Are JS promises compatible with any function in Angular?

Question

Are JS promises compatible with any function in Angular?

I have a question: Can you use then() on any function?

In my Angular app, I'm encountering an error ('cannot read property then of undefined') when attempting to utilize then.

For instance, take this function:

self.getCommentsData = function() {
  commentsService.getComments($routeParams.id)
  .then(function (data){
    //Perform some operations and finally push to a scope array
    $scope.commentsList.push(someValue);
  });
}

Later on, I want to invoke this method and only proceed with another line of code once it has completed. This is where I'm using the then:

self.getCommentsData()
      .then(function(){
        $location.hash('goTotrue');
        $anchorScroll();
});

However, I am running into an error - not sure what I'm missing here.

Thank you

javascript angularjs es6-promise

Answer 1

Answer №1

It is important to ensure that your function returns a promise for proper chaining.

self.fetchCommentData = function() {
  return commentsService.retrieveComments($routeParams.id)
  .then(function (data){
    //Perform necessary operations and add values to the scope array
    $scope.commentsList.push(newValue);
  });
}

Answer 2

It is important to ensure that your function returns a promise for proper chaining.

self.fetchCommentData = function() {
  return commentsService.retrieveComments($routeParams.id)
  .then(function (data){
    //Perform necessary operations and add values to the scope array
    $scope.commentsList.push(newValue);
  });
}

Answer 3

Answer №2

.then() is only applicable to functions that return some form of a Promise, not just any function.

For more information on Promises, Promise.prototype.then, and working with Promises in Angular, I recommend checking out the MDN documentation on Promises, Promise.prototype.then, and Angular's implementation using $q.

In your scenario, only

commentsService.getComments($routeParams.id)

returns a Promise, allowing you to use .then() with it.

When you chain

commentsService.getComments($routeParams.id).then()

, it also creates a new Promise that resolves after the code in the .then() block executes. You can further chain this by adding a return statement, such as:

commentsService.getComments($routeParams.id).then().then();

Your updated code with a return:

  self.getCommentsData = function() {
    return commentsService.getComments($routeParams.id)
      .then(function (data){
        //Do some processing and eventually add to a scope array
        $scope.commentsList.push(someValue);
      });
}

Answer 4

.then() is only applicable to functions that return some form of a Promise, not just any function.

For more information on Promises, Promise.prototype.then, and working with Promises in Angular, I recommend checking out the MDN documentation on Promises, Promise.prototype.then, and Angular's implementation using $q.

In your scenario, only

commentsService.getComments($routeParams.id)

returns a Promise, allowing you to use .then() with it.

When you chain

commentsService.getComments($routeParams.id).then()

, it also creates a new Promise that resolves after the code in the .then() block executes. You can further chain this by adding a return statement, such as:

commentsService.getComments($routeParams.id).then().then();

Your updated code with a return:

  self.getCommentsData = function() {
    return commentsService.getComments($routeParams.id)
      .then(function (data){
        //Do some processing and eventually add to a scope array
        $scope.commentsList.push(someValue);
      });
}

Answer 5

Answer №3

It is crucial to ensure that your function returns a promise to avoid encountering the displayed error message repeatedly. Information from the angular documentation highlights the importance of utilizing the deferred object and Promise instance for task completion status.

The deferred object serves the purpose of presenting the associated Promise instance, along with functionalities for indicating successful or unsuccessful completion, as well as the task's status.

An example has been provided to demonstrate this concept:

angular
  .module("myApp", [])
  .controller("myCtrl", myCtrl)
  .service("myService", myService)
  
  myCtrl.$inject  = ["myService"];
  myService.$inject  = ["$q", "$timeout"];
  
  function myCtrl(myService){
    var vm = this;
    vm.data = [{user: "empty"}, {user: "empty"}, {user: "empty"}]
    vm.message = "Just wait 3 seconds to retrieve the data";
    
    myService.myPromise().then(function(res){
      //successful scenario. The promise has been fulfilled
      vm.data = res;
      vm.message = "Promise fulfilled!";
      console.log("result");
      console.log(res);
    });
    
    
  }
  
  function myService($q, $timeout){
    this.myPromise = function(){
      var promiseObj = $q.defer(); //promise
      var dummyData = [{user: 1}, {user: 2}, {user: 3}];
      
      //the timeout demonstrates that the promise may require some time
      //you are not obligated to use the $timeout service
      $timeout(function(){
        promiseObj.resolve(dummyData); //promise fulfilled
      }, 3000);
      
      //promise fulfilled
      return promiseObj.promise;
      
    }
  }

<!DOCTYPE html>
<html ng-app="myApp">

<head>
  <script data-require="<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="84e5eae3f1e8e5f6aaeef7c4b5aab7aab5b0">[email protected]</a>" data-semver="1.3.14" src="https://code.angularjs.org/1.3.14/angular.js"></script>
  <link rel="stylesheet" href="style.css" />
</head>

<body ng-controller="myCtrl as ctrl">
  <h1>Promise test</h1>
  <h2>Dummy data</h2>
  <h4>{{ctrl.message}}</h4>
  <ul>
    <li ng-repeat="data in ctrl.data">{{data.user}}</li>
  </ul>
</body>

</html>

Answer 6

It is crucial to ensure that your function returns a promise to avoid encountering the displayed error message repeatedly. Information from the angular documentation highlights the importance of utilizing the deferred object and Promise instance for task completion status.

The deferred object serves the purpose of presenting the associated Promise instance, along with functionalities for indicating successful or unsuccessful completion, as well as the task's status.

An example has been provided to demonstrate this concept:

angular
  .module("myApp", [])
  .controller("myCtrl", myCtrl)
  .service("myService", myService)
  
  myCtrl.$inject  = ["myService"];
  myService.$inject  = ["$q", "$timeout"];
  
  function myCtrl(myService){
    var vm = this;
    vm.data = [{user: "empty"}, {user: "empty"}, {user: "empty"}]
    vm.message = "Just wait 3 seconds to retrieve the data";
    
    myService.myPromise().then(function(res){
      //successful scenario. The promise has been fulfilled
      vm.data = res;
      vm.message = "Promise fulfilled!";
      console.log("result");
      console.log(res);
    });
    
    
  }
  
  function myService($q, $timeout){
    this.myPromise = function(){
      var promiseObj = $q.defer(); //promise
      var dummyData = [{user: 1}, {user: 2}, {user: 3}];
      
      //the timeout demonstrates that the promise may require some time
      //you are not obligated to use the $timeout service
      $timeout(function(){
        promiseObj.resolve(dummyData); //promise fulfilled
      }, 3000);
      
      //promise fulfilled
      return promiseObj.promise;
      
    }
  }

<!DOCTYPE html>
<html ng-app="myApp">

<head>
  <script data-require="<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="84e5eae3f1e8e5f6aaeef7c4b5aab7aab5b0">[email protected]</a>" data-semver="1.3.14" src="https://code.angularjs.org/1.3.14/angular.js"></script>
  <link rel="stylesheet" href="style.css" />
</head>

<body ng-controller="myCtrl as ctrl">
  <h1>Promise test</h1>
  <h2>Dummy data</h2>
  <h4>{{ctrl.message}}</h4>
  <ul>
    <li ng-repeat="data in ctrl.data">{{data.user}}</li>
  </ul>
</body>

</html>

Answer 7

Answer №4

Unfortunately, using them on $promise is the only option available.

Therefore, you are limited to this approach:

commentsService.retrieveComments($routeParams.id).then(function (data){
    $scope.commentsList.addComment(someValue);
});

Answer 8

Unfortunately, using them on $promise is the only option available.

Therefore, you are limited to this approach:

commentsService.retrieveComments($routeParams.id).then(function (data){
    $scope.commentsList.addComment(someValue);
});

Are JS promises compatible with any function in Angular?

Answer №1

Answer №2

Answer №3

Answer №4

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