What are the consequences of not subscribing to an HttpClient request that returns observables in an Angular application?

Question

What are the consequences of not subscribing to an HttpClient request that returns observables in an Angular application?

Exploring Angular and TypeScript, I am currently delving into the concepts of HttpClient, observables, and subscribe.

When I include the following code in a component function:

console.log(this.http.get('assets/user.json'));

I receive an object but do not see any network request to

https://localhost:4200/assets/user.json

in the debugger's network tab. However, if I modify the code to:

this.http.get('assets/userDetail.json').subscribe(data => this.user = { name: data['name'] });

I can observe the network request being made to the specified URL. Why does this occur? My initial assumption was that

this.http.get('assets/userDetail.json')

would trigger the request even without explicitly subscribing to the response data stream.

angular typescript rxjs observable angular-httpclient

Answer 1

Answer №1

Understanding the concept involves differentiating between warm and chilly observables - chilly ones require subscription to be triggered, while warm ones fire regardless of being subscribed to.

The Angular HttpClient exemplifies a chilly Observable, as it remains inactive until someone subscribes to it. To identify whether an observable is warm or chilly, refer to the respective documentation, such as HttpClient.post:

Produces an Observable which, upon subscription, triggers the configured POST request on the server.

An instance of a warm observable in Angular is for instance ActivatedRoute.params - how to use - where no explicit subscription is present.

The distinction between warm and chilly observables goes beyond mere subscription requirements:

When subscribing, it's advisable to unsubscribe in certain cases to prevent memory leaks, with Angular offering the async pipe for automated management; read more about this in Don't forget to unsubscribe (Angular specific).
A comprehensive article by Ben Lesh delves into the topic from a broader perspective: Hot vs Cold Observables (not limited to Angular).

Answer 2

Understanding the concept involves differentiating between warm and chilly observables - chilly ones require subscription to be triggered, while warm ones fire regardless of being subscribed to.

The Angular HttpClient exemplifies a chilly Observable, as it remains inactive until someone subscribes to it. To identify whether an observable is warm or chilly, refer to the respective documentation, such as HttpClient.post:

Produces an Observable which, upon subscription, triggers the configured POST request on the server.

An instance of a warm observable in Angular is for instance ActivatedRoute.params - how to use - where no explicit subscription is present.

The distinction between warm and chilly observables goes beyond mere subscription requirements:

When subscribing, it's advisable to unsubscribe in certain cases to prevent memory leaks, with Angular offering the async pipe for automated management; read more about this in Don't forget to unsubscribe (Angular specific).
A comprehensive article by Ben Lesh delves into the topic from a broader perspective: Hot vs Cold Observables (not limited to Angular).

Answer 3

Answer №2

It is important to note that when implementing code like this with a cold observable:

(pseudo-code)

Component:

ngOnInit(): void {
    console.log('Before, in ngInit')
    usersService.usersGet();
    console.log('After, in ngInit')
}

Service:

public usersGet(){
   console.log("I'm in service");
   let data = this.http.get('assets/user.json')
   console.log("I'm already here")
   return data;
}

You will notice the following output in the browser developer tools:

However, your backend endpoint will not receive any request.

Your backend will only receive the request when you update the code to:

ngOnInit(): void {
    console.log('Before, in ngInit')
    usersService.usersGet().subscribe( x=> {
        console.log("I'm inside of subscribe")
        console.log(x)
    });
    console.log('After, in ngInit')
}

The new output will be:

Answer 4

It is important to note that when implementing code like this with a cold observable:

(pseudo-code)

Component:

ngOnInit(): void {
    console.log('Before, in ngInit')
    usersService.usersGet();
    console.log('After, in ngInit')
}

Service:

public usersGet(){
   console.log("I'm in service");
   let data = this.http.get('assets/user.json')
   console.log("I'm already here")
   return data;
}

You will notice the following output in the browser developer tools:

However, your backend endpoint will not receive any request.

Your backend will only receive the request when you update the code to:

ngOnInit(): void {
    console.log('Before, in ngInit')
    usersService.usersGet().subscribe( x=> {
        console.log("I'm inside of subscribe")
        console.log(x)
    });
    console.log('After, in ngInit')
}

The new output will be:

Answer 5

Answer №3

The reason why it won't function without any subscribers was already clarified in the earlier response. As a cold observer, it will only be triggered once it detects at least one subscriber.

As an alternative, you can simply use:

 this.http.get('assets/user.json').subscribe();

Answer 6

The reason why it won't function without any subscribers was already clarified in the earlier response. As a cold observer, it will only be triggered once it detects at least one subscriber.

As an alternative, you can simply use:

 this.http.get('assets/user.json').subscribe();

Answer 7

Answer №4

Summary: This snippet of code showcases the creation of a webapi service using injectable that is visible throughout the application. By subscribing to the observable returned by http.get from the calling function, the result of IView is bound to the data list in a manner similar to a promise. As a result, the WebApiService can now be injected into other components' constructors.

myComponent.component.ts

 public data: IDataView[];

Calling the observable: this.app.getMyViews('123').subscribe(result=>{this.data=result;});

WebApi.service.ts

@Injectable({ providedIn: 'root' })

export class WebApiService
{
   constructor(private http: HttpClient,private env: EnvironmentUrlService) { }

getMyViews(id)
    {
        var path = this.env.urlAddress + '/my_class/GetData/'+id;
        return this.http.get<IDataView[]>(path);

  }

}

Answer 8

Summary: This snippet of code showcases the creation of a webapi service using injectable that is visible throughout the application. By subscribing to the observable returned by http.get from the calling function, the result of IView is bound to the data list in a manner similar to a promise. As a result, the WebApiService can now be injected into other components' constructors.

myComponent.component.ts

 public data: IDataView[];

Calling the observable: this.app.getMyViews('123').subscribe(result=>{this.data=result;});

WebApi.service.ts

@Injectable({ providedIn: 'root' })

export class WebApiService
{
   constructor(private http: HttpClient,private env: EnvironmentUrlService) { }

getMyViews(id)
    {
        var path = this.env.urlAddress + '/my_class/GetData/'+id;
        return this.http.get<IDataView[]>(path);

  }

}

What are the consequences of not subscribing to an HttpClient request that returns observables in an Angular application?

Answer №1

Answer №2

Answer №3

Answer №4

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