A step-by-step guide on utilizing Observables to extract JSON data from a file

Currently, I am experimenting with reading data from a JSON file that contains information about countries and states. Previously, I used a method to achieve this but now I want to switch to using httpClient for fetching the data. Below are the methods I have used in the past and the new approach that I am trying:

<pre><code><pre><code><pre><code><pre><code><pre>[
    {
        "label": "Select State",
        "value": null
     },
     {
         "label": "Adeje",
         "value": {
              "county": "Gran Canaria",
              "state": "Islas Canarias",
               "country": "España"
           }
        },
        {
            "label": "Agaete",
            "value": {
                 "county": "Gran Canaria",
                 "state": "Islas Canarias",
                 "country": "España"
             }
      }
 ]
 </pre>

Please note that I am working with strict mode enabled (--strict).

jsonangulartypescript

Answer №1

To effectively address your issue, it is important to acknowledge that the recommended approach when utilizing angular is to utilize Observables instead of Promises

The Challenge at Hand

Take a look at the following code snippet:

getC3(): Observable<Country[]> {
  return this.http.get<Country[]>(this.link)
    .toPromise()
    .then(res => <Country[]>res.countries)
    .then(data => {return data});
  }

The line 

return this.http.get<Country[]>(this.link)
will give back an Observable. When you call .toPromise() on an Observable, it converts it into a Promise. This will result in an error stating that 
Promise<Country[]> does not contain properties ... from type Observable<Country[]>

The Resolution

One solution (the preferred one) to this problem is to maintain the type as Observable.

getC3(): Observable<Country[]> { return this.http.get<Country[]>(this.link)}

I advise against the following, altering the return type to promise

getC3(): Promise<Country[]> {
  return this.http.get<Country[]>(this.link)
    .toPromise()
    .then(res => <Country[]>res.countries)
    .then(data => {return data});
  }

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