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Using $resource in a promise chain (fixing deferred anti-pattern)

I have a service with a method that gets me a list of project types using a $resource. It's working well for me, except that if I make multiple nearly simultaneous calls (from say, two directives) each will create another request instead of using the same response/$promise/data.
I found this which led me to this and TL;DR, apparently it's creating a redundant $q.defer() and is actually considered to be a deferred anti-pattern.
The code below works well if the calls to get project types are significantly staggered (like more than milliseconds apart). The consecutive calls are resolved with the shared.projectTypes. It also works in the sense that if the request to get project types fails, the dfr.reject() will be triggered and be caught by .catch in the calling controller.
angular.module('projects')
.factory('projectService', function(notificationService){
// an object to share data gathered by this service
var shared = {};
// $resource for projects API
var projectResource = $resource(baseApiPath + 'projects', {}, {
...,
getProjectTypes: {
method: 'GET',
url: baseApiPath + 'projects/types'
},
...
});
// loads a list of project types
var loadProjectTypes = function(){
var dfr = $q.defer();
// if we've already done this, just return what we have.
if(shared.projectTypes){
dfr.resolve(shared.projectTypes);
}
else {
// begin anti-pattern (?)
projectResource.getProjectTypes(null,
function(response){
shared.projectTypes = response.result.projectTypes;
dfr.resolve(response);
},
function(errResponse){
console.error(errResponse);
notificationService.setNotification('error', errResponse.data.messages[0]);
dfr.reject(errResponse);
});
}
return dfr.promise;
};
return {
shared: shared,
project: projectResource,
loadProjectTypes: loadProjectTypes
};
});
So, I read that having this extra var dfr = $q.defer() is not necessary as the $resource would provide all that for me. With a bit of refactoring, I ended up with this:
...
// $resource for projects API
var projectResource = $resource(baseApiPath + 'projects', {}, {
...,
getProjectTypes: {
method: 'GET',
url: baseApiPath + 'projects/types',
isArray: true,
transformResponse: function(response){
return JSON.parse(response).result.projectTypes;
}
},
...
});
// loads a list of project types
var loadProjectTypes = function(){
return shared.projectTypes || (shared.projectTypes = projectResource.getProjectTypes());
};
...
To clarify, I have added isArray and transformResponse to the resource because my API returns a lot of extra meta information and all I wanted was an array of types. In my loadProjectTypes method, I'm including the same caching we originally had, but I'm caching the result of projectResource.getProjectTypes() instead of the actual response data (even though that might be exactly what I'm caching because of the transformResponse).
This works on the happy path (reduced calls to API, returns the same thing to everyone, etc) but my main problem is with the chaining and catching of errors.
In my original anti-pattern example, if there is an error with GET /project/types, I'm using dfr.reject() which is then passed back to my controller where I have a .catch().
This is code from the controller which actually makes the original request to get project types:
$q.all([
projectService.loadProjects(),
userService.loadUserRole('project_manager'),
userService.loadUserRole('sales_representative'),
projectService.loadProjectTypes(),
clientService.loadClients()
])
.then(function(response){
// doing stuff with response
})
.catch(function(errResponse){
// expecting errors from service to bubble through here
console.error(errResponse);
});
With the anti-pattern example, the dfr.reject is causing the error to show up here in the catch, but in my supposed non-anti-pattern example, it's not happening. I'm not sure how to reject or resolve the $resource results in the same way I was before. If one of the points of promise chaining is to have one spot to handle errors from any chain link, I was doing it right.
I tried to use $q.resolve()/reject(), since I don't have dfr anymore, but this seems dumb and doesn't work anyway.
return shared.projectTypes || (shared.projectTypes = projectResource.getProjectTypes(null,
function(response){
return $q.resolve(response);
},
function(errResponse){
return $q.reject(errResponse);
}));
How do I get the chain to work so that .catch() in the controller is where the errors get handled?
Did I actually implement the anti-pattern in my original code, or was that one of the accepted ways to use $q.defer() and it wasn't an anti-pattern at all?
In the second link I posted, there is an answer that says:
"What's wrong with it? But the pattern works! Lucky you.
Unfortunately, it probably doesn't, as you likely forgot some edge
case. In more than half of the occurrences I've seen, the author has
forgotten to take care of the error handler."
However, my original code was addressing the errors. It was working, except that each caller was getting it's own promise. I feel that's where I missed something.
I might be confused, but I'm thinking that the loadProjectTypes method should return the same promise/data to anyone who calls it, no matter when it's called. It should be the one true source of anything projectTypes and only make the call once, the very first time.
Any time I look for any of this (lots of purple/visited google links on these subjects), everyone is either showing chaining with contrived examples, or only using $http, or something else. I haven't found anyone doing error catching in a promise chain that uses $resource.
UPDATE: Adding my requirements for the solution. I posted them in my answer, but wanted to include them in the original post too.
Requirement 1: Allows multiple calls to the method, but only makes one API request which updates all callers with the same data.
Requirement 2: Must be able to use result of method as actual data, just as the promise spec intends. var myStuff = service.loadStuff() should actually set myStuff to be "stuff".
Requirement 3: Must allow promise chaining so that all errors in any part of the chain can be caught by a single catch at the end of the chain. As I've found in my solution, there can be more than one chain, and more than one catch, but the point is that each chain has a catch, and any "links" in the chain that break should all report their errors to their respective catch.

Isn't that always the way, as soon as you speak your problems, you come across your solution.
Requirement 1: Only make one request per method call. This is solved with the original fix to the anti-pattern. This will either always return the $resource result by either returning the cached $resource or returning and caching at the same time.
var loadProjectTypes = function(){
return shared.projectTypes || (shared.projectTypes = projectResource.getProjectTypes());
};
Requirement 2: Be able to use the service method as a promise where I can set the value of a $scope variable directly to the result of loadProjectTypes(). Using the revised method above, I can simply state $scope.theTypes = projectService.loadProjectTypes() and it'll automatically be filled with the list of types when they come in, just as the promise spec intends.
Requirement 3: Be able to chain together multiple $resource calls and have their errors be caught by a single .catch(). By using the $promise of the result of loadProjectTypes within $q.all(), I can catch any errors in any catch I want.
$q.all([
...,
projectService.loadProjectTypes().$promise,
...
])
.then(function(response){
// my project types comes in as response[n]
})
.catch(function(errResponse){
// but any errors will be caught here
});
Technically, I can put catches in different places and they'll all work the same. Anytime I have loadProjectTypes(), I can use a .catch() and my errors will be handled there. Each loader of types can handle the API being down in it's own way. This could be really good actually. A controller might get the UI to display a message and a small directive might just display something else, or nothing at all. They each can handle the bad in their own way.
My service, directive and controller look like this now:
angular.module('projects')
.factory('projectService', function(notificationService){
// an object to share data gathered by this service
var shared = {};
// $resource for projects API
var projectResource = $resource(baseApiPath + 'projects', {}, {
...,
getProjectTypes: {
method: 'GET',
url: baseApiPath + 'projects/types',
isArray: true,
transformResponse: function(response){
return JSON.parse(response).result.projectTypes;
}
},
...
});
// loads a list of project types
var loadProjectTypes = function(){
return shared.projectTypes || (shared.projectTypes = projectResource.getProjectTypes());
};
return {
shared: shared,
project: projectResource,
loadProjectTypes: loadProjectTypes
};
});
angular.module('projects')
.directive('projectPageHeader', ['projectService', function(projectService){
return {
restrict: 'E',
scope: {
active: '#',
},
templateUrl: 'src/js/apps/projects/partials/dir_projectPageHeader.html',
replace: true,
controller: function($scope){
$scope.projectService = projectService;
// sets the types to the array of types
// as given by the transformResponse
$scope.types = projectService.getProjectTypes();
// could also do a .$promise.catch here if I wanted.
// all catches will fire if get projectTypes fails.
}
};
}]);
angular.module('projects')
.controller('projectListPageController', [
'$scope','projectService',
function($scope, projectService){
// load it all up
$q.all([
projectService.loadProjectDetails($routeParams.projectId).$promise,
userService.loadUserRole('project_manager').$promise,
userService.loadUserRole('sales_representative').$promise,
projectService.loadProjectStatuses().$promise,
projectService.loadProjectTypes().$promise,
clientService.loadClients().$promise
])
.then(function(response){
// do work with any/all the responses
})
.catch(function(errResponse){
// catches any errors from any of the $promises above.
})
}]);
Since the loadProjectTypes (or any other load_____ method) saves the types within the service it comes from, I don't really need to do any storing on the controller. projectService.shared.projectTypes is universal across the entire app. The .then() method in my controller could potentially be noop if all the services were storing the results of their loads internally (which is how I like it) unless there was some view specific thing I needed to do with them. I typically only use controllers for entire pages, or $modals. Everything else is broken up into directives and most information and logic is in services.
I'm leaving the question open in case someone has a better solution. I like the one that Jack A. posted, but I feel it makes my load___ methods more verbose than they already are. Since there are a few of them with slight differences, it leads to a lot of redundant code, or complex 'smart' methods in my actual code. It definitely solves Requirement 1 and possibly 2 and 3 though.
UPDATE (GOTCHA):
So, I've been using this pattern for a few days now and it's working really exactly as I intend. It's really streamlined our process; however, I recently came upon a gotcha when using a method like loadProjectTypes in a singular context (i.e.: outside of $q.all()).
If you just use the load method like so:
// This code is just placed in your controllers init section
loadProjectTypes()
.$promise
.then(function(response){
// ... do something with response (or noop)
})
.catch(function(errResponse){
// ... do something with error
});
You will run into a situation when that controller 'refreshes'. For example, you have the code above in controllerA, you change "pages" which uses controllerB, then you go back to the first "page" and controllerA refreshes and tries to run this again. The error you get is that "there is no .then of undefined."
Inspecting this in the console, the first time loadProjectTypes() runs, it returns the response from the $resource (which includes $promise AND all the projectType data). The second time - coming back from controllerB - it will only hold the projectType data. There is no more $promise because you are not returning the result of a $resource, you returned the cached shared.projectTypes that you set after the first time. That's why we did all this, remember? I'm not sure why this goes away since that's what you saved to shared.projectTypes, but it does, and it doesn't actually matter.
return shared.projectTypes || (shared.projectTypes = projectResource.getProjectTypes());
For me, the easiest fix was to just have loadProjectTypes().$promise as the only member of a $q.all() set:
// again, this code is just placed somewhere near the top of your controller
$q.all([
loadProjectTypes().$promise
])
.then(...)
.catch(...);
In most cases, my controllers will be getting more than one thing so this would've happened eventually, but there will always be a situation where you only need to load one thing. Using a single item set in $q.all() is the only way to have no issues when using this solution. It's not that bad really, could've been worse.

I wrote something very similar to this a while ago, with a couple key differences:
I only create the promise when the data is already in the cache and return the native promise when an actual request is initiated.
I added a third state for when a request for the resource is already pending.
A simplified version of the code looks like this:
module.factory("templateService", function ($templateCache, $q, $http) {
var requests = {};
return {
getTemplate: function getTemplate(key, url) {
var data = $templateCache.get(key);
// if data already in cache, create a promise to deliver the data
if (data) {
var deferred = $q.defer();
var promise = deferred.promise;
deferred.resolve({ data: data });
return promise;
}
// else if there is an open request for the resource, return the existing promise
else if (requests[url]) {
return requests[url];
}
// else initiate a new request
else {
var req = $http.get(url);
requests[url] = req;
req.success(function (data) {
delete requests[url];
$templateCache.put(key, data);
});
return req;
}
},
};
});

The promise of a promise again (Angular JS)

Updated with HTTP and initial code based on requests/Please look at the bottom of the post:
I've been posting several questions on my AngularJS learning curve of late and the SO community has been fantastic. I've been a traditional C programmer when I used to program and have recently started writing my own ionic/Angular JS app. I'm struggling with the promise version of traditional async calls when it comes to converting a custom function to a promise. I don't think I really understood and I find various examples very contrived. I'd appreciate some help. I have some code which is not working, and I have some conceptual questions:
Let's take this simple function:
angular.module('zmApp.controllers').service('ZMDataModel', function() { return { getMonitors: function () { return monitors; } }
getMonitors is a simple function that basically returns an array of monitors. But here is the rub: When the app first starts, I call an http factory that does an http get and goes about populating this monitor list. This http factory is different from this service but invokes a setMonitor method in this service to populate the array. When the array is populated, a variable called 'monitorsLoaded' is set to 1. When this variable is set to 1, I know for sure monitors is loaded.
Now, I have a view with a controller called "MontageCtrl". I want to wait for the monitors to load before I show the view. In a previous post, one person suggested I use route resolve, but I had to first convert my getMonitors to a promise. So here is what I did:
angular.module('zmApp.controllers').service('ZMDataModel', function($q) {
getMonitors: function () {
var _deferred = $q.defer();
if (monitorsLoaded!=0)
{
console.log ("**** RETURNING MONITORS *****");
_deferred.resolve(monitors);
}
console.log ("*** RETURNING PROMISE ***");
return _deferred.promise;
},
Next up, in app.js I connected the route as follows:
.state('app.montage', {
data: {requireLogin:false},
resolve: {
message: function(ZMDataModel)
{
console.log ("Inside app.montage resolve");
return ZMDataModel.getMonitors();
}
},
Finally I modified my controller to grab the promise as such:
angular.module('zmApp.controllers').controller('zmApp.MontageCtrl', function($scope,$rootScope, ZMHttpFactory, ZMDataModel,message) {
//var monsize =3;
console.log ("********* Inside Montage Ctrl");
It seems based on logs, I never go inside Montage Ctrl. Route resolve seems to be waiting for ever, whereas my logs are showing that after a while, monitorLoaded is being set to 1.
I have several conceptual questions:
a) In function getMonitors, which I crafted as per examples, why do people return a _deferred.promise but only assign a _deferred.resolve? (i.e. why not return it too?). Does it automatically return?
b) I noticed that if I moved var _deferred definition to my service and out of its sub function, it did work, but the next view that had the same route dependency did not. I'm very confused.
c) Finally I ready somewhere that there is a distinction between a service and a factory when it comes to route resolve as a service is only instantiated once. I am also very confused as in some route resolve examples people use when, and I am using .state.
At this stage, I'm deep into my own confusion. Can someone help clarify? All I really want is for various views to wait till monitorsLoaded is 1. And I want to do it via route resolves and promises, so I get the hang of promises once and for all.
Added: Here is the HTTP factory code as well as the app.run code that calls this when the app first starts. FYI, the http factory works well - the problems started when I crafted ZMDataModel - I wanted this to be a central data repository for all controllers to use -- so they did not have to call HTTP Factory each time to access data, and I could control when HTTP factory needs to be called
angular.module('zmApp.controllers').factory('ZMHttpFactory', ['$http', '$rootScope','$ionicLoading', '$ionicPopup','$timeout','ZMDataModel',
function($http, $rootScope, $ionicLoading, $ionicPopup, $timeout,ZMDataModel) {
return {
getMonitors: function() {
var monitors = [];
var apiurl = ZMDataModel.getLogin().apiurl;
var myurl = apiurl+"/monitors.json";
return $http({
url: myurl,
method: 'get'
}) //http
.then(function(response) {
var data = response.data;
//console.log("****YAY" + JSON.stringify(data));
// $rootScope.$broadcast ('handleZoneMinderMonitorsUpdate',monitors);
$ionicLoading.hide();
ZMDataModel.setMonitors(data.monitors);
ZMDataModel.setMonitorsLoaded(1);
//monitors = data.monitors;
return ZMDataModel.getMonitors();
},
function (result)
{
console.log ("**** Error in HTTP");
$ionicLoading.hide();
ZMDataModel.setMonitorsLoaded(1);
//$ionicPopup.alert ({title: "Error", template:"Error retrieving Monitors. \nPlease check if your Settings are correct. "});
return ZMDataModel.getMonitors();
}
); //then
}, //getMonitors
And here is the code in app.run that first calls this:
.run(function($ionicPlatform, $ionicPopup, $rootScope, $state,ZMDataModel, ZMHttpFactory)
{
ZMDataModel.init();
var loginData = ZMDataModel.getLogin();
if ( loginData.username && loginData.password && loginData.url && loginData.apiurl)
{
console.log ("VALID CREDENTIALS. Grabbing Monitors");
// this calls http factory getMonitors that eventually populated the ZMDataModel
// monitors array and sets monitorsLoaded to 1
ZMHttpFactory.getMonitors();
}
}

I finally solved all the problems. There were various issues with my initial attempts. My final resolved solution is here Am I returning this promise correctly?
The learnings:
a) Separating the HTTP get into a factory and the data model into another service was unnecessarily complicating life. But that separation was not the problem. Infact, the way the promise was coded above, on first run, if monitorsLoaded was 0, it would simply return the deferred promise and there was no ".success" or similar construct for me to get into the resolve code block again.
b) The biggest thing that was making me run around in loops was deferring or rejecting was simply setting a state. the return always has to be the promise - and it would return the state you set. I assumed return d.promise always means returning "in progress".

Angular/Promises - Multiple Controllers waiting for the same $promise? [duplicate]

I want to implement a dynamic loading of a static resource in AngularJS using Promises. The problem: I have couple components on page which might (or not, depends which are displayed, thus dynamic) need to get a static resource from the server. Once loaded, it can be cached for the whole application life.
I have implemented this mechanism, but I'm new to Angular and Promises, and I want to make sure if this is a right solution \ approach.
var data = null;
var deferredLoadData = null;
function loadDataPromise() {
if (deferredLoadData !== null)
return deferredLoadData.promise;
deferredLoadData = $q.defer();
$http.get("data.json").then(function (res) {
data = res.data;
return deferredLoadData.resolve();
}, function (res) {
return deferredLoadData.reject();
});
return deferredLoadData.promise;
}
So, only one request is made, and all next calls to loadDataPromise() get back the first made promise. It seems to work for request that in the progress or one that already finished some time ago.
But is it a good solution to cache Promises?

Is this the right approach?
Yes. The use of memoisation on functions that return promises a common technique to avoid the repeated execution of asynchronous (and usually expensive) tasks. The promise makes the caching easy because one does not need to distinguish between ongoing and finished operations, they're both represented as (the same) promise for the result value.
Is this the right solution?
No. That global data variable and the resolution with undefined is not how promises are intended to work. Instead, fulfill the promise with the result data! It also makes coding a lot easier:
var dataPromise = null;
function getData() {
if (dataPromise == null)
dataPromise = $http.get("data.json").then(function (res) {
return res.data;
});
return dataPromise;
}
Then, instead of loadDataPromise().then(function() { /* use global */ data }) it is simply getData().then(function(data) { … }).
To further improve the pattern, you might want to hide dataPromise in a closure scope, and notice that you will need a lookup for different promises when getData takes a parameter (like the url).

For this task I created service called defer-cache-service which removes all this boiler plate code. It writted in Typescript, but you can grab compiled js file. Github source code.
Example:
function loadCached() {
return deferCacheService.getDeferred('cacke.key1', function () {
return $http.get("data.json");
});
}
and consume
loadCached().then(function(data) {
//...
});
One important thing to notice that if let's say two or more parts calling the the same loadDataPromise and at the same time, you must add this check
if (defer && defer.promise.$$state.status === 0) {
return defer.promise;
}
otherwise you will be doing duplicate calls to backend.

This design design pattern will cache whatever is returned the first time it runs , and return the cached thing every time it's called again.
const asyncTask = (cache => {
return function(){
// when called first time, put the promise in the "cache" variable
if( !cache ){
cache = new Promise(function(resolve, reject){
setTimeout(() => {
resolve('foo');
}, 2000);
});
}
return cache;
}
})();
asyncTask().then(console.log);
asyncTask().then(console.log);
Explanation:
Simply wrap your function with another self-invoking function which returns a function (your original async function), and the purpose of wrapper function is to provide encapsulating scope for a local variable cache, so that local variable is only accessible within the returned function of the wrapper function and has the exact same value every time asyncTask is called (other than the very first time)

AngularJs: Have method return synchronously when it calls $http or $resource internally

Is there a way to wait on a promise so that you can get the actual result from it and return that instead of returning the promise itself? I'm thinking of something similar to how the C# await keyword works with Tasks.
Here is an example of why I'd like to have a method like canAccess() that returns true or false instead of a promise so that it can be used in an if statement. The method canAccess() would make an AJAX call using $http or $resource and then somehow wait for the promise to get resolved.
The would look something like this:
$scope.canAccess = function(page) {
var resource = $resource('/api/access/:page');
var result = resource.get({page: page});
// how to await this and not return the promise but the real value
return result.canAccess;
}
Is there anyway to do this?

In general that's a bad idea. Let me tell you why. JavaScript in a browser is basically a single threaded beast. Come to think of it, it's single threaded in Node.js too. So anything you do to not "return" at the point you start waiting for the remote request to succeed or fail will likely involve some sort of looping to delay execution of the code after the request. Something like this:
var semaphore = false;
var superImportantInfo = null;
// Make a remote request.
$http.get('some wonderful URL for a service').then(function (results) {
superImportantInfo = results;
semaphore = true;
});
while (!semaphore) {
// We're just waiting.
}
// Code we're trying to avoid running until we know the results of the URL call.
console.log('The thing I want for lunch is... " + superImportantInfo);
But if you try that in a browser and the call takes a long time, the browser will think your JavaScript code is stuck in a loop and pop up a message in the user's face giving the user the chance to stop your code. JavaScript therefore structures it like so:
// Make a remote request.
$http.get('some wonderful URL for a service').then(function (results) {
// Code we're trying to avoid running until we know the results of the URL call.
console.log('The thing I want for lunch is... " + results);
});
// Continue on with other code which does not need the super important info or
// simply end our JavaScript altogether. The code inside the callback will be
// executed later.
The idea being that the code in the callback will be triggered by an event whenever the service call returns. Because event driven is how JavaScript likes it. Timers in JavaScript are events, user actions are events, HTTP/HTTPS calls to send and receive data generate events too. And you're expected to structure your code to respond to those events when they come.
Can you not structure your code such that it thinks canAccess is false until such time as the remote service call returns and it maybe finds out that it really is true after all? I do that all the time in AngularJS code where I don't know what the ultimate set of permissions I should show to the user is because I haven't received them yet or I haven't received all of the data to display in the page at first. I have defaults which show until the real data comes back and then the page adjusts to its new form based on the new data. The two way binding of AngularJS makes that really quite easy.

Use a .get() callback function to ensure you get a resolved resource.
Helpful links:
Official docs
How to add call back for $resource methods in AngularJS

You can't - there aren't any features in angular, Q (promises) or javascript (at this point in time) that let do that.
You will when ES7 happens (with await).
You can if you use another framework or a transpiler (as suggested in the article linked - Traceur transpiler or Spawn).
You can if you roll your own implementation!

My approach was create a function with OLD javascript objects as follows:
var globalRequestSync = function (pUrl, pVerbo, pCallBack) {
httpRequest = new XMLHttpRequest();
httpRequest.onreadystatechange = function () {
if (httpRequest.readyState == 4 && httpRequest.status == 200) {
pCallBack(httpRequest.responseText);
}
}
httpRequest.open(pVerbo, pUrl, false);
httpRequest.send(null);
};

I recently had this problem and made a utility called 'syncPromises'. This basically works by sending what I called an "instruction list", which would be array of functions to be called in order. You'll need to call the first then() to kick things of, dynamically attach a new .then() when the response comes back with the next item in the instruction list so you'll need to keep track of the index.
// instructionList is array.
function syncPromises (instructionList) {
var i = 0,
defer = $q.defer();
function next(i) {
// Each function in the instructionList needs to return a promise
instructionList[i].then(function () {
var test = instructionList[i++];
if(test) {
next(i);
}
});
}
next(i);
return defer.promise;
}
This I found gave us the most flexibility.
You can automatically push operations etc to build an instruction list and you're also able to append as many .then() responses handlers in the callee function. You can also chain multiple syncPromises functions that will all happen in order.

Caching a promise object in AngularJS service

I want to implement a dynamic loading of a static resource in AngularJS using Promises. The problem: I have couple components on page which might (or not, depends which are displayed, thus dynamic) need to get a static resource from the server. Once loaded, it can be cached for the whole application life.
I have implemented this mechanism, but I'm new to Angular and Promises, and I want to make sure if this is a right solution \ approach.
var data = null;
var deferredLoadData = null;
function loadDataPromise() {
if (deferredLoadData !== null)
return deferredLoadData.promise;
deferredLoadData = $q.defer();
$http.get("data.json").then(function (res) {
data = res.data;
return deferredLoadData.resolve();
}, function (res) {
return deferredLoadData.reject();
});
return deferredLoadData.promise;
}
So, only one request is made, and all next calls to loadDataPromise() get back the first made promise. It seems to work for request that in the progress or one that already finished some time ago.
But is it a good solution to cache Promises?

Is this the right approach?
Yes. The use of memoisation on functions that return promises a common technique to avoid the repeated execution of asynchronous (and usually expensive) tasks. The promise makes the caching easy because one does not need to distinguish between ongoing and finished operations, they're both represented as (the same) promise for the result value.
Is this the right solution?
No. That global data variable and the resolution with undefined is not how promises are intended to work. Instead, fulfill the promise with the result data! It also makes coding a lot easier:
var dataPromise = null;
function getData() {
if (dataPromise == null)
dataPromise = $http.get("data.json").then(function (res) {
return res.data;
});
return dataPromise;
}
Then, instead of loadDataPromise().then(function() { /* use global */ data }) it is simply getData().then(function(data) { … }).
To further improve the pattern, you might want to hide dataPromise in a closure scope, and notice that you will need a lookup for different promises when getData takes a parameter (like the url).

For this task I created service called defer-cache-service which removes all this boiler plate code. It writted in Typescript, but you can grab compiled js file. Github source code.
Example:
function loadCached() {
return deferCacheService.getDeferred('cacke.key1', function () {
return $http.get("data.json");
});
}
and consume
loadCached().then(function(data) {
//...
});
One important thing to notice that if let's say two or more parts calling the the same loadDataPromise and at the same time, you must add this check
if (defer && defer.promise.$$state.status === 0) {
return defer.promise;
}
otherwise you will be doing duplicate calls to backend.

This design design pattern will cache whatever is returned the first time it runs , and return the cached thing every time it's called again.
const asyncTask = (cache => {
return function(){
// when called first time, put the promise in the "cache" variable
if( !cache ){
cache = new Promise(function(resolve, reject){
setTimeout(() => {
resolve('foo');
}, 2000);
});
}
return cache;
}
})();
asyncTask().then(console.log);
asyncTask().then(console.log);
Explanation:
Simply wrap your function with another self-invoking function which returns a function (your original async function), and the purpose of wrapper function is to provide encapsulating scope for a local variable cache, so that local variable is only accessible within the returned function of the wrapper function and has the exact same value every time asyncTask is called (other than the very first time)