What is .subscribe in Angular?

I'm going through angular-tour-of-heroes app, and I encountered .subscribe method in routing. Can someone explain what's going on here?

Link for the app: https://embed.plnkr.co/?show=preview

Inside hero-detail.component.ts file,

ngOnInit(): void {
  this.route.paramMap
    .switchMap((params: ParamMap) => this.heroService.getHero(+params.get('id')))
    .subscribe(hero => this.hero = hero);
}

.subscribe is not an Angular2 thing.

It's a method that comes from rxjs library which Angular is using internally.

If you can imagine yourself subscribing to a newsletter, every time there is a new newsletter, they will send it to your home (the method inside subscribe gets called).

That's what happens when you subscribing to a source of magazines ( which is called an Observable in rxjs library)

All the AJAX calls in Angular are using rxjs internally and in order to use any of them, you've got to use the method name, e.g get, and then call subscribe on it, because get returns and Observable.

Also, when writing this code <button (click)="doSomething()">, Angular is using Observables internally and subscribes you to that source of event, which in this case is a click event.

Back to our analogy of Observables and newsletter stores, after you've subscribed, as soon as and as long as there is a new magazine, they'll send it to you unless you go and unsubscribe from them for which you have to remember the subscription number or id, which in rxjs case it would be like :

 let subscription = magazineStore.getMagazines().subscribe(
   (newMagazine)=>{

         console.log('newMagazine',newMagazine);

    }); 

And when you don't want to get the magazines anymore:

   subscription.unsubscribe();

Also, the same goes for

 this.route.paramMap

which is returning an Observable and then you're subscribing to it.

My personal view is rxjs was one of the greatest things that were brought to JavaScript world and it's even better in Angular.

There are 150~ rxjs methods ( very similar to lodash methods) and the one that you're using is called switchMap

In Angular (currently on Angular-6) .subscribe() is a method on the Observable type. The Observable type is a utility that asynchronously or synchronously streams data to a variety of components or services that have subscribed to the observable.

The observable is an implementation/abstraction over the promise chain and will be a part of ES7 as a proposed and very supported feature. In Angular it is used internally due to rxjs being a development dependency.

An observable itself can be thought of as a stream of data coming from a source, in Angular this source is an API-endpoint, a service, a database or another observable. But the power it has is that it's not expecting a single response. It can have one or many values that are returned.

Link to rxjs for observable/subscribe docs here: https://rxjs-dev.firebaseapp.com/api/index/class/Observable#subscribe-

Subscribe takes 3 methods as parameters each are functions:

• next: For each item being emitted by the observable perform this function
• error: If somewhere in the stream an error is found, do this method
• complete: Once all items are complete from the stream, do this method

Within each of these, there is the potentional to pipe (or chain) other utilities called operators onto the results to change the form or perform some layered logic.

In the simple example above:

.subscribe(hero => this.hero = hero); basically says on this observable take the hero being emitted and set it to this.hero.

Adding this answer to give more context to Observables based off the documentation and my understanding.

A Subscription is an object that represents a disposable resource, usually the execution of an Observable. A Subscription has one important method, unsubscribe, that takes no argument and just disposes of the resource held by the subscription.

import { interval } from 'rxjs';

const observable = interval(1000);
const subscription = observable.subscribe(a=> console.log(a));

/** This cancels the ongoing Observable execution which
was started by calling subscribe with an Observer.*/

subscription.unsubscribe();

A Subscription essentially just has an unsubscribe() function to release resources or cancel Observable executions.

import { interval } from 'rxjs';

const observable1 = interval(400);
const observable2 = interval(300);

const subscription = observable1.subscribe(x => console.log('first: ' + x));
const childSubscription = observable2.subscribe(x => console.log('second: ' + x));

subscription.add(childSubscription);

setTimeout(() => {
// It unsubscribes BOTH subscription and childSubscription
subscription.unsubscribe();
}, 1000);

According to the official documentation, Angular should unsubscribe for you, but apparently, there is a bug.