Angular 6: saving data to local storage

I have a data table which display data from external API, I want the number of items /element on the table page should be saved in local storage

Here is what I have tried so far:

ngOnInit() {
  this.moviesService.getPopularTVShows().subscribe(res => {
    this.dataSource = new MatTableDataSource(res.results);
    this.dataSource.paginator = this.paginator;
    this.dataSource.sort = this.sort;
    localStorage.setItem(this.dataSource, this.dataSource.length);
    console.log(localStorage.length);
  });
}

When I run my app, the console displays undefined

What is wrong with my code? any help or suggestion is welcomed, newbie trying new stuff.