Unwrapping Promises in Angular for Seamless Resolution

Question

Unwrapping Promises in Angular for Seamless Resolution

I recently started working with Angular and found myself in a large project. I encountered a simplified version of my code below:

var beforeClose = function() {
    var closeDeferred = $q.defer(),
    a = $q.defer(),
    b = $q.defer(),
    c = $q.defer(),
    promiseArray = [a.promise, b.promise, c.promise];

    /* logic that resolves or rejects a, b, and c */

    $q.all(promiseArray).then(
        function() {
            closeDeferred.resolve();
        },
        function() {
            closeDeferred.reject();
        }
    );
    return closeDeferred.promise;
}

var modalClose = function() {
    beforeClose().then(
        function() {
            //close the modal
        },
        function() {
            //warn and don't close modal
        }
    )
}

While using Chrome's DevTools, I noticed that the code is returning closeDeferred promise before promiseArray is fully resolved or rejected. So my question is,

When is the promise returned? Can it be returned before being resolved or rejected? If not, how can I prevent it from being returned prematurely?

Moving the return statement into a separate function (e.g., within the resolve and reject functions of promiseArray) does make that function return the promise, but it doesn't necessarily pass that promise to modalClose (which initiated the first promise).

Or perhaps I am misunderstanding the behavior of my code. When I placed the closeDeferred return into another function, I received errors indicating that .then() cannot be called on undefined (pointing at the beforeClose().then() line).

Edit: After reviewing Andrew's answer, I made updates to the code and need to clarify what's happening. The issue is not necessarily that the promise is returned sooner than desired, but rather that it executes the resolve function for that promise -- closing the modal even though promiseArray() hasn't finished processing.

Edit 2: It appears that by simplifying the code structure, I inadvertently removed the actual problem. In setting up the promises differently, I neglected to define closeDeferred as a defer or a promise before the return statement. As a result, when the loop executed its task, it also returned closeDeferred.promise as undefined, causing the modalClose function to run before any promises were resolved/rejected. Only in the subsequent loop was closeDeferred finally defined and the promises behaved correctly.

I still greatly appreciate the assistance provided. @andrew-tomlinson and @georgeawg both enhanced my understanding of promises significantly. While it did not directly solve this specific issue (due to my misidentification), the guidance offered was incredibly valuable. Thank you!

javascript angularjs angular-promise

Answer 1

Answer №1

Upon initial inspection, it appears that the promiseArray is being loaded with Defer objects. However, based on the information provided in the documentation (available at: https://docs.angularjs.org/api/ng/service/$q), it seems that the all() method expects an array of Promise objects.

To retrieve the promise from a Defer object, you can access it as a property:

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

Subsequently, you would utilize the promise objects in your promiseArray. If implemented in your example, it would transform to resemble:

a = $q.defer().promise,
b = $q.defer().promise,
c = $q.defer().promise,
promiseArray = [a,b,c];

Please note that I have not personally tested this yet, but feel free to give it a try if you are still facing issues. Best of luck!

UPDATE: Upon reevaluating some of your inquiries more attentively, I believe it is necessary to adjust my response to address certain specifics. I apologize if you are already familiar with this information and my explanation misses the mark of your questions.

"When is the promise returned?"

A Promise object is returned from a function when the return statement is executed. Returning a promise object does not equate to resolving or rejecting a promise. Typically, returning a Promise object is done to pass its reference within an application. In the context of your example, you are passing a Promise object from the beforeClose() function to the modalClose() function. The callback functions attached to that Promise object using .then(function(){},function(){}) in modalClose() will only execute once the promise is resolved or rejected.

"Can it be returned before it is resolved or rejected?"

Yes, the Promise object closeDeferred.promise will indeed be returned immediately from beforeClose(), however, based on the sample code provided, it should not be resolved or rejected until the promises within the promiseArray are collectively resolved or rejected. Essentially, return closeDeferred.promise; does not block execution and functions similarly to a regular return statement at the end of a function.

An easily understandable illustration of the Promise concept can be found here, explained in a cartoon format.

Answer 2

Upon initial inspection, it appears that the promiseArray is being loaded with Defer objects. However, based on the information provided in the documentation (available at: https://docs.angularjs.org/api/ng/service/$q), it seems that the all() method expects an array of Promise objects.

To retrieve the promise from a Defer object, you can access it as a property:

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

Subsequently, you would utilize the promise objects in your promiseArray. If implemented in your example, it would transform to resemble:

a = $q.defer().promise,
b = $q.defer().promise,
c = $q.defer().promise,
promiseArray = [a,b,c];

Please note that I have not personally tested this yet, but feel free to give it a try if you are still facing issues. Best of luck!

UPDATE: Upon reevaluating some of your inquiries more attentively, I believe it is necessary to adjust my response to address certain specifics. I apologize if you are already familiar with this information and my explanation misses the mark of your questions.

"When is the promise returned?"

A Promise object is returned from a function when the return statement is executed. Returning a promise object does not equate to resolving or rejecting a promise. Typically, returning a Promise object is done to pass its reference within an application. In the context of your example, you are passing a Promise object from the beforeClose() function to the modalClose() function. The callback functions attached to that Promise object using .then(function(){},function(){}) in modalClose() will only execute once the promise is resolved or rejected.

"Can it be returned before it is resolved or rejected?"

Yes, the Promise object closeDeferred.promise will indeed be returned immediately from beforeClose(), however, based on the sample code provided, it should not be resolved or rejected until the promises within the promiseArray are collectively resolved or rejected. Essentially, return closeDeferred.promise; does not block execution and functions similarly to a regular return statement at the end of a function.

An easily understandable illustration of the Promise concept can be found here, explained in a cartoon format.

Answer 3

Answer №2

Retrieve the $q.all promise.

var beforeClosing = function() {

    //var closeDeferred = $q.defer(),
    var x = $q.defer(),
    var y = $q.defer(),
    var z = $q.defer(),
    var promisesArray = [x.promise, y.promise, z.promise];

    /* process that resolves or rejects x, y, and z */

    var allPromises = $q.all(promisesArray).then(
                  function(resultsArr) {
                      resultsX = resultsArr[0];
                      resultsY = resultsArr[1];
                      resultsZ = resultsArr[2];
                      return anotherThing;
                  },
                  function(errorMessage) {
                      throw errorMessage;
                  }
    );
    return allPromises;
}

It should be noted that $q.all is not robust. Should any of the promises generate an error, it will resolve (as rejected) with the initial error.

The $q.all method will provide a promise right away. This promise will resolve (with data or error) at some point in the future. The $q service postpones executing the function within the .then method until the promise completes with data; within the .catch method for errors.

var closingModal = function() {
    //allPromises receives a promise immediately
    var allPromises = 
        beforeClosing()
           .then( function(anotherThing) {
                  //awaits resolution of x, y, z
                  //close the modal
           }) .catch( function (errorMessage) {
                  //triggers on first error   
                  //alert and do not close modal 
           });
}

Answer 4

Retrieve the $q.all promise.

var beforeClosing = function() {

    //var closeDeferred = $q.defer(),
    var x = $q.defer(),
    var y = $q.defer(),
    var z = $q.defer(),
    var promisesArray = [x.promise, y.promise, z.promise];

    /* process that resolves or rejects x, y, and z */

    var allPromises = $q.all(promisesArray).then(
                  function(resultsArr) {
                      resultsX = resultsArr[0];
                      resultsY = resultsArr[1];
                      resultsZ = resultsArr[2];
                      return anotherThing;
                  },
                  function(errorMessage) {
                      throw errorMessage;
                  }
    );
    return allPromises;
}

It should be noted that $q.all is not robust. Should any of the promises generate an error, it will resolve (as rejected) with the initial error.

The $q.all method will provide a promise right away. This promise will resolve (with data or error) at some point in the future. The $q service postpones executing the function within the .then method until the promise completes with data; within the .catch method for errors.

var closingModal = function() {
    //allPromises receives a promise immediately
    var allPromises = 
        beforeClosing()
           .then( function(anotherThing) {
                  //awaits resolution of x, y, z
                  //close the modal
           }) .catch( function (errorMessage) {
                  //triggers on first error   
                  //alert and do not close modal 
           });
}

Answer 5

Answer №3

It appears that sending $q.all promise objects and utilizing the inherent .catch functionality of promises might be a better approach:

var prepareToClose = function() {
    var closeDeferred = $q.defer(),
    x = $q.defer().promise, // send promise objects to promiseArray
    y = $q.defer().promise, // send promise objects to promiseArray
    z = $q.defer().promise, // send promise objects to promiseArray
    promiseArray = [x, y, z];

    /* logic that resolves or rejects x, y, and z */

    $q.all(promiseArray)
    .then(closeDeferred.resolve();)
    .catch(closeDeferred.reject();)

    return closeDeferred.promise;
}

var handleModalClose = function() {
    prepareToClose()
    .then( //execute modal closure )
    .catch( //display warning and prevent modal closure );
}

Answer 6

It appears that sending $q.all promise objects and utilizing the inherent .catch functionality of promises might be a better approach:

var prepareToClose = function() {
    var closeDeferred = $q.defer(),
    x = $q.defer().promise, // send promise objects to promiseArray
    y = $q.defer().promise, // send promise objects to promiseArray
    z = $q.defer().promise, // send promise objects to promiseArray
    promiseArray = [x, y, z];

    /* logic that resolves or rejects x, y, and z */

    $q.all(promiseArray)
    .then(closeDeferred.resolve();)
    .catch(closeDeferred.reject();)

    return closeDeferred.promise;
}

var handleModalClose = function() {
    prepareToClose()
    .then( //execute modal closure )
    .catch( //display warning and prevent modal closure );
}

Unwrapping Promises in Angular for Seamless Resolution

Answer №1

Answer №2

Answer №3

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