How can I save the response of an HTTP get request into a json file in Angular 7?

I have written a library called ngx-rxcache which is exactly for this sort of thing.

https://github.com/adriandavidbrand/ngx-rxcache

I have written an article on it here https://medium.com/@adrianbrand/angular-state-management-with-rxcache-468a865fc3fb

@Injectable()
export class DataService{

  get data$(): Observable<DataType> {
    return this.dataCache.value$;
  }

  private dataCache = this.cache.get<DataType>({
    id: 'some unique identifer',
    construct: () => this.http.get<DataType>('/dataUrl'),
    autoload: true
  });

  constructor(private http: HttpClient, private cache: RxCacheService) {}
}

and in a component use the data$ observable

data$ = this.dataService.data$;

and in the template use the async pipe.

{{ data$ | async }}

You can not save JSON data to local file and load data because of security reason.

You can use localStorage or sessionStorage to save JSON object.

@Injectable()
export class YourService{

constructor(private http: HttpClient){
}

 getData(){
       var cached = JSON.parse(localStorage.setItem('yourkey'));
       if(cached == null || cached == undefined){
        this.http.get('yoururl').subscribe(data=>{
            localStorage.setItem('yourkey', JSON.stringify(data);
            return data;    
        });
      }else{
         return cached;
      }
    }
}