Promise API - combining results of 2 asynchronous call

angularjs, javascript, promise

Solution

As @Matt said, you need to use `$q.all`, but the usage isn't quite right. AngularJS doesn't support `.done` and `.fail` and they don't work quite like that anyway in that there's no such thing as a promise for multiple values, instead you just have a promise for an array.

If you were writing this using the full Q we would write:

var get = function (id) {
    return Q.all([Db.get(id, "abc"), Db.get(id, "def")])
        .spread(function (res1, res2) {
            return {res1: res1, res2: res2};
        });//the error case is handled automatically
};

In this case, `.spread` acts like `.then` except that it spreads the results of an array for a promise out over the arguments of its `onFulfilled` function. To change this to use the promise methods from AngularJS we just need to make do without `.spread`. This leads to the following solution:

var get = function (id) {
    return $q.all([Db.get(id, "abc"), Db.get(id, "def")])
        .then(function (res) {
            return {res1: res[0], res2: res[1]};
        });//the error case is handled automatically
};

The beauty of this is that we are freed from handling all the nitty grity of error propagation and storing the partial results because `.then` acts as a filter. If you leave out the error handler, it automatically propagates any errors. This means that if either of the input promises are rejected, the result will be rejected. If both promises are fulfilled successfully, res is the array of those resolution values.

Problem

With promise API, how to send two asynchronous request in parallel, and resolve the combined result as the response. ``` var get = function(id){ var res1, res2; var deferred = $q.defer(); Db.get(id, "abc") .then(function (d) { //deferred.resolve(d)); res1 = d; }, function (e) { //error }); Db.get(id, "def") .then(function (d) { //deferred.resolve(d)); res2 = d; }, function (e) { //error }); //?????? how to return {res1:res1 , res2: res2} return deferred.promise; }; ``` now, when I call get() like ``` get(123).then(function(d)){ // d= {res1: res1, res2: res2} }, ... ``` I need to get the combined result as indicated. How to do this with Angular promise API?

Original source