

  });

​