Helper method for combining latest values from different RxJS observable streams into a new stream based on a JS object template.

combinedMappedStreamBasedOn.js

// construct function copied from
// http://stackoverflow.com/questions/1606797/use-of-apply-with-new-operator-is-this-possible/#1608546
function construct(constructor, args) {
    function F() {
        return constructor.apply(this, args);
    }
    F.prototype = constructor.prototype;
    return new F();
}

function combinedMappedStreamBasedOn(template) {
  var keys = _.keys(template);
  var streams = _.values(template);
  var generator = function () {
    var args = Array.prototype.slice.call(arguments);
    var json = {};
    _.each(args, function (arg, index) {
      json[ keys[index] ] = arg;
    });
    return json;
  };
  var argumentsForCombineLatest = streams.concat([generator]);
  var combinedStream = construct(Rx.Observable.combineLatest, argumentsForCombineLatest);

  var replaySubject1 = new Rx.ReplaySubject(1);
  combinedStream.subscribe( replaySubject1 );
  return replaySubject1;
}

// Usage example
// -------------

var combinedStream = combinedMappedStreamBasedOn({
  traveller: travellerS,
  country: selectedCountryS
});

combinedStream.subscribe(logWith('expense claim'));

// logs the expense claim as a JS object:
// {
//   traveller: /* latest value from the travellerS stream */ ,
//   country: /* latest value from the selectedCountryS stream */
// }