Editing JavaScript/JScript (section)

====Observables and Observers====
The observable (e.g. see [https://angular.io/guide/observables here] type can be used to model push-based data sources such as events, timer intervals and sockets. The '''Observer''' pattern is a design pattern in which '''Subjects''' maintain a list of '''Observers''' (consumers) and notifies them of state changes. It is heavily used in [[Angular|Angular]]. The biggest difference between observables and [[JavaScript/JScript#Promises|promises]] is that promises won't change their value once they have been fulfilled. They can only emit (reject, resolve) a single value. On the other hand, observables can emit multiple results. The subscriber will be receiving results until the observer is completed or unsubscribed from.
A handler for receiving observable notifications implements the '''Observer''' interface. It is an object that defines callback methods to handle the three types of notifications that an observable can send:
 Observer = {
   next: ... // required
   error: ... // optional
   complete: ... // optional 


* An observable is said to be '''cold''' because it does not generate new values if no subscriptions exist
* '''$''' suffix (popularized by Cycle.js) is used to indicate that the variable is an Observable.
* '''Creation Operators''' are the other kind of operator, which can be called as standalone functions to create a new Observable.

 // create an observable
 timer$ = new Observable<string>(                      // new observable
   observer => {                                       // '''subscriber function''' is executed when a consumer calls subscribe() and desribes how to generate values
     setInterval(
       () => observer.next(new Date().toString()),
       10000);
     console.log("someone subscribed");
   }
 );
 
 var subscription: Subscription = timer$.subscribe({   // passing an '''observer''' which is an objects that defines the handler for the values it receives
   next(datestring) {                                  // implements the Observer interface with callback methods for the 3 types of notifications 
     console.log(datestring);                          // next, error and complete
   }
 });