0-promises.js

var models = [{
    name: 'Jonathan',
    location: 'Earth'
}, {
    name: 'Joe',
    location: 'Mars'
}]

var ps = models.map(function(model) {

    // These are two independent operations
    var p1 = (model.name)?
             nameFormatter.format(model.name)
             : Promise.of();

    var p2 = (model.location)?
             geocorder.geocode(model.location)
             : Promise.of();
    
    //Don't fire until async operations have updated the model.
    return Promise.all([p1, p2]).then(function([name, location]) {
      if (name) model.name = name;
      if (location) model.location = location;
      return model.save();
    })

});

Promises.all(ps).then(...)

1-rx.js

var models = [{
    name: 'Jonathan',
    location: 'Earth'
}, {
    name: 'Joe',
    location: 'Mars'
}]

var rs = Rx.from(models)
           .flatMap(function(model) {
             var r1 = (model.name)? Rx.fromNodeCallback(nameFormatter.format)(model.name)
                                  : Rx.Observable.return(null);
             var r2 = (model.location)? Rx.fromNodeCallback(geocorder.geocode)(model.location)
                                      : Rx.Observable.return(null);

             var r3 = r1.zip(r2, function(name, location) {
               if (name) model.name = name;
               if (location) model.location = location;
             })
             r3.flatMap(function(){ return model.save() }) // model.save() needs to return a Rx.Observable
           })

rs.subscribeOnCompleted(function() { ... })