Let’s fix the multiple HTTP requests problem with a promise: So, while handling a HTTP request, Promise can manage a single response for the same request, but what if there are multiple responses to the same request, then we have to use Observable. Then we can do nice things on it, like.every (...), which will result in a single boolean observable according to whether all the promises satisfy a … Promises have their own methods which are then and catch..then () is called when success comes, else the catch () method calls. You have to call subscribe() on an observable before the code will actually execute. Promise.resolve(): It returns a new Promise object that is resolved with the given value. We can solve this by either using an existing rxjs operator, in combination with the from operator we're already using or you can decide to build the observable from scratch. The above code will create an observable based on the promise and only subscribe to it after 5000 ms. First of all, let’s recall what promises and observables are all about: handling asynchronous execution. You can see this in action here: https://stackblitz.com/edit/rxjs-bb626s. Here are some of the operators 1. create 2. defer 3. empty 4. from 5. fromEvent 6. interval 7. of 8. range 9. thr… Notice that the only difference is that now the subscription is notified with the pending observables, still without waiting. So it makes sense to convert a list of promises into an observable. But what if I told you this probably isn't what you want (yet)? Promises are created using the promise constructor. This makes observables useful for defining recipes that can be run whenever you need the result. The above code is the promise representation of the snippet that we want to convert to using observables in such a way that we can integrate it with other, existing, observables. Converting Observable Sequences to Promises. To support this, we provide the Rx.Observable.fromPromisemethod which calls the thenmethod of the promise to handle both success and error cases. Built on Forem — the open source software that powers DEV and other inclusive communities. The function is a Producer of data, and the code that calls the function is consuming it by "pulling" out a singlereturn value from its call. On an Observable object, RxJS toPromise() method is called which converts the observable to Promise object. Observables provide many values. * * **WARNING**: Only use this with observables you *know* will complete. We strive for transparency and don't collect excess data. As you might notice, it is also using the AbortController to cancel the HTTP call when unsubscribed from the Observable (even tho it's slightly more complicated because this article sticks to the basics of promise cancelation). You can read more about Aborting a fetch on https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API. Promise.reject(): It returns a new Promise object that is rejected with the given reason. That’s because the first boolean is associated to the longer running promise, which exhausts the processing capacity of the resulting observable until it gets resolved (and completed). Notice that above and in the next snippets I’m going to show the console output corresponding to the subscription defined earlier and using the last source. Turn an array, promise, or iterable into an observable. Made with love and Ruby on Rails. Notice that we are now getting reversed logs because, to get a better sense of promises, we are creating them so that they will resolve in reversed order. Converts a higher-order Observable into a first-order Observable by concatenating the inner Observables in order. But first, let me introduce some snippets I’ll be using later on. We're still missing one crucial part in our Promise to Observable conversion. In the Observable we call observer.next() to trigger and emit our value to Also notice that the fact that the notification happens only after all the promises have resolved, is coincidental (because we made it happen by forcing each promise to complete sooner than the previous one). This is a fairly common pattern when handling observables. Even tho I'd recommend using existing rxjs operators when possible, I think for converting a Promise to an Observable it's worth taking control over the Observable creation ourselves so that we have more control over what happens when we unsubscribe from the Observable (which we will cover in promise cancellation). Promise emits a single value while Observable emits multiple values. When using observables, it's not expected that anything happens for as long as there is no active subscription. Here's a stackblitz containing the functionality to abort the HTTP call: https://stackblitz.com/edit/rxjs-7wc1rb. Rxjs has built-in support for converting the fetch API to an observable (see: https://github.com/ReactiveX/rxjs/blob/0e4849a36338133ac3c1b890cd68817547177f44/src/internal/observable/dom/fetch.ts This operator can be used to convert a promise to an observable! combineAll(project: function): Observable. However, this article is intended to give you an example on how we can convert any promise to an Observable. Just as you can convert a Promise to an Observable sequence, you can also convert an Observable sequence to a Promise. We're a place where coders share, stay up-to-date and grow their careers. Even though a lot of async operations might require a custom AbortController implementation, the fetch API supports the AbortController by default. Yes, and there are many ways to bring a higher order back to the first one. With a simple .map(...) we can convert our booleans to promises. Notice how the subscription is notified once per resolved promise, as soon as each promise is resolved. With you every step of your journey. In order to embrace the full reactivity, it's a good idea to convert that promise into an observable so we can easily pipe other operators or even combine it with other streams. Notify me of follow-up comments by email. How to Subscribe to Observables in Angular Templates On the Promise object, the method then is invoked which returns the Promise. Promises execute immediately on creation. However, it's still not a bad idea to use defer either way to ensure the Promise is lazy, no matter how it's used. 3. In a nutshell, the main differences between a Promise and an Observable are as follows: a Promise is eager, whereas an Observable is lazy, a Promise is … An Observable is an array or a sequence of … For arrays and iterables, all contained values will be emitted as a sequence! Save my name, email, and website in this browser for the next time I comment. Previously, rxjs had an operator that was specifically designed for this use-case: fromPromise. 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