Javascript Angular: how to chain unknown number of promises

Question

I'm writing an Angular.js service to pull some JSON feeds. Initially, the service does not know which/how many resources to request; they are dependent on ids returned by a another request.

I'm having a headache chaining the $http service requests together. Is there a commonly used pattern to do this?

I've tried the Array.reduce technique suggested on another thread, but had problems syncing the ids and the requested data.

This is what I have so far. Does anyone have any suggestions?

aService.factory('dummy', function($http){
    // Dummy resources.
    var resources = [   
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js'
    ];
    return {
        data: function(callback){

            var jquerySources = []

            var promiseChain = $http({method: 'GET', url: resources[0]});
            var l = resources.length
            for (var i = 1; i < l; i++ ){
                promiseChain.then(function(data){

                    jquerySources.push({
                        url: resources[i],
                        source: data
                    });

                    promise_chain = $http({method: 'GET', url: resources[i]});
                    if (i === l){
                        return callback(jquerySources);
                    }
               });
            }
        }
    }
});

Thankyou.

Answer 1

If you would need to do requests sequentially, you would create a promise which you could use as the head of your chain. Then, you could chain $http calls up to that head and resolve the head promise:

aService.factory('seq', function($http, $q){
    // Dummy resources.
    var resources = [   
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js'
    ];
    var deferred = $q.defer();
    var resourcePromise = deferred.promise;
    var res = [];
    angular.forEach(resources, function(resource){
      return resourcePromise.then(function(){
        return $http({ method: 'GET', url: resource });
      }).then(function(data){
        res.push({res: resource, data : data});
      });
    });

    deferred.resolve();

    return {
        getResource: resourcePromise.then(function(){
          return res;
        })
    };
});

but if requests would be in parallel - then it would be simplier solution. Just array of promises and simply call $q.all function for waiting to resolve all promises.

aService.factory('par', function($http, $q){
    // Dummy resources.
    var resources = [   
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js',
      'http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js'
    ];
    var promises = [];
    var res = [];
    angular.forEach(resources, function(resource){
      promises.push(
        $http({ method: 'GET', url: resource }).then(
          function(data){
            res.push({res: resource, data : data});
          })
        );
    });

    return {
        getResource: $q.all(promises).then(function(){
          return res;
        })
    };
});

Also note that in both cases we have res array for collecting results of requests.

EDIT: Plunker with example

Answer 2

You can indeed do this with reduce:

var chain = resources.reduce(function (sourcesPromise, url) {
    return sourcesPromise.then(function (sources) {
        return $http({method: 'GET', url: url})
        .then(function (data) {
            sources.push({url: url, source: data});

            return sources;
        });
    });
}, $q.when([]));

chain.then(function (sources) {
    // [{url, source}, ...]
});

based on How to chain a variable number of promises in Q, in order?