var requestStream, responseStream;

requestStream = Rx.Observable.from(['a','b','c','d','e']);

​

responseStream = requestStream.concatMap(

  (val)=>{

    return Rx.Observable.defer(()=>{

      return sendRequest(val);

    });

  },

  (val, response)=>{ return {val, response}; }

);

​

responseStream.subscribe(

  item=>{

    console.log(item);

  },

  err => {

    console.log(err.message);

  },

  ()=>{

    console.log('Done');

  }

);

​

function sendRequest(val) {

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

    console.log('sending '+val);

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

  });

};