online and Offline.ts

  public onlineOffline: boolean = navigator.onLine;
  constructor(.......)
  ................
  ................
  
    ngOnInit() {
    window.addEventListener('online', () => {
      this.onlineOffline = true;
    });
    window.addEventListener('offline', () => {
      this.onlineOffline = false;
    });
  }
  
  
  ...............
  ...............
  
  <mat-card *ngIf="!onlineOffline" class="internet-error">
 No Internet connection. Please connect to the Internet and try again.
</mat-card>