Angular Promise vs Observable: Ultimate Differences You Must Know

JavaScript strictly follows the asynchronous concurrent language and its engine instantly moves to execute the next statement without waiting for the concurrent statement to finish.

Such a concept is called the asynchronous function, and you need to deal with the callbacks approach. So, I know it’s a challenging task to manage the codes, making the callback hell.

Angular Promises and Observables are the one-stop solutions to overcome such situations. Their implementations help us deal with that asynchronous code more cleanly. However, they have different APIs, and their motivation is slightly different.

And we have come up with an article that depicts the technical difference between Angular Promise vs Observable — one of the most common comparisons developers encounter when working with asynchronous programming in Angular.

What is Promise in Angular?

One of the best ways to execute asynchronous functions in your application that typically uses callbacks is opting for a Promise in Angular. Many developers search for what is promise in Angular or how promises in Angular differ from other asynchronous patterns, so let’s break it down.

The promise in Angular takes place while emitting and completing (resolving or rejecting) one value at a time. So if you’re using an Angular Promise, it becomes easy for you to emit a single event from the API.

Then, the controller’s (directive) sole responsibility is to register up to three callbacks – success, error, and notifications.

Generally, four states of the Angular Promise are available for your Angular application development.

• fulfilled
• rejected
• pending
• settled

Remember that Angular Promise is not as active as an Observable. It means AngularJS developers are not allowed to cancel any specific event once it starts. So, passing the callback to the Promise constructor (controller or directive) will provide you with two options: either resolve or reject the function, depending on your needs.

If you’ve ever explored Angular 2 Promises or looked at a simple Angular Promise example, you’ll notice that promises are straightforward to use when your app only needs to return a single value from an async operation.

Get Custom-Tailored Web App Development Solutions For Your Business Needs

We strictly adhere to the standard software development lifecycle to provide the best possible IT solutions for your business success.

Contact Us

What is an Observable in Angular?

Many developers wonder what are Observables in Angular or what is Observable in Angular and how they differ from Promises. Observables in Angular are preferable to Promises as they allow you to run functions asynchronously and synchronously.

Furthermore, it means that you can deliver multiple values over time with the help of Observables. Such values are actively responsible for handling 0, 1, or multiple events, and that too by using the same API for every event.

The excellent part of Angular Observables is the number of operators actively simplifying coding, including retry(), replay(), map, forEach, reduce, and others.

With the help of Observables, the first thing that you need to perform is to have a subscription to start emitting values. If you do not perform this step, it’s just a blueprint of code responsible for handling future emits.

So, when comparing Promises and Observables in Angular, remember: Promises emit a single value once, whereas Observables can emit multiple values over time — making them ideal for streaming data or event-based systems.Related Post: Resolver in Angular

Angular Promise vs Observable: Differences You Must Know

Now, it’s high time to see the technical differences between Angular Promise vs Observable. Developers often ask, “What is the difference between Promise and Observable?” or “Promise vs Observable in Angular – which is better?”

I have gathered a list of examples to clarify doubts about the difference between Promise and Observable in Angular applications.

1. Angular Promise handles one value; Observables handles multiple values.

One of the significant differences between Observable vs Angular Promise is that you are not allowed to change the fulfilled value. Instead, you can just emit (either reject or resolve) a single value for your Angular application.

To understand this concept, you need to have basic information about what Angular is and how it manages asynchronous operations.

And if we talk about Observables, you can freely emit multiple results. The subscriber will receive results until the observer is completed or unsubscribed.

Let’s see an example of handling values in terms of Angular Promise vs Observable.

// Promises
const x = new Promise((resolve) => {
    setInterval(() => {
        // ✅ when resolve is called, the promise will become
        // fullfilled and it's value won't ever change
        resolve(Math.random() * 10)
    }, 3000);
});

// Observables
const observable$ = rxjs.Observable.create((observer) => {
  // ✅ you can pass multiple values by using `next` method
  observer.next(1);
  observer.next(2);
  observer.next(3);
  observer.complete();
})

// ✅ base will be receiving values untils it's completed
observable$
    .subscribe(
      v => console.log(v),
      err => console.log(err),
      done => console.log("completed")
    );

Observables vs Promises is one of the core concepts in Angular’s asynchronous design. Observables are responsive tools that quickly listen to data streams. In addition, there exists a bidirectional kind of Observable: Subjects — they are a perfect use case for those web sockets. The RxJS library helps you deal with shipping using a thin wrapper on web sockets.

2. Observable subscriptions are cancellable; promises aren’t

Once an Angular Promise is started, it seems impossible to cancel the event. The callback passed to the Promise constructor is solely responsible for resolving or rejecting the promise.

The subscriber reacts to the result once fired; hence, it’s completely passive.

Compared to Angular Promise, Observables are less passive, and on subscription creation, you can quickly opt out of the observer at any time.

So, if you are no longer interested in getting the response, you can opt for such scenarios. The best example is when a user leaves a page.

Yes, you’ll find multiple ways to cancel/complete subscribers. Some of the common ones to cancel subscribers are mentioned below:

• unsubscribe: It allows you to cancel the subscription from the Observation manually.
• take: It enables you to take number X of elements and easily cancel the subscription process.
• takeUntil: It makes it easy for you to take values until the passed Observable emits any value.

Let’s go through an example to demonstrate it.

// ✅ 1. Cancelling by using the unsubscribe method
const observable1$ = rxjs.interval(1000);
const subscription1$ = observable1$
  .subscribe(x => console.log(x));

subscription1$.unsubscribe();

// ✅ 2. Cancelling by taking a number of elements
const observable2$ = rxjs.interval(1000);

observable2$
  .pipe(rxjs.operators.take(5))
  .subscribe(x => console.log(x));

// ✅ 3. Conditionally stop listineing for more elements
const subject3$ = new rxjs.Subject();
const observable3$ = rxjs.interval(1000);

observable3$
  .pipe(rxjs.operators.takeUntil(subject3$))
  .subscribe(x => {
    console.log(x);
    if (Math.random() > 0.5) {
      subject3$.next(1);
      subject3$.complete();
      console.log('complete');
    }
  });

This is one of the most practical distinctions when comparing Observable and Promise in Angular — Observables are cancellable, but Promises are not.

The operators of Observable are essential as they allow you to compose declaratively complex asynchronous operations quickly.

Planning To Develop a Robust Single-Page Application?

Our Angular development team has core expertise in designing and building feature-rich applications for all your business needs.

Contact Us

3. Eager vs lazy execution

In the discussion about Angular Promise vs Observable, we need to understand the process of execution. That is done by using the Eager and Lazy execution methods.

Let’s put it simply. You can execute Angular Promises eagerly. On the other hand, Observables are executed lazily.

What is Lazy and Eager execution?

• Eager: It allows you to execute the Promise callback, especially at the constructor level.
• Lazy: The Producer function will only trigger after a subscription is created for that Observable. Otherwise, it will stay idle.

I’ll make it easier for you by demonstrating it with a good example.

const foo = new Promise((resolve) => {
  console.log('1. Callback execution');
  resolve(true);
});

foo.then(() => {
  console.log('2. Promise resolution');
});

console.log('3. Pre exit method statement');

// ✅ console output
// 1. Callback execution
// 3. Pre exit method statement
// 2. Promise resolution

const observable$ = new rxjs.Observable(observer => {
  console.log('1. Execution of observable body');
  observer.next(1);
  observer.complete();
});

console.log('2. Pre Subscribe statement');

observable$.subscribe(() => {
  console.log('3. Execution of subscribe callback')
});

// ✅ console output
// 2. Pre Subscribe statement
// 1. Execution of observable body
// 3. Execution of subscribe callback

This difference between Promise and Observable execution style — eager vs lazy — is one of the fundamental factors developers consider when deciding between the two.

4. Runtime execution

Once ES Promises enter the resolved state, it becomes pretty easy for you to queue the callback in the microtask queue. Simply put, the execution takes place after the current macro task has been completed.

Let’s see an example:

console.log('1. Start');

setTimeout(() => {
    console.log('2. Timeout callback')
}, 0);

Promise.resolve().then(() => {
    console.log('3. Promise callback')
});

console.log('4. End');

// ✅ outputs
// 1. Start
// 4. End
// 3. Promise callback
// 2. Timeout callback

You can’t change the above behavior.

Related Post: Angular Ivy

However, Observables enable you to effortlessly fine-tune the runtime execution by strictly using schedulers. The primary role of a scheduler is to control the state, especially when a subscription starts and you receive notifications for the same.

Here are some common scheduling options:

• null: You’ll receive notifications synchronously and recursively by default.
  
• queueScheduler: It allows you to schedule quickly on a queue in the current event frame.
  
• asapScheduler: You can efficiently schedule it on the microtask queue — and it uses the same queue as Promises.
  
• asyncScheduler: This concept is similar to scheduling a task using setInterval. So, you need to schedule it in the macro task queue.
  
• animationFrameScheduler: Relies on the requestAnimationFrame API.

I’ll demonstrate an example by using asapScheduler:

const observable1$ = rxjs.interval(1000)
  // ✅ using the asapScheduler scheduler
  .pipe(rxjs.operators.observeOn(rxjs.asapScheduler));

const subscription1$ = observable1$
  .subscribe(x => console.log(x));

This flexibility makes Observables in Angular much more powerful for handling complex asynchronous flows. When developers compare Observable vs Promise in Angular, this is one of the decisive differences — Promises have fixed timing, while Observables can adjust execution timing dynamically.

5. Interoperability

So, we have seen many differences between Angular Promise and Observables, but the biggest question developers ask is — can you use both together? Or do you have to select only one of them?

Well, the answer is no — you can use both together.

Observables can easily be created from a Promise, and you can also convert Observable to Promise efficiently. This ability to switch between the two makes it easier to integrate different data streams or external APIs.

If you’re working with Angular 9 promise examples or experimenting with rxjs promise to observable conversions, you’ll often find that both are interoperable with just a few lines of code.

Promises take only one value, so you must choose whether you want the first or last value to be dumped on the Promise.

In the RxJS library, you can use:

• firstValueFrom — to get the first emitted value from an Observable as a Promise.
  
• lastValueFrom — to get the last emitted value.
  

This is one of the most useful features when bridging older async code (which uses Promises) with newer reactive designs (which use Observables). In practice, developers often need both — so Angular Promise to Observable conversions are quite common.

Example of converting Observable to Promise in Angular:

import { firstValueFrom } from ‘rxjs’;

const promise = firstValueFrom(myObservable);

Example of converting Promise to Observable:

import { from } from ‘rxjs’;

const observable$ = from(myPromise);Both promise angular examples and angular promise to observable conversions are essential when integrating multiple data sources, especially when you rely on both synchronous and streaming responses.

Let me demonstrate you with a simple example.

// ✅ Observable -> Promise
const source$ = rxjs.interval(2000).pipe(rxjs.operators.take(10));
// toPromise is deprecated in favor of firstValueFrom and lastValueFrom
// outputs 9
const result = await source$.toPromise();

// ✅ Promise -> Observable
const promise = Promise.resolve(10);
rxjs.from(promise)
  // outputs 10
  .subscribe(item => console.log(item));

6. Summary: Angular Promises vs Observables

Let’s quickly summarize the Angular Promises vs Observables differences:

Whether you’re exploring Angular Observable vs Promise, Observable and Promise in Angular, or Promise vs Observable in Angular, the takeaway is that each has its own purpose.

7. When to Use Promise vs Observable in Angular

If your task emits a single value and then completes — for example, making a one-time HTTP request — a Promise is simple and effective.

However, if you’re dealing with multiple asynchronous values over time, such as live updates or streams, Observables are far superior.

This decision becomes crucial in modern Angular development. Many developers learning Promises and Observables in Angular start with Promises for their simplicity and then move toward Observables as their apps grow more reactive.

Conclusion

I hope you clearly understand the technical difference between Angular Promise and Observable. It entirely depends on your business use case on when to use one or the other.

But they are going to play a vast role in solving the async paradigm in JavaScript. Moreover, observables are cost-effective as the Browser does not natively support them.

If you’re planning to develop a robust web application using the Angular framework, we are a renowned AngularJS development company. We have a team of Angular developers who are proficient in providing the best possible web app solutions for your business needs.