Exploring the concept of JavaScript nested promise scopes within the context of AngularJS

Question

Exploring the concept of JavaScript nested promise scopes within the context of AngularJS

I've been struggling with JavaScript promises for the past few hours, trying to fix a problem that I just can't seem to solve. My knowledge of promises is limited, so I'm open to the possibility that my approach might be incorrect.

Currently, I'm working on an app that involves a two-step process:

Connecting to an external PaaS service, which returns a promise
Retrieving some data within that promise

Here's a snippet of the factory I have created:

app.factory('serviceFactory', [
    function() {
        var getData = function getData() {
            service.connect(apiKey).then(function() {
                    service.getData('dataStore').then(function(result) {
                        // Retrieve data
                        return result;
                    }, errorFunction);
                },
                errorFunction);
        };
        return {
                getData: getData
        };
    }

]);

In this code, there are nested promises. The issue arises when I attempt to use the data from the innermost promise in an AngularJS view. Specifically, I want to incorporate the data into an ng-repeat statement. However, no matter what I try, the data doesn't display. I've even tried assigning the data within the promise instead of returning it, like this:

service.getData('dataStore').then(function(result) {
    // Retrieve data
    // Assigned the enclosing scope's this to 'self'
    self.data = result;
}, errorFunction);

Unfortunately, this method also fails to work. I've experimented with various other approaches, but I just can't figure out how to get the data to appear in the view. Despite successfully displaying the data with a console.log(data) call, indicating that the data is indeed being retrieved properly. Has anyone encountered and resolved a similar issue?

javascript angularjs promise angular-promise

Answer 1

Answer №1

To improve your code, it is advisable to avoid using nested promises. Take a look at this blog post that discusses how you can eliminate 'promise soup' and utilize promise chaining.

In response to your query, here are some recommendations:

A quick fix for your issue would involve correcting the return statement in your factory method:

app.factory('serviceFactory', [
    function() {
        var getData = function() {
            return service.connect(apiKey).then(function() {
                    service.getData('dataStore').then(function(result) {
                        // Retrieve data
                        return result;
                    }, errorFunction);
                },
                errorFunction);
        }; // make sure to close the 'getData' method here
        return {
            getData: getData
        };   
    }
]);

Alternatively, you can refactor your code to chain promises like this:

app.factory('serviceFactory', [
    function() {
        var connect = function() {
            return service.connect(apiKey);
        };
        var getData = function(data) {
            return service.getData(data);
        };       
        return {
            getData: getData,
            connect: connect
        };
    }
]);

Now you can execute something similar to this:

serviceFactory.connect(apiKey)
     .then(serviceFactory.getData)
     .then(function(result){
           //use data here
      })

Make sure to test all of this thoroughly - feel free to provide a plunker or jsbin for a functional solution...

EDIT:

It seems there may be another issue with your code. There appears to be confusion between serviceFactory and service. It's unclear if these refer to the same service or different ones. Consider sharing more detailed code or providing a plunker/jsbin for clarification.

Answer 2

To improve your code, it is advisable to avoid using nested promises. Take a look at this blog post that discusses how you can eliminate 'promise soup' and utilize promise chaining.

In response to your query, here are some recommendations:

A quick fix for your issue would involve correcting the return statement in your factory method:

app.factory('serviceFactory', [
    function() {
        var getData = function() {
            return service.connect(apiKey).then(function() {
                    service.getData('dataStore').then(function(result) {
                        // Retrieve data
                        return result;
                    }, errorFunction);
                },
                errorFunction);
        }; // make sure to close the 'getData' method here
        return {
            getData: getData
        };   
    }
]);

Alternatively, you can refactor your code to chain promises like this:

app.factory('serviceFactory', [
    function() {
        var connect = function() {
            return service.connect(apiKey);
        };
        var getData = function(data) {
            return service.getData(data);
        };       
        return {
            getData: getData,
            connect: connect
        };
    }
]);

Now you can execute something similar to this:

serviceFactory.connect(apiKey)
     .then(serviceFactory.getData)
     .then(function(result){
           //use data here
      })

Make sure to test all of this thoroughly - feel free to provide a plunker or jsbin for a functional solution...

EDIT:

It seems there may be another issue with your code. There appears to be confusion between serviceFactory and service. It's unclear if these refer to the same service or different ones. Consider sharing more detailed code or providing a plunker/jsbin for clarification.

Answer 3

Answer №2

After revisiting and refining my initial response, I realized that the lack of clarity in my explanation may have contributed to receiving downvotes without any explanations. To provide a more comprehensive answer, here are some additional insights.

It seems like the issue you're facing stems from a disconnect between the PaaS platform you are utilizing and Angular. If Angular is unaware of the promises returned by the PaaS methods, it won't be able to update the DOM when these promises resolve. Angular triggers a digest cycle to monitor changes within its watched items; however, this mechanism only initiates automatically for promises generated through Angular services like $q or $http, not for externally created promises.

In your scenario, the promises from the PaaS are resolving correctly (as evidenced by console logs), but the HTML isn't reflecting these updates.

To illustrate this behavior, I've adapted our shared plunkr to showcase a mock PaaS setup using jQuery to create and resolve promises. Despite seeing the promise results in the console, the DOM remains unchanged.

// Angular module, controller, factory, and directive implementations

Previously, I had suggested rectifying this situation by calling $scope.$apply() after capturing the promise result. By referencing the updated Plunker link, you can observe how this approach successfully triggers DOM updates.

// Updated DataController implementation with $scope.$apply()

Alternatively, a cleaner solution exists. When integrating external promises into Angular, consider wrapping them with $q.when to convert them into Angular-aware promises. This methodology should prompt Angular to initiate its digest cycle upon resolution of external promises.

// Enhanced dataFactory implementation incorporating $q.when to normalize promises

If you'd like a detailed explanation of this concept, Ben Nadel provides valuable insights in his article here.

Answer 4

After revisiting and refining my initial response, I realized that the lack of clarity in my explanation may have contributed to receiving downvotes without any explanations. To provide a more comprehensive answer, here are some additional insights.

It seems like the issue you're facing stems from a disconnect between the PaaS platform you are utilizing and Angular. If Angular is unaware of the promises returned by the PaaS methods, it won't be able to update the DOM when these promises resolve. Angular triggers a digest cycle to monitor changes within its watched items; however, this mechanism only initiates automatically for promises generated through Angular services like $q or $http, not for externally created promises.

In your scenario, the promises from the PaaS are resolving correctly (as evidenced by console logs), but the HTML isn't reflecting these updates.

To illustrate this behavior, I've adapted our shared plunkr to showcase a mock PaaS setup using jQuery to create and resolve promises. Despite seeing the promise results in the console, the DOM remains unchanged.

// Angular module, controller, factory, and directive implementations

Previously, I had suggested rectifying this situation by calling $scope.$apply() after capturing the promise result. By referencing the updated Plunker link, you can observe how this approach successfully triggers DOM updates.

// Updated DataController implementation with $scope.$apply()

Alternatively, a cleaner solution exists. When integrating external promises into Angular, consider wrapping them with $q.when to convert them into Angular-aware promises. This methodology should prompt Angular to initiate its digest cycle upon resolution of external promises.

// Enhanced dataFactory implementation incorporating $q.when to normalize promises

If you'd like a detailed explanation of this concept, Ben Nadel provides valuable insights in his article here.

Exploring the concept of JavaScript nested promise scopes within the context of AngularJS

Answer №1

Answer №2

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