const Observable = Rx.Observable;

​

function mockHTTPRequest(id) {

  return Observable.of(id).delay(100);

}

​

let parameter1 = 'a';

let parameter2 = false; 

​

let sources = [

  parameter1 ? mockHTTPRequest('rest/fillin/ids/' + parameter1) : Observable.of(null),

  parameter2 ? mockHTTPRequest('rest/textitem/ids/' + parameter2) : Observable.of(null)

];

​

Observable.forkJoin(...sources)

  .map(data => {

    console.log(data.length);

    return data;

  })

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

​