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Below are the advantages i have read in URL: Angular - Promise vs Observable

promise:

  1. returns a single value
  2. not cancelled
  3. more readable code with try/catch and async/await

observable

  1. works with multiple values over time
  2. cancellable
  3. supports map, filter, reduce and similar operators
  4. use Reactive Extensions (RxJS)
  5. an array whose items arrive asynchronously over time

In observable i see 3 & 4 Point as supports operators and RXJS. I just have a basic question like can't i use RXJS and operators with promises? what is the meaning of point 5

Jay
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2 Answers2

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In short, no you can't use those operators (like map, filter) directly on a Promise.

Rxjs does provide an operator toPromise which does convert an Observable to a Promise - if that is your preference.

I think point 5 is actually conflated with point 1. Point 1 is the crux of what Observables are all about: dealing with 0 to n values over time.

You may not think that to be useful if you're used to using Promises simply for Ajax requests - e.g. hit an endpoint and get a value back. But in the case of Observables, you can use them in any context - for example, DOM events.

If you were to create an Observable via listening to a Mouseover event, then you'd be receiving n values over any given length of time - and then you could react to these events.

When thinking in terms of Ajax requests, the classic example is that of the look ahead search input which is detailed in the link of your question.

Rich
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As @Rich mentioned, rxjs Operators are aimed for continues data streams (e.g. take takes the first n next of an Observable). As such, not all operators are useful for Promise-based results.

However, given that some operators are compact/neat even for Promise, you can use the following:

import { from, firstValueFrom, delay } from "rxjs";
...

// converts to Observable and back to Promise
firstValueFrom(from(myPromise).pipe(delay(1000))
r3mark
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