// Mocks an http call that takes 1 second to complete

function fakeRequest(id) {

  console.log('doing the request for id ' + id);

  return Rx.Observable.of(id).delay(1000);

}

​

let items = ["1", "2", "3"];

​

// create an observable from the array that emits every item one after one

Rx.Observable.from(items)

   // use concatMap, this operator accepts a function that returns an observable.

   // It will subscribe to this observable and not take on any next values

   // untill that observable completes.

   // So it will get '1' and perform the request. As soon as this one completes

   // it will handle the next value, which will be '2' and so on.

  .concatMap(

    (val) => {

      return fakeRequest(val)

   })

   // Use combine all to make sure the subscription below isn't call untill

   // every element in the original array is handled.

  .combineAll()

   // Get back an array with the results from every call, just take the last one

   // if you really need it

  .subscribe((val) => console.log(val));