var data, responseStream;

data = ['a','b','c','d','e'];

responseStream = Rx.Observable.create(observer=>{

  var sendNext = function(){

    var val = data.shift();

    if (!val) {

      observer.onCompleted();

      return;

    }

    sendRequest(val).then(response=>{

      observer.onNext({val, response});

      sendNext();

    });

  };

  sendNext();

});

​

responseStream.subscribe(

  item=>{

    console.log(item);

  },

  err => {

    console.log(err);

  },

  ()=>{

    console.log('Done');

  }

);

​

function sendRequest(val) {

  return new Promise((resolve,reject)=>{

    setTimeout(()=>{resolve('response for '+val);}, Math.random() * 2500 + 500);

  });

};