Rx.Observable.prototype.groupBy(keySelector, [elementSelector], [comparer])
Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.
Arguments
keySelector(Function): A function to extract the key for each element.[elementSelector](Function): A function to map each source element to an element in an observable group.[comparer](Any): Used to compare objects. If not specified, the default comparer is used.
Returns
(Observable): A sequence of observable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.