Run and stop timer using class field in TypeScript

I would like to create the class containing a field with timer. The main problem is that I can't set the timer default value undefined or null, because TypeScript doesn't allow this. I need to create an empty timer and run or stop it with relevant class methods. Now this script doesn't even run necessary timer with entered interval when I call start method.

class Test {
  timer: NodeJS.Timer = setInterval(() => { console.log('1') }, 1000);

  start(interval: number) {
    this.timer = setInterval(() => console.log('Timer is working!'), interval);
  }

  stop() { clearInterval(this.timer); }
}

const test = new Test();

test.start(5000);

test.stop();

Define the timer as NodeJS.Timer | null and init it with null. Check that the timer is initialized when calling stop, and also assign null to timer after clearing it:

class Test {
  timer: NodeJS.Timer | null = null;

  start(interval: number) {
    this.stop();
    this.timer = setInterval(() => console.log('Timer is working!'), interval);
  }

  stop() {
    if(this.timer) {
      clearInterval(this.timer);
      this.timer = null;
    }
  }
}