App init for getting dynamic configuration on app startup

app.init.ts

import { Injectable } from '@angular/core';;
import { from } from 'rxjs';
import { filter, map, switchMap, tap } from 'rxjs/operators';
declare var window: any;

@Injectable()
export class AppInitService {

  // This is the method you want to call at bootstrap
  // Important: It should return a Promise
  public init() {
    return from(
        fetch('assets/app-config.json').then(function(response) {
          return response.json();
        })
      ).pipe(
        map((config) => {
        window.config = config;
        return 
      })).toPromise();
  }
}