Understanding the status of HTTP requests or observing the updates of observables in Angular2/Typescript is essential

Question

Understanding the status of HTTP requests or observing the updates of observables in Angular2/Typescript is essential

I've been working on an Angular2 and Typescript application where I'm utilizing Angular2's HTTP methods to retrieve data from a database within a service. The service is triggered inside a component's onInit() function and I'm able to successfully load the data. However, I'm facing an issue where I want to also use this loaded data within the onInit() function itself. Whenever I attempt to do this, I encounter an error similar to the one below:

Error: Uncaught (in promise): TypeError: Cannot read property 'user_id' of undefined
TypeError: Cannot read property 'user_id' of undefined

Below is a snippet of the component calling the service:

export class ProfileComponent implements OnInit {

public profile: StaffProfile[];

constructor(private userService: UserService) {}

ngOnInit() {
    this.userService.fetchProfile();
    this.profile = this.userService.getProfile();
    // I want to perform operations once the data is loaded
    console.log(this.profile[0].user_id);
}
}

Below is a snippet of the service:

@Injectable()
export class WorkforceUserService implements OnInit {

private Profile: Profile[];

constructor(private http: Http) {
    this.Profile = [];
}

public getProfile(){
    return this.Profile;
}

public fetchStaffProfile(){
return this.http.get('http://localhost:3000/api/staff/1')
  .map((response: Response) => response.json())
  .subscribe(
    (data) => {
      var user_id = data.user_id || null;

      var loadedProfile = new Profile(user_id);

      this.Profile.push(loadedProfile);
    }
  );
}
}

I'm looking for a way to trigger a function in my component once the data has been retrieved from the server or has been updated. Any insights on how I can achieve this would be greatly appreciated.

Thank you in advance.

javascript angular typescript angular2-observables

Answer 1

Answer №1

When sync & async worlds collide, a classic scenario unfolds. (TL;DR - See my proposed solutions below)

Expected flow of ngOnInit() execution:

1. (Component) Request service to fetch profile  
2. (Service) Fetch profile  
3. (Service) Extract user_id from received profile and create new profile  
4. (Service) Add profile to this.Profile  
5. (Component) Set this.profile as service's Profile  
6. (Component) Display first entry of profile fetched and configured in service

Actual flow observed:

1 => 2 => 5 => 6 (fails, hypothetically) => 4 => 5

In the synchronous world:

The fetch method runs and returns a Subscription for the http call. At this point, the fetch method is completed. Subsequently, ngOnInit proceeds with
```
this.profile = this.userService.getProfile();
```

In the asynchronous world:

The http request is initiated, and will populate this.Profile at some point in the future.

However, before this occurs, ngOnInit attempts to access the user_id property of an undefined initial element.

In such scenarios, staying in the asynchronous realm is crucial, and rxjs offers a robust and well-documented toolkit for handling such situations.

Here are my suggestions:

Naive solution - Instead of returning a subscription, modify the fetch method to return a Promise resolved in ngOnInit.

// WorkforceUserService

public fetchStaffProfile() {
    return this.http.get('http://localhost:3000/api/staff/1')
        .map((response: Response) => response.json())
        .toPromise()
        .then((data) => {
            var user_id = data.user_id || null;
            var loadedProfile = new Profile(user_id);
            this.Profile.push(loadedProfile);
        });
       // Note: Handle promise errors appropriately
}

// ProfileComponent

ngOnInit() {
    this.userService.fetchProfile().then(() => { 
        this.profile = this.userService.getProfile();
        console.log(this.profile[0].user_id);
    });
            
}

Rxjs style solution - Maintain a Subject of type profile array, which the component can subscribe to:

// WorkforceUserService

this.Profile = new Subject<Profile[]>(); // Keep it private and avoid direct subscriptions
this.Profile$ = this.Profile.asObservable(); // Expose an observable for subscribers

public fetchStaffProfile(){
    return this.http.get('http://localhost:3000/api/staff/1')
        .map((response: Response) => response.json())
        .subscribe(
            (data) => {
                var user_id = data.user_id || null;
                var loadedProfile = new Profile(user_id);
                this.Profile.next([loadedProfile]);
    });
}

// ProfileComponent

export class ProfileComponent implements OnInit {

    public profile: StaffProfile[];

    constructor(private userService: UserService) {
        this.profile = this.userService.getProfile();
        console.log(this.profile[0].user_id);  
    }

    ngOnInit() {
        this.userService.fetchProfile();
    }
}

Some additional pointers to consider:

this.Promise should be this.promise for better adherence to naming conventions in javascript.
Replace var with let or const instead of the oldschool var. Review Angular styleguide

For further insights on using observables, refer to this article which provides detailed examples and explanations.

Answer 2

When sync & async worlds collide, a classic scenario unfolds. (TL;DR - See my proposed solutions below)

Expected flow of ngOnInit() execution:

1. (Component) Request service to fetch profile  
2. (Service) Fetch profile  
3. (Service) Extract user_id from received profile and create new profile  
4. (Service) Add profile to this.Profile  
5. (Component) Set this.profile as service's Profile  
6. (Component) Display first entry of profile fetched and configured in service

Actual flow observed:

1 => 2 => 5 => 6 (fails, hypothetically) => 4 => 5

In the synchronous world:

The fetch method runs and returns a Subscription for the http call. At this point, the fetch method is completed. Subsequently, ngOnInit proceeds with
```
this.profile = this.userService.getProfile();
```

In the asynchronous world:

The http request is initiated, and will populate this.Profile at some point in the future.

However, before this occurs, ngOnInit attempts to access the user_id property of an undefined initial element.

In such scenarios, staying in the asynchronous realm is crucial, and rxjs offers a robust and well-documented toolkit for handling such situations.

Here are my suggestions:

Naive solution - Instead of returning a subscription, modify the fetch method to return a Promise resolved in ngOnInit.

// WorkforceUserService

public fetchStaffProfile() {
    return this.http.get('http://localhost:3000/api/staff/1')
        .map((response: Response) => response.json())
        .toPromise()
        .then((data) => {
            var user_id = data.user_id || null;
            var loadedProfile = new Profile(user_id);
            this.Profile.push(loadedProfile);
        });
       // Note: Handle promise errors appropriately
}

// ProfileComponent

ngOnInit() {
    this.userService.fetchProfile().then(() => { 
        this.profile = this.userService.getProfile();
        console.log(this.profile[0].user_id);
    });
            
}

Rxjs style solution - Maintain a Subject of type profile array, which the component can subscribe to:

// WorkforceUserService

this.Profile = new Subject<Profile[]>(); // Keep it private and avoid direct subscriptions
this.Profile$ = this.Profile.asObservable(); // Expose an observable for subscribers

public fetchStaffProfile(){
    return this.http.get('http://localhost:3000/api/staff/1')
        .map((response: Response) => response.json())
        .subscribe(
            (data) => {
                var user_id = data.user_id || null;
                var loadedProfile = new Profile(user_id);
                this.Profile.next([loadedProfile]);
    });
}

// ProfileComponent

export class ProfileComponent implements OnInit {

    public profile: StaffProfile[];

    constructor(private userService: UserService) {
        this.profile = this.userService.getProfile();
        console.log(this.profile[0].user_id);  
    }

    ngOnInit() {
        this.userService.fetchProfile();
    }
}

Some additional pointers to consider:

this.Promise should be this.promise for better adherence to naming conventions in javascript.
Replace var with let or const instead of the oldschool var. Review Angular styleguide

For further insights on using observables, refer to this article which provides detailed examples and explanations.

Answer 3

Answer №2

To receive updates, simply subscribe to the outcome as shown below:

ngOnInit() {
    this.userService.fetchProfile().subscribe(() => {
      this.profile = this.userService.getProfile();
      console.log(this.profile[0].user_id);
   });
}

Answer 4

To receive updates, simply subscribe to the outcome as shown below:

ngOnInit() {
    this.userService.fetchProfile().subscribe(() => {
      this.profile = this.userService.getProfile();
      console.log(this.profile[0].user_id);
   });
}

Answer 5

Answer №3

When you invoke fetchStaffProfile as an async process and immediately call getProfile, the returned value is empty. Instead, consider changing it so that fetch returns an observable/promise. Then, when you invoke it, subscribe to it.

@Injectable()
export class WorkforceUserService {

constructor(private http: Http) {
}

public fetchStaffProfile(){
return this.http.get('http://localhost:3000/api/staff/1')
  .map((response: Response) => response.json());
}
}

For example, in a component:

export class ProfileComponent implements OnInit {

public profile: StaffProfile;

constructor(private userService: UserService) {}

ngOnInit() {
    this.userService.fetchStaffProfile()
      .subscribe(res => { 
        // do some data transformation
       this.profile = res; 
       console.log(this.profile);
     }
  }
}

Answer 6

When you invoke fetchStaffProfile as an async process and immediately call getProfile, the returned value is empty. Instead, consider changing it so that fetch returns an observable/promise. Then, when you invoke it, subscribe to it.

@Injectable()
export class WorkforceUserService {

constructor(private http: Http) {
}

public fetchStaffProfile(){
return this.http.get('http://localhost:3000/api/staff/1')
  .map((response: Response) => response.json());
}
}

For example, in a component:

export class ProfileComponent implements OnInit {

public profile: StaffProfile;

constructor(private userService: UserService) {}

ngOnInit() {
    this.userService.fetchStaffProfile()
      .subscribe(res => { 
        // do some data transformation
       this.profile = res; 
       console.log(this.profile);
     }
  }
}

Understanding the status of HTTP requests or observing the updates of observables in Angular2/Typescript is essential

Answer №1

Here are my suggestions:

Answer №2

Answer №3

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