setInterval（）似乎将数字更改为NaN-如何停止此操作？

Question

我正在计时器。我有一个保存秒值的变量。如果我使用onclick（）手动从变量中减去一个，它将起作用。但是，当我通过setInterval运行该函数时，它将返回NaN。

我尝试过在更改值时在变量前添加“ +”，还尝试了Number（）函数，但无济于事。

    export class CountdownPage implements OnInit {

  countdown: any;
  tabata: any;
  id: any;
  set: any;
  currentSet: any;
  setTest: any;
  phaseCountdown: any;
  currentPhase: any;
  phaseWork: any;
  phaseRest: any;
  phaseEnd: any;
  intervalOne: any;
  authState$: Observable<boolean>;


  constructor(private Auth: AuthService, public http: HttpClient, private 
router: Router, private route: ActivatedRoute) {

  this.route.queryParams.subscribe(params => {
      if (this.router.getCurrentNavigation().extras.state) {
    this.countdown = 
this.router.getCurrentNavigation().extras.state.tabataData[0].substring(17, 19);
    this.tabata = this.router.getCurrentNavigation().extras.state.tabataData[1];
    this.id = this.router.getCurrentNavigation().extras.state.tabataData[2];


            console.log(this.countdown);
        }
    });
 }

  repeat() {
      this.intervalOne = setInterval(this.begin, 1000);
  }

  begin() {
     let first = document.querySelector('#counter');
     this.countdown = this.countdown - 1;
     Number(this.countdown);
first.innerText = this.countdown;
console.log(this.countdown);

}

有关信息：this.countdown变量是通过服务传递的（我正在使用Ionic AngularJS）。

我只是想将存储在变量中的数字每秒减少一个值。

谢谢。