我如何访问类中的函数？

Question

我正在做一个Web应用程序： Pomodoro 。

我创建了一个课程。但是我无法访问此this.countdown = setInterval(timer,1000)中的函数。

我写了“ this.timer”。这样，错误不会返回，但是函数无法正常工作。我该怎么办？

class Pomodoro{
    constructor(){
        this.countdown = 0;
        this.seconds = 1500;
        this.workTime = 25;
        this.breakTime = 5;
        this.isBreak = true;
        this.isPaused = true; 
        this.minutes = Math.floor(this.seconds / 60);
        this.remainderSeconds = this.seconds % 60;
    };
    start(){
        clearInterval(this.countdown);
        this.isPaused = !this.isPaused;
        if(!this.isPaused){
            this.countdown = setInterval(timer,1000);
        };
    };
    reset(){
        clearInterval(this.countdown);
        this.seconds = this.workTime*60;
        this.countdown = 0;
        this.isBreak = true;
        this.isPaused = true; 
    };
    timer(){
        this.seconds --;
        if(this.seconds < 0){
            clearInterval(this.countdown);
            this.seconds = (this.isBreak ? this.breakTime : this.workTime) * 60;
        }
    };
}

let pomodoro = new Pomodoro();
function Update(){
    pomodoro.start();
    pomodoro.timer();
}
window.setInterval(Update, 100);