What if we'd like to set off a bunch of async calls at once, and wait until they all finish (at various times, in any order) to set off the next promise in the chain? Q provides a simple method that takes an array of promises: Q.all()

all-promise.js

function get_all_the_things(things) {
    var the_promises = [];

    for (var i = 0; i < things.length; i++) {
        (function(i) {  // keep i in scope
            var deferred = Q.defer();
            get_a_thing(things[i], function(result) {
                deferred.resolve(result);
            });
            the_promises.push(deferred.promise);
        })(i);
    }

    return Q.all(the_promises);
}