q.js demo for merging the results of an arbitrary number of promises generated by an initial promise.

map_reduce.js

// @cpdean
// demonstrates how to have one promise kick off
// an additional arbitrary number of promises, and
// then merge their results down again after
// all the promises are complete.
var Q = require('q'); // "q": "~1.0.0"


// initial query that generates seed data for more work to be done
function fakeQuery(){
    var deferred = Q.defer();
    setTimeout(function(){
        console.log("first query complete");
        deferred.resolve([{id: 1}, {id: 2}, {id: 3}]);
    }, 1e3);
    return deferred.promise;
}

// second query
function deferredPropertyLookup(x) {
    var deferred = Q.defer();
    setTimeout(function(){
        console.log("next query complete on " + x); 
        deferred.resolve(x);
    }, 1e3);
    return deferred.promise;
}

fakeQuery().then(function(results){
    // `results` could be an arbitrary number of things
    // to do more work on

    // then concurrently run the lookup function with its
    // post processing step in then()
    var with_properties = results.map(function(e){
        return deferredPropertyLookup(e.id).then(function(found){
            return {id: e.id, prop: found};
        }); 
    }); 


    return Q.all(with_properties).done(function(a){
        // print the results when the lookups and processing are done
        console.log(a);
    }); 
});

Here is a snippet I use when I need to run an arbitrary number of promises one at a time sequentially. This is handy if you are making a lot of API calls and don't want to exceed rate or concurrent request limits.

function runPromiseSequence(items) {
  return items.map(function(item) {
    // Have to wrap this in a function to generate a promise for each item
    return function() {
      // return a value or promise
      return item.doSomethingAsync();
    };
  }).reduce(Q.when, Q());
}

The function will return a promise chain for each item in the array, like ...then(function(item[0] { ... }).then(function(item[1]) { ... }) etc and you can add to the end of the chain to do something after all the other promises are resolved sequentially.