Angular 11 is throwing an error stating that the type 'Observable<Object>' is lacking certain properties as required by its type definition

My code is producing the following error:

TS2739  (TS) Type 'Observable<Object>' is missing the following properties from type 'WeatherForecast': ID, date, temperatureC, temperatureF, summary

I'm puzzled as to why this error is occurring since my object contains all the necessary variables. The ApiService I've created includes a WeatherForecast object that does indeed have the specified variables like this.

export class WeatherForecast {
  ID: number
  date: number
  temperatureC: number
  temperatureF: number
  summary: string
}

This component file was created for editing the weather forecast table.

export class EditWeatherForecastComponent implements OnInit {
  title = 'Edit Data';
  baseUrl: string;
  ID: number;

  forecast: WeatherForecast = {
    ID: 0,
    date: 0,
    temperatureC: 0,
    temperatureF: 0,
    summary: ''
  };

  constructor(private service: ApiService, @Inject('BASE_URL') baseUrl: string, private formBuilder: FormBuilder, public http: HttpClient, private router: ActivatedRoute) {
    this.baseUrl = "https://localhost:44347/WeatherForecast";
  }

  forecasts: any = [];

  ngOnInit(): void {
    this.refreshWeatherList();
    this.ID = +this.router.snapshot.paramMap.get('ID');

    this.forecast = this.service.getWeatherListID(this.ID);  //ERROR HERE
  }

  refreshWeatherList() {
    this.service.getForecast().subscribe(data => {
      this.forecasts = data;
    });
  }

  onSubmit(form: NgForm) {
    let forecasts: WeatherForecast = {
      ID: form.value.id,
      date: form.value.id,
      temperatureC: form.value.id,
      temperatureF: form.value.id,
      summary: form.value.id
    }
    
  }


}

Here's the service

getWeatherListID(ID: number) {
    return this.http.get(this.baseURL + '/?ID=' + ID)
  }

Answer №1

When you use a service, it's common to return an Observable.

return this.http.get(this.baseURL + '/?ID=' + ID)

However, in your component, you declare a variable 'forecast' with the type WeatherForecast.

WeatherForecast

Then, you try to define the forecast variable as type WeatherForecast and assign it an Observable result, which isn't possible.

There are different ways to handle this situation, but one approach is:

this.service.getWeatherListID(this.ID).subscribe((response:WeatherForecast) => {
    this.forecast = response;
});

By following this method, your code will wait for the HTTP call to complete before assigning the response to your forecast variable.

Answer №2

To rephrase the pertinent sections of your inquiry,

ModifyWeatherForecastComponent is declaring a attribute in this manner:

forecast: WeatherForecast;

The specification for WeatherForecast is

export class WeatherForecast {
  ID: number
  date: number
  temperatureC: number
  temperatureF: number
  summary: string
}

In the service, the specific function is described as follows:

retrieveWeatherListID(ID: number) {
    return this.http.get(this.baseURL + '/?ID=' + ID)
}

Angular's HttpClient offers several (overloaded) methods, with get being one of them. Angular doesn't have knowledge on what kind of object your api returns. However, the two relevant functions are the following.

https://i.sstatic.net/hoivk.png

Given that get can take a type but isn't mandatory, when it's not specified, the framework defaults to an Object. An Object in JavaScript does not contain the attributes expected by the forecast property in the ModifyWeatherForecastComponent, namely those outlined in the WeatherForecast class (e.g., ID, date, temperatureC, temperatureF, summary), resulting in the error. To rectify this issue, you should utilize one of the other function definitions for get, specifically the one where you provide type information. This can be achieved as shown below:

return this.http.get<WeatherForecast>(this.baseURL + '/?ID=' + ID)

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