Difference Between Promise and Observable Subscription

Jul 17, 2021 / Updated on Jul 17, 2021

Anyone who looks Promise and Observable at first it might looks like they both solve the same purpose i.e.they both used for asynchronous functionality, and immediately question arises what is the difference? which one is best? Which one I should use?

Promise

  • Promise is simple and strightforward to use compare to observable
  • It is a better option when you want to do something asynchrously for one time
  • Code looks cleaner with async/await

Observable

  • It is slightly complex to use compare with Promise however it is much more powerful than Promise
  • It can be used for getting multiple responses i.e. unlike Promise it keeps the subscription alive and keep getting response whenever something available

So which one I use?

It depends on the functionality and your preference.

Promise is better in situations like getting onetime response e.g. HTTP call

e.g. angular http client respond a obserable

httpClient.get('https://example.com/api/users').subscribe(res => {
  // do you logic here with res
})
// here you won't get the response.

here for whatever reason if you want process the response sequentially then it is better to use the promise as follows

const res = await httpClient.get('https://example.com/api/users/').toPromise()
// do whatever you want here with res

In this example if you use Observable you may fall into callback hell kind of situation, so Promise is a better option.

On the other hand Obervable is better choice when you want keep sending responses e.g. user login, logout events etc

e.g.

Consider you have Auth class which takes care of your login and logout process.

class Auth {
  public static authEvents$ = new Subject()
  
  login() {
    // do login process here and then publish a event
    Auth.authEvents$.next('login')
  }

  login() {
    // do logout process here and then publish a event
    Auth.authEvents$.next('logout')
  }}
}

You can use this in any component as

Auth.authEvents$.subscribe(event => {
  if (event === 'login') {
    // do something
  }
  if (event === 'logout') {
    // do something
  }
})
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