Angular async notes

* Promise vs Observable
  * Promise
    * Single async event that can either complete or fail once.
    * Can be used with async/await keywords.
  * Observable
    * Can emit multiple events async. 
    * Offers a bit more control over the async task, for example cancelling. 
    * Cannot be used with async/await keywords.
  * Think of Observables as a stream of data instead of an async task.
  * Observable is the default used in Angular, but promises are still fine to use. 
  * You can convert an Observable into a promise by calling toPromise.
* Hot vs Cold
  * Just know that Cold observables will not emit events until something has subscribed to them.
  * All Observables returned from httpClient are cold.
* subscriptions vs the async pipe
  * Generally using the async pipe is prefered over managing subscriptions
  * Subscription can introduce memory leaks if not managed properly.
  * If you must use a subscription, unsubscribe from it in ngOnDestroy.
* Piping
  * switchMap
    * "switches" to a new observable
    * Useful when one observable requires input from another
  * map
    * simply maps the data to a new structure.
  * startWith
    * Emits the provided data before anything else in the observable.
    * Useful when your observable represents state, and you need an initial value for the ui.
  * shareReplay
    * "replays" the last x number of events when something subsscribes to the observable.
    * Useful for when you use a single observable in multiple places in your template.
  * tap
    * Runs the provided function for each event, but does not modify the observable.
    * Usefule for debugging.
* Tips & Tricks
  * ngIf - ngIf="user$ | async as user; else guestUser"
* Resources
  * https://rxjs-dev.firebaseapp.com/operator-decision-tree