What is the best way to determine if an item qualifies as an Angular $q promise?

Question

What is the best way to determine if an item qualifies as an Angular $q promise?

In my project, I have an existing API library that is not Angular-based. This library contains a method called .request which returns promises using jQuery.Deferred. To integrate this with Angular, I created a simple service that wraps the .request method to convert its output into Angular $q promises. Here's how it looks:

var module = angular.module('example.api', []);

module.factory('api', function(
    $q,
    $window
) {
    function wrappedRequest() {
        var result = $window.API.request.apply($window.API, arguments);
        return $q.when(result);
    };

    return {
        request: wrappedRequest
    };
});

I want to write a test to ensure that this service works correctly. The test should involve creating a mock $window object with an API property that has a request method returning jQuery.Deferred promises. The goal is to confirm that the returned objects are indeed Angular $q promises.

Is there a way to determine whether an object is an Angular $q promise?

While it's important for the given scenario to distinguish between jQuery.Deferred and Angular $q promises, ideally we should be able to identify Angular $q promises in any context.

javascript angularjs promise

Answer 1

Answer №1

Typically, the recommended approach is to convert any object you have into an Angular promise.

The idea of incorporating thenables is a key component of the Promises/A+ specification. Most promise libraries offer a method to accomplish this, facilitating seamless compatibility between different promise implementations and ensuring a consistent API.

In $q, this can be achieved using .when:

Wraps an object that could be a value or a third-party thenable promise into a $q promise. This is particularly useful when working with objects that may or may not be promises, or if the promise originates from an untrustworthy source.

By leveraging thenables, $q can transform an 'untrusted' promise into a reliable $q promise.

All you need to do is:

var p = $q.when(value); // p will always be a $q promise
                        // no issue if it was already one

Answer 2

Typically, the recommended approach is to convert any object you have into an Angular promise.

The idea of incorporating thenables is a key component of the Promises/A+ specification. Most promise libraries offer a method to accomplish this, facilitating seamless compatibility between different promise implementations and ensuring a consistent API.

In $q, this can be achieved using .when:

Wraps an object that could be a value or a third-party thenable promise into a $q promise. This is particularly useful when working with objects that may or may not be promises, or if the promise originates from an untrustworthy source.

By leveraging thenables, $q can transform an 'untrusted' promise into a reliable $q promise.

All you need to do is:

var p = $q.when(value); // p will always be a $q promise
                        // no issue if it was already one

Answer 3

Answer №2

For a simple solution specific to the example at hand, differentiating between jQuery.Deferred promises and Angular promises can be achieved by comparing specific methods. In Angular, $q promises utilize the catch method for error handling, while jQuery.Deferred promises use the fail method.

function determinePromiseType(promise) {
    if (typeof promise.fail === 'function') {
        return 'jQuery';
    } else if (typeof promise.catch === 'function') {
        return 'Angular';
    } else {
        throw new Error("Unable to identify the type of promise!");
    }
}

However, attempting to use this methodology for distinguishing between various types of promises or even identifying non-promises could become convoluted. Different implementations often share common method names, leading to potential confusion. While possible, it may not be worth pursuing down this path.

An alternative approach exists that can reliably recognize $q promises in suitable conditions, such as working with trusted objects in a controlled environment with a single version of Angular. Despite its effectiveness, some may view this technique as too unconventional for serious applications.

By converting a function to a string using the String() function, we can access the source code for that function. This allows us to compare the .then method of a presumed promise object against that of a confirmed $q promise:

function isAngularPromise(value) {
    if (typeof value.then !== 'function') {
        return false;
    }
    var promiseThenSrc = String($q.defer().promise.then);
    var valueThenSrc = String(value.then);
    return promiseThenSrc === valueThenSrc;
}

Answer 4

For a simple solution specific to the example at hand, differentiating between jQuery.Deferred promises and Angular promises can be achieved by comparing specific methods. In Angular, $q promises utilize the catch method for error handling, while jQuery.Deferred promises use the fail method.

function determinePromiseType(promise) {
    if (typeof promise.fail === 'function') {
        return 'jQuery';
    } else if (typeof promise.catch === 'function') {
        return 'Angular';
    } else {
        throw new Error("Unable to identify the type of promise!");
    }
}

However, attempting to use this methodology for distinguishing between various types of promises or even identifying non-promises could become convoluted. Different implementations often share common method names, leading to potential confusion. While possible, it may not be worth pursuing down this path.

An alternative approach exists that can reliably recognize $q promises in suitable conditions, such as working with trusted objects in a controlled environment with a single version of Angular. Despite its effectiveness, some may view this technique as too unconventional for serious applications.

By converting a function to a string using the String() function, we can access the source code for that function. This allows us to compare the .then method of a presumed promise object against that of a confirmed $q promise:

function isAngularPromise(value) {
    if (typeof value.then !== 'function') {
        return false;
    }
    var promiseThenSrc = String($q.defer().promise.then);
    var valueThenSrc = String(value.then);
    return promiseThenSrc === valueThenSrc;
}

Answer 5

Answer №3

My current solution involves utilizing the instanceof operator:

var AngularPromise = $q.resolve().constructor;

console.log($q.resolve() instanceof AngularPromise);  // true

This method ensures that the result will be true only if the object is an actual Angular Promise.

To see a demonstration, visit: https://jsfiddle.net/DerekL/cmzp7ovj/

Answer 6

My current solution involves utilizing the instanceof operator:

var AngularPromise = $q.resolve().constructor;

console.log($q.resolve() instanceof AngularPromise);  // true

This method ensures that the result will be true only if the object is an actual Angular Promise.

To see a demonstration, visit: https://jsfiddle.net/DerekL/cmzp7ovj/

What is the best way to determine if an item qualifies as an Angular $q promise?

Answer №1

Answer №2

Answer №3

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