How to get previous page url while navigating to new page in angular 4

Question

I tried given below code. But previous url can't get in service pages

previousUrl: string;
constructor(router: Router) {
  router.events
  .filter(event => event instanceof NavigationEnd)
  .subscribe(e => {
    console.log('prev:', this.previousUrl);
    this.previousUrl = e.url;
  });
}