The parameter failed to initialize causing the HTTP service to fire prematurely

Question

The parameter failed to initialize causing the HTTP service to fire prematurely

In my project, I am utilizing Angular 10. Within the ngOnInit function, I have nested HTTP calls structured like this:

ngOnInit(): void {
  
  let communetyid;
  
    this.route.data.subscribe(data => { 
      this.route.params.subscribe(params => { 
        
      if(data.category === "code")
      {
        this.dataService.getCode(params.code)
          .subscribe(resp => { communetyid = resp.results.communityId });
      }
      else
      {
        communetyid = params.id
      }

      this.dataService.getCommunityById(communetyid)
        .subscribe(response => { this.community = response.results; 
        
            ///other http calls
        
       }) 
     })
   })
})

The issue arises where dataService.getCommunityById is executed before the value of communetyid is set due to asynchronous operations, resulting in an error message in the console.

Is there a way to modify the code so that when dataService.getCommunityById is called, the value of communetyid is initialized?

I understand that one approach would be to include dataService.getCommunityById within the subscribe method of dataService.getCode, but I am looking for a solution that avoids duplicating code.

angular typescript asynchronous rxjs

Answer 1

Answer №1

To maintain the original pattern of nesting subscriptions, simply relocate the "getCommunityById" HTTP call within the callback of "dataService.getCode." The crucial point to grasp here is that "dataService.getCode" will provide an observable (data not yet available), while your else statement functions with a value that already exists (params.id). The correct approach to resolving this issue involves utilizing concatMap() within a pipe() and chaining these operations together. Within the concatMap() callback, manage your if-else branch logic by returning either this.dataService.getCode or Observable.of(params.id). Subsequently, in the following concatMap(), use this value as an argument for this.dataService.getCommunityById.

Answer 2

To maintain the original pattern of nesting subscriptions, simply relocate the "getCommunityById" HTTP call within the callback of "dataService.getCode." The crucial point to grasp here is that "dataService.getCode" will provide an observable (data not yet available), while your else statement functions with a value that already exists (params.id). The correct approach to resolving this issue involves utilizing concatMap() within a pipe() and chaining these operations together. Within the concatMap() callback, manage your if-else branch logic by returning either this.dataService.getCode or Observable.of(params.id). Subsequently, in the following concatMap(), use this value as an argument for this.dataService.getCommunityById.

Answer 3

Answer №2

Consider enhancing your Observables by using advanced operators

export class ChildComponent implements OnInit {
  constructor(
    private route: ActivatedRoute,
    private dataService: DataService
  ) {}

  // Extracting Parameters from the Activated Route
  params$ = this.route.paramMap.pipe(
    map(params => ({
      id: params.get("id"),
      code: params.get("code"),
      category: params.get("category")
    }))
  );

  // Retrieve Community Id
  communityId$ = this.params$.pipe(
    mergeMap(({ category, code, id }) =>
      category === "code"
        ? this.dataService
            .getCode(code)
            .pipe(map(({ results }) => results.communityId))
        : of(id)
    )
  );

  // Get Community Information
  community$ = this.communityId$.pipe(
    switchMap((communetyId) => this.dataService.getCommunityById(communetyId))
  )

  ngOnInit() {
    this.community$.subscribe({
      next: console.log
    });
  }
}

Explanation of the Code

Parameter Extraction

  params$ = this.route.paramMap.pipe(
    map(params => ({
      id: params.get("id"),
      code: params.get("code"),
      category: params.get("category")
    }))
  );

The paramMap method is used to extract parameters from the ActivatedRoute. According to Angular Docs, the params method is either deprecated or will be deprecated in the future.

ActivatedRoute contains two properties that are less capable than their replacements and may be deprecated in a future Angular version.

In the next step, we define a property called communityId, which depends on mapping the params while checking the value of the category property. The mergeMap operator is used here to combine the subscriptions of params$ and communityId$.

Lastly, we obtain information about the community with the use of switchMap. This operator ensures that we do not continue with an initial request if a new request has been made, making it the most suitable choice for this scenario.

View a demo example here

Answer 4

Consider enhancing your Observables by using advanced operators

export class ChildComponent implements OnInit {
  constructor(
    private route: ActivatedRoute,
    private dataService: DataService
  ) {}

  // Extracting Parameters from the Activated Route
  params$ = this.route.paramMap.pipe(
    map(params => ({
      id: params.get("id"),
      code: params.get("code"),
      category: params.get("category")
    }))
  );

  // Retrieve Community Id
  communityId$ = this.params$.pipe(
    mergeMap(({ category, code, id }) =>
      category === "code"
        ? this.dataService
            .getCode(code)
            .pipe(map(({ results }) => results.communityId))
        : of(id)
    )
  );

  // Get Community Information
  community$ = this.communityId$.pipe(
    switchMap((communetyId) => this.dataService.getCommunityById(communetyId))
  )

  ngOnInit() {
    this.community$.subscribe({
      next: console.log
    });
  }
}

Explanation of the Code

Parameter Extraction

  params$ = this.route.paramMap.pipe(
    map(params => ({
      id: params.get("id"),
      code: params.get("code"),
      category: params.get("category")
    }))
  );

The paramMap method is used to extract parameters from the ActivatedRoute. According to Angular Docs, the params method is either deprecated or will be deprecated in the future.

ActivatedRoute contains two properties that are less capable than their replacements and may be deprecated in a future Angular version.

In the next step, we define a property called communityId, which depends on mapping the params while checking the value of the category property. The mergeMap operator is used here to combine the subscriptions of params$ and communityId$.

Lastly, we obtain information about the community with the use of switchMap. This operator ensures that we do not continue with an initial request if a new request has been made, making it the most suitable choice for this scenario.

View a demo example here

Answer 5

Answer №3

Revise your code as shown below:-

ngOnInit(): void  {
  
  let communityId;
  
    this.route.data.subscribe(data => { 
      this.route.params.subscribe(params => { 
        
      const communityIdObs = date.category === 'code' ? this.dataService.getCode(params.code).pipe(map(resp => resp.results.communityId)) : of(params.id);
      communityIdObs.pipe(mergeMap(res => {
         return this.dataService.getCommunityById(communityId);
      }).subscribe(response => { 
            this.community = response.results; 
            ///other http calls
      });
})

The merger operator ensures that your calls are executed in sequence and I have also made the code shorter.

Answer 6

Revise your code as shown below:-

ngOnInit(): void  {
  
  let communityId;
  
    this.route.data.subscribe(data => { 
      this.route.params.subscribe(params => { 
        
      const communityIdObs = date.category === 'code' ? this.dataService.getCode(params.code).pipe(map(resp => resp.results.communityId)) : of(params.id);
      communityIdObs.pipe(mergeMap(res => {
         return this.dataService.getCommunityById(communityId);
      }).subscribe(response => { 
            this.community = response.results; 
            ///other http calls
      });
})

The merger operator ensures that your calls are executed in sequence and I have also made the code shorter.

Answer 7

Answer №4

If you're looking to streamline your code, consider leveraging the concatMap Operator.

The concatMap Operator is perfect for transforming one response into parameters needed for the next request in line.

When using the concatMap() Operator, it internally subscribes to the Observable returned from its projection function and patiently waits for it to complete before re-emitting all of its values.

Check out this practical example below:

function simulateHTTPRequest(url) {
    return Observable.of(`Response from ${url}`)
      .delay(1000);
}

let urls = ['url-1', 'url-2', 'url-3', 'url-4'];
let startTime = (new Date()).getTime();

Observable.from(urls)
  .concatMap(url => simulateHTTPRequest(url))
  .timestamp()
  .map(timestampData => [timestampData.timestamp - startTime, timestampData.value])
  .subscribe(data => console.log(data));

Answer 8

If you're looking to streamline your code, consider leveraging the concatMap Operator.

The concatMap Operator is perfect for transforming one response into parameters needed for the next request in line.

When using the concatMap() Operator, it internally subscribes to the Observable returned from its projection function and patiently waits for it to complete before re-emitting all of its values.

Check out this practical example below:

function simulateHTTPRequest(url) {
    return Observable.of(`Response from ${url}`)
      .delay(1000);
}

let urls = ['url-1', 'url-2', 'url-3', 'url-4'];
let startTime = (new Date()).getTime();

Observable.from(urls)
  .concatMap(url => simulateHTTPRequest(url))
  .timestamp()
  .map(timestampData => [timestampData.timestamp - startTime, timestampData.value])
  .subscribe(data => console.log(data));

Answer 9

Answer №5

Although rxjs can be used for chaining observables, there is a simpler approach to resolve the issue at hand. By moving anything inside the subscription of an observable outside as a separate function, you can prevent unnecessary calls to your dataService.getCommunityById function.

ngOnInit(): void  {

let communityId;

this.route.data.subscribe(data => { 
  this.route.params.subscribe(params => { 
    
  if(data.category === "code")
  {
    this.dataService.getCode(params.code)
      .subscribe((resp) => 
      { 
          communityId = resp.results.communityId;
          getCommunityData(communityId);
    });
  }
  else  
  {
    communityId = params.id;
    getCommunityData(communityId);
  }

  })
 })  
})

getCommunityData(communityId) {
  this.dataService.getCommunityById(communityId)
  .subscribe(response => { 
     this.community = response.results;
     //other http calls
  })
}

This method offers an alternative to using rxjs and is easier to comprehend. Additionally, rxjs operators like concatMap and of can also be utilized, as suggested by Medhat Mahmoud and Muhammet Can TONBUL.

Answer 10

Although rxjs can be used for chaining observables, there is a simpler approach to resolve the issue at hand. By moving anything inside the subscription of an observable outside as a separate function, you can prevent unnecessary calls to your dataService.getCommunityById function.

ngOnInit(): void  {

let communityId;

this.route.data.subscribe(data => { 
  this.route.params.subscribe(params => { 
    
  if(data.category === "code")
  {
    this.dataService.getCode(params.code)
      .subscribe((resp) => 
      { 
          communityId = resp.results.communityId;
          getCommunityData(communityId);
    });
  }
  else  
  {
    communityId = params.id;
    getCommunityData(communityId);
  }

  })
 })  
})

getCommunityData(communityId) {
  this.dataService.getCommunityById(communityId)
  .subscribe(response => { 
     this.community = response.results;
     //other http calls
  })
}

This method offers an alternative to using rxjs and is easier to comprehend. Additionally, rxjs operators like concatMap and of can also be utilized, as suggested by Medhat Mahmoud and Muhammet Can TONBUL.

The parameter failed to initialize causing the HTTP service to fire prematurely

Answer №1

Answer №2

Explanation of the Code

Parameter Extraction

Answer №3

Answer №4

Answer №5

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