How can I initialize an empty promise using AngularJS?

Question

How can I initialize an empty promise using AngularJS?

I am looking for a way to achieve the following:

var promise = IAmAEmptyPromise;

if(condition){
    promise = ApiService.getRealPromise();
}

promise.then(function(){
    //perform some actions
});

I want to define a promise that can be resolved with a then function. However, this promise might get replaced by another promise that returns some data. After that, I would like to resolve the promise regardless of whether it has any data or not. Is this feasible? My attempt was:

var promise = $q.defer().promise;

if(!$scope.user){
    promise = UserService.create(params);
}

promise.then(function(){
   //handle the scenario where a new user is created or an existing user is found
});

The above code does not work as expected when a user already exists. Any suggestions on how to make it work in all cases?

javascript angularjs

Answer 1

Answer №1

As Bixi mentioned, you have the option to utilize $q.when(), which allows you to encapsulate a promise or a value within a promise. When you pass a promise into when(), it will be returned as is. If a non-promise value is passed, a new promise will be generated and resolved immediately with that value. Here is an example:

var promise;
if(!$scope.user){
  promise = UserService.create(params);
} else {
  promise = $q.when($scope.user);
}

promise.then(function(user){
  //The user will either be newly created or already exist.
});

Answer 2

As Bixi mentioned, you have the option to utilize $q.when(), which allows you to encapsulate a promise or a value within a promise. When you pass a promise into when(), it will be returned as is. If a non-promise value is passed, a new promise will be generated and resolved immediately with that value. Here is an example:

var promise;
if(!$scope.user){
  promise = UserService.create(params);
} else {
  promise = $q.when($scope.user);
}

promise.then(function(user){
  //The user will either be newly created or already exist.
});

Answer 3

Answer №2

Utilizing native es-6 promises allows you to easily generate an instantaneously resolved promise by using Promise.resolve(). This feature proves to be beneficial when chaining promises together.

var p = Promise.resolve();
for (var i=0; i<something.length; i++) {
    p = p.then(UserService.create(newUsers[i]));
}
p.catch(function(e) {
    console.error('oops: ', e);
}).then(function() {
    console.log('done.');
});

Answer 4

Utilizing native es-6 promises allows you to easily generate an instantaneously resolved promise by using Promise.resolve(). This feature proves to be beneficial when chaining promises together.

var p = Promise.resolve();
for (var i=0; i<something.length; i++) {
    p = p.then(UserService.create(newUsers[i]));
}
p.catch(function(e) {
    console.error('oops: ', e);
}).then(function() {
    console.log('done.');
});

How can I initialize an empty promise using AngularJS?

Answer №1

Answer №2

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