https://medium.com/@gmurop/managing-dependencies-among-app-initializers-in-angular-652be4afce6f

config.service.ts

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { AppConfig } from '../model/config.model';

const CONFIG_PATH = '/assets/config.json';

@Injectable()
export class ConfigurationService {
  private appConfig: AppConfig;

  constructor(private http: HttpClient) {}

  public async loadAppConfig(configPath: string = CONFIG_PATH) {
    return this.http
      .get(configPath)
      .toPromise()
      .then((data: AppConfig) => {
        this.appConfig = data;
      });
  }

  public get(key: string): any {
    if (!this.appConfig) {
      throw Error('Config file not loaded!');
    }

    return this.appConfig[key];
  }

  public set(key: string, value: any) {
    this.appConfig[key] = value;
  }
}

config.model.ts

export interface AppConfig {
  apiEndpoint: string;
}

config.module.ts

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { ConfigurationService } from './service/config.service';

@NgModule({
  declarations: [],
  providers: [ConfigurationService],
  imports: [CommonModule],
})
export class ConfigModule {}

Your core/app module where you want to load the configuration

  imports: [
    ...,
    ConfigModule,
  ],
  providers: [
    ...,
    {
      provide: APP_INITIALIZER,
      multi: true,
      deps: [ConfigurationService],
      useFactory: (configurationService: ConfigurationService) => {
        return () => {
          return configurationService.loadAppConfig('/assets/config.json');
        };
      },
    },
  ],