what different between observable and promise angular 9?

In Angular 9 (and subsequent versions), both observables and promises are used for handling asynchronous operations, but they have different characteristics and functionalities.

Observables:

1. Streams of Data: Observables represent a stream of data over time. They can emit multiple values asynchronously.
2. Lazy Execution: They are lazy, meaning the data is only emitted when something subscribes to the observable.
3. Cancellation and Disposal: Observables support cancellation using the unsubscribe() method. This helps in managing resources and preventing memory leaks.
4. Operators: They offer a wide range of operators (like map, filter, etc.) that can be used to transform, combine, or manipulate the emitted data streams.
5. Supports Multiple Values: They can emit multiple values over time, making them suitable for handling events, user input, and continuous data streams.

Promises:

1. Single Value: Promises represent a single asynchronous operation that will eventually be resolved with a value or rejected with an error.
2. Eager Execution: They are eager, meaning the asynchronous operation starts as soon as the promise is created.
3. No Cancellation: Once a promise is initiated, it cannot be canceled. This is one of the key differences from observables.
4. Simpler API: Promises have a simpler API compared to observables, with then() for handling a successful response and catch() for handling errors.
5. Not Composable: They don't have built-in operators like observables, so chaining and manipulating operations isn’t as straightforward.

Summary:

• Observables are streams that can emit multiple values over time, are lazy, support cancellation, have a variety of operators for data transformation, and are suitable for handling continuous data.
• Promises represent the result of a single asynchronous operation, are eager, cannot be canceled, have a simpler API, and are suitable for handling a single value asynchronously.

In Angular, both observables and promises are used in different scenarios. Observables are extensively used with Angular's HttpClient for handling HTTP requests, managing event streams, and handling other asynchronous tasks. Promises are used when dealing with APIs that return promises or when integrating with existing promise-based libraries. However, Angular generally promotes the use of observables due to their flexibility and additional capabilities.

In Angular 9 (and in JavaScript/TypeScript in general), Observables and Promises are both used for handling asynchronous operations, but they have some differences in their usage and behavior.

Promises:

1. Single Value: Promises represent a single value that will be resolved or rejected in the future.
2. Eager Execution: When a Promise is created, it starts executing immediately.
3. Once Resolved: A Promise resolves only once with a single value or gets rejected with an error.
4. No Cancellation: Once a Promise is created, it cannot be canceled.
5. Methods: Promises have methods like .then() and .catch() to handle successful resolution or errors respectively.
6. Non-Cancellable: Once a Promise is settled (resolved or rejected), its state cannot be changed.

Example of a Promise in JavaScript:  

const myPromise = new Promise((resolve, reject) => 
{ 
// Perform asynchronous operation 
setTimeout(() => 
{ 
// Simulate a successful result 
resolve('Data fetched successfully!'); 
// Simulate an error 
// reject('Error fetching data!'); }, 2000); }); 
myPromise .then((result) => 
{ console.log(result); 
// 
Output: Data fetched successfully! }) .catch((error) => 
{ 
console.error(error); 
// Output: Error fetching data! });

Observables:

1. Multiple Values: Observables are streams of multiple values over time.
2. Lazy Execution: Observables are lazy; they don’t start executing until they have a subscription.
3. Multiple Emits: They can emit multiple values over time (zero, one, or many).
4. Cancellation: Observables can be cancelled by unsubscribing, helping to prevent memory leaks.
5. Methods: Observables have operators like subscribe() to handle emitted values and other operators to transform, filter, or manipulate the stream of values.
6. Cancelable: Observables can be canceled by unsubscribing from them, preventing further emissions.

Example of an Observable in Angular using RxJS (a popular library for reactive programming):

import { Observable } from 'rxjs'; 
const myObservable = new Observable((observer) => 
{ 
// Perform asynchronous operation 
setTimeout(() => { 
// Emit a value 
observer.next('Data fetched successfully!'); 
// Simulate an error 
observer.error('Error fetching data!'); }, 2000); }); 
const subscription = myObservable.subscribe( (value) => console.log(value), 
// Output: Data fetched successfully! (error) => console.error(error) 
// Output: Error fetching data! ); 
// To unsubscribe (cancel) the Observable s
ubscription.unsubscribe();

In Angular applications, Observables are widely used, especially with RxJS, because they provide powerful features for handling asynchronous data and are extensively used in services, HTTP requests, event handling, and more. Promises are also supported in Angular and can be used, but Observables are often preferred due to their flexibility and additional capabilities for handling streams of data.

Promise

A Promise handles a single event when an async operation completes or fails.

Note: There are Promise libraries out there that support cancellation, but ES6 Promise doesn't so far.

Observable

An Observable is like a Stream (in many languages) and allows to pass zero or more events where the callback is called for each event.

Often Observable is preferred over Promise because it provides the features of Promise and more. With Observable it doesn't matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.

Observable also has the advantage over Promise to be cancellable. If the result of an HTTP request to a server or some other expensive async operation isn't needed anymore, the Subscription of an Observable allows to cancel the subscription, while a Promise will eventually call the success or failed callback even when you don't need the notification or the result it provides anymore.

you can find more from below link

https://www.youtube.com/watch?v=pzu17v2SeSM&t=61s

Promise 

A Promise handles a single event when an async operation completes or fails.Note: There are Promise libraries out there that support cancellation, but ES6 Promise doesn't so far.

Observable

An Observable is like a Stream (in many languages) and allows to pass zero or more events where the callback is called for each event.

Often Observable is preferred over Promise because it provides the features of Promise and more. With Observable it doesn't matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.

Observable also has the advantage over Promise to be cancellable. If the result of an HTTP request to a server or some other expensive async operation isn't needed anymore, the Subscription of an Observable allows to cancel the subscription, while a Promise will eventually call the success or failed callback even when you don't need the notification or the result it provides anymore.

While a Promise starts immediately, an Observable only starts if you subscribe to it. This is why Observables are called lazy.

Let’s see the difference between observable and promise (observable vs promise)

Now let’s see code snippets / examples of a few operations defined by observables and promises. 

Use Promise instead of an Observable, when:

• You need to handle the (future response) event no matter what (no unsubscribe, no cancel: after you subscribe, there will be an answer, 100%, and you will have to handle it, 100%, the code will get executed)
• One Subscription = One Event handling: there will be only one event from the source, so the future response and the completition is the same event.

Use Observable instead of a Promise, when:

• You want to have the ability to accept multiple events from the same source
• You need a "I am finished with this event stream" handler
• You want to be able to unsubscribe from a possibly never ending stream of data, and re-subscribe anytime (meaning also that you might don't really need to fulfill the subscription at all: for instance, if nothing happens in 10 sec, let's unsubscribe, nobody will ever handle the late answer)
• You want to use the RxJS "Stream API" to preprocess the data of your responses.

For more you can below link

https://www.youtube.com/watch?v=pzu17v2SeSM

Promises deal with one asynchronous event at a time, while observables handle a sequence of asynchronous events over a period of time. Emit multiple values over a period of time. Emit a single value at a time. Observables provide the map for forEach, filter, reduce, retry, and retryWhen operators. Promises doen't provide ant operations.

This diagram can give some more insights 

Hi Ahmed,

Below are the basic differences between observable and promises. check the below links for a better understanding.

https://stackoverflow.com/questions/37364973/what-is-the-difference-between-promises-and-observables

https://www.syncfusion.com/blogs/post/angular-promises-versus-observables.aspx

https://dzone.com/articles/what-is-the-difference-between-observable-and-prom