Fetching data from an API using Observables in Angular

Question

Fetching data from an API using Observables in Angular

I am facing a challenge with an API endpoint that returns an array of strings in JSON format. My goal is to display these contents on a webpage using an Angular Service. Below is the code snippet I have implemented so far (working with Angular 7):

export class FieldService {
  constructor(private httpClient: HttpClient) { }

  fieldTypesURL = 'http://localhost:3000/fields/types';

  public getTypes(): Observable<any[]> {
    return this.httpClient.get<any[]>(this.fieldTypesURL)
    .pipe(map((response: any) => response.json()),
      catchError((error: any) => Observable.throw(error.json().error || 'Server error')));
  }
}

The issue I'm encountering during compilation is as follows:

Type 'Observable<any[]>' is missing the following properties from type 'Promise<string[]>': then, catch, [Symbol.toStringTag], finally

I find it confusing why the error references a Promise when my intention is to utilize an Observable. Any insights on how to resolve this would be greatly appreciated!

angular typescript

Answer 1

Answer №1

While utilizing httpClient, it automatically interprets the response as JSON which means that

.pipe(map((response: any) => response.json())

might be where the issue lies.

In addition, consider changing 'any' to 'string'

Try this out:

public getTypes(): Observable<string[]> {
return this.httpClient.get<string[]>(this.fieldTypesURL)
  .catch((error: any) => Observable.throw(( error && JSON.parse(error).error) || 'Server error')));
}

The .json() method carries out a similar task as illustrated here Angular Response.json() not documented

Answer 2

While utilizing httpClient, it automatically interprets the response as JSON which means that

.pipe(map((response: any) => response.json())

might be where the issue lies.

In addition, consider changing 'any' to 'string'

Try this out:

public getTypes(): Observable<string[]> {
return this.httpClient.get<string[]>(this.fieldTypesURL)
  .catch((error: any) => Observable.throw(( error && JSON.parse(error).error) || 'Server error')));
}

The .json() method carries out a similar task as illustrated here Angular Response.json() not documented

Answer 3

Answer №2

It was mentioned by @hamzakhan that parsing the response as JSON is not necessary. However, if your code is still not functioning, you may want to try a different approach. I suggest modifying Observable to Observable in the return type of the getTypes() function. Hopefully, this adjustment will solve the issue. Best of luck!

export class FieldService {
  constructor(private httpClient: HttpClient) { }

  fieldTypesURL = 'http://localhost:3000/fields/types';

  public getTypes(): Observable<any> {
    return this.httpClient.get(this.fieldTypesURL)
    .pipe(map((response: any) => console.log(response),
      catchError((error: any) => Observable.throw(( error && JSON.parse(error).error) || 'Server error')));
  }
}

Answer 4

It was mentioned by @hamzakhan that parsing the response as JSON is not necessary. However, if your code is still not functioning, you may want to try a different approach. I suggest modifying Observable to Observable in the return type of the getTypes() function. Hopefully, this adjustment will solve the issue. Best of luck!

export class FieldService {
  constructor(private httpClient: HttpClient) { }

  fieldTypesURL = 'http://localhost:3000/fields/types';

  public getTypes(): Observable<any> {
    return this.httpClient.get(this.fieldTypesURL)
    .pipe(map((response: any) => console.log(response),
      catchError((error: any) => Observable.throw(( error && JSON.parse(error).error) || 'Server error')));
  }
}

Answer 5

Answer №3

1) Utilizing the httpClient method with a generic parameter takes care of the json conversion and type casting on your behalf.

2) It's worth noting that Observable#throw is now deprecated, so opt for using the throwError operator instead. Remember to handle errors properly and consider defining a specific type for error handling to enhance overall type safety.

3) Always ensure you are invoking your service method correctly...

// field.service.ts
export class FieldService {

  constructor(private httpClient: HttpClient) { }

  fieldTypesURL = 'http://localhost:3000/fields/types';

  public getTypes(): Observable<string[]> {
    return this.httpClient.get<string[]>(this.fieldTypesURL).pipe(
      catchError((r: HttpErrorResponse) => throwError(r.error || 'Server error'))
    );
  }
}

// your.component.ts
export class YourComponent {

  constructor(private fieldService: FieldService){}

  // ... component logic ...

  public types: string[];
  public types$: Observable<string[]>;

  public callService() {

    // An example of a correct invocation
    this.fieldService.getTypes().subscribe(types => this.types = types)

    // Another valid usage with additional observable pipeline operations
    this.types$ = this.fieldService.getTypes().pipe(
      catchError((err: any) => {
        // Error handling at the component level can be implemented here
        return throwError(err);
      }),
      flatMap(types => types),
      filter(type => !!type),
      map(type => type.trim()),
      toArray()
    );

    // Incorrect calls leading to compiler errors
    this.fieldService.getTypes().then(types => /* whatever */);
    this.fieldService.getTypes().catch(err => /* whatever */);
    let r: Promise<string[]> = this.fieldService.getTypes();
  }
}

Hopefully, this provides some clarity and assistance :-)

Answer 6

1) Utilizing the httpClient method with a generic parameter takes care of the json conversion and type casting on your behalf.

2) It's worth noting that Observable#throw is now deprecated, so opt for using the throwError operator instead. Remember to handle errors properly and consider defining a specific type for error handling to enhance overall type safety.

3) Always ensure you are invoking your service method correctly...

// field.service.ts
export class FieldService {

  constructor(private httpClient: HttpClient) { }

  fieldTypesURL = 'http://localhost:3000/fields/types';

  public getTypes(): Observable<string[]> {
    return this.httpClient.get<string[]>(this.fieldTypesURL).pipe(
      catchError((r: HttpErrorResponse) => throwError(r.error || 'Server error'))
    );
  }
}

// your.component.ts
export class YourComponent {

  constructor(private fieldService: FieldService){}

  // ... component logic ...

  public types: string[];
  public types$: Observable<string[]>;

  public callService() {

    // An example of a correct invocation
    this.fieldService.getTypes().subscribe(types => this.types = types)

    // Another valid usage with additional observable pipeline operations
    this.types$ = this.fieldService.getTypes().pipe(
      catchError((err: any) => {
        // Error handling at the component level can be implemented here
        return throwError(err);
      }),
      flatMap(types => types),
      filter(type => !!type),
      map(type => type.trim()),
      toArray()
    );

    // Incorrect calls leading to compiler errors
    this.fieldService.getTypes().then(types => /* whatever */);
    this.fieldService.getTypes().catch(err => /* whatever */);
    let r: Promise<string[]> = this.fieldService.getTypes();
  }
}

Hopefully, this provides some clarity and assistance :-)

Answer 7

Answer №4

To handle errors and failed Http requests, simply add catchError(this.handleError) to your code.

export class DataService {
  constructor(private httpClient: HttpClient) { }

  dataURL = 'http://localhost:3000/data';

  public getData(): Observable<any[]> {
    return this.httpClient.get<any[]>(this.dataURL)
    .pipe(
      catchError(this.handleErrors)
    );
  }

  private handleErrors(error: HttpErrorResponse) {

    if (error.error instanceof ErrorEvent) {
      // Handle client-side or network error
      console.log('An error occurred:', error.error.message);
    } else {
      // Handle unsuccessful response code from backend
      console.log(
        `Backend returned code ${error.statusText}, ` +
        `body was: `,error.message);
    }
    // Return observable with user-friendly error message
    return _throw('Something went wrong. Please try again later.');
  };
}

I made a small adjustment by removing .map() from the code since it wasn't necessary for processing the response data.

Answer 8

To handle errors and failed Http requests, simply add catchError(this.handleError) to your code.

export class DataService {
  constructor(private httpClient: HttpClient) { }

  dataURL = 'http://localhost:3000/data';

  public getData(): Observable<any[]> {
    return this.httpClient.get<any[]>(this.dataURL)
    .pipe(
      catchError(this.handleErrors)
    );
  }

  private handleErrors(error: HttpErrorResponse) {

    if (error.error instanceof ErrorEvent) {
      // Handle client-side or network error
      console.log('An error occurred:', error.error.message);
    } else {
      // Handle unsuccessful response code from backend
      console.log(
        `Backend returned code ${error.statusText}, ` +
        `body was: `,error.message);
    }
    // Return observable with user-friendly error message
    return _throw('Something went wrong. Please try again later.');
  };
}

I made a small adjustment by removing .map() from the code since it wasn't necessary for processing the response data.

Fetching data from an API using Observables in Angular

Answer №1

Answer №2

Answer №3

Answer №4

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