How can I compare the length of an Observable array with previous one?

Question

So I have a service http function which return an Observable<any[]>. I call this function every minute in the component and I want to compare the length of the previously returned array with the length of the newly returned array.How can I do this?

This is the service method:

public getMatchIncident(matchId: number): Observable<any[]>{
    return  this.http.get<any>(this.url + matchId);
}

This is the component function:

private getMatchIncident(matchId: number): void {
    this.matchService.getMatchIncident(matchId).subscribe(event => {
      const previousLength = this.incidents.length;
      const newLength = event.length;
      this.incidents = event;
    });
}

But it doesn't work well... How can I solve this problem?