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

@Injectable()
export class LazyLoadingScriptService {
    _loadedLibraries: { [url: string]: ReplaySubject<any> } = {};

    constructor(@Inject(DOCUMENT) private readonly document: any) { }

    loadScript(url: string): Observable<any> {
        if (this._loadedLibraries[url]) {
            return this._loadedLibraries[url].asObservable();
        }

        this._loadedLibraries[url] = new ReplaySubject();

        const script = this.document.createElement('script');
        script.type = 'text/javascript';
        script.src = url;
        script.onload = () => {
            this._loadedLibraries[url].next();
            this._loadedLibraries[url].complete();
        };

        this.document.body.appendChild(script);

        return this._loadedLibraries[url].asObservable();
    }
}

      
/* Usage */
this.lazyLoadService.loadScript('/assets/scripts/some-script.js').subscribe(() => {
   // code
});