How to do the chain sequence in rxjs

The equivalent of then for observables would be flatMap. You can see some examples of use here :

• RxJS Promise Composition (passing data)
• Why we need to use flatMap?
• RxJS sequence equvalent to promise.then()?

For your example, you could do something like :

this._myService.doSomething()
  .flatMap(function(x){return functionReturningObservableOrPromise(x)})
  .flatMap(...ad infinitum)
  .subscribe(...final processing)

Pay attention to the types of what your functions return, as to chain observables with flatMap you will need to return a promise or an observable.