Angular lazy library loader

lazylibs.service.ts

import { Injectable, Inject } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { ReplaySubject } from 'rxjs/ReplaySubject';

import { DOCUMENT } from '@angular/platform-browser';

@Injectable()
export class LazylibsService {
  private chartJs?: ReplaySubject<any>;

  // see http://stackoverflow.com/questions/37521298/how-to-inject-document-in-angular-2-service
  // can't use Document type here due to: https://github.com/angular/angular/issues/12631
  constructor(@Inject(DOCUMENT) private readonly document: any) { }

  public loadChartsJs(): Observable<any> {
    if (this.chartJs) {
      return this.chartJs.asObservable();
    }

    this.chartJs = new ReplaySubject();

    const script = this.document.createElement('script');
    script.type = 'text/javascript';
    script.src = '/lazylib/Chart.bundle.min.js';
    script.onload = () => {
      this.chartJs!.next('');
      this.chartJs!.complete();
    };

    this.document.body.appendChild(script);
    return this.chartJs.asObservable();
  }
}