Rx.Observable.for(sources, resultSelector, [thisArg])
Concatenates the observable sequences or Promises obtained by running the specified result selector for each element in source.
There is an alias for this method called forIn for browsers <IE9
Arguments
sources(Array): An array of values to turn into an observable sequence.resultSelector(Function): A function to apply to each item in the sources array to turn it into an observable sequence. The resultSelector is called with the following information:- the value of the element
 - the index of the element
 - the Observable object being subscribed
 
[thisArg](Any): Object to use asthiswhen executingresultSelector.
Returns
(Observable): An observable sequence from the concatenated observable sequences or Promises.