Rx.Observable.prototype.map(selector, [thisArg])
Projects each element of an observable sequence into a new form by incorporating the element's index.  This is an alias for the select method.
Arguments
selector(Function): Transform function to apply to each source element. The selector 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 executing the predicate.
Returns
(Observable): An observable sequence which results from the comonadic bind operation.