Mobx test timers

counter.test.ts

import { TimeTrackerStore } from './time-tracker-store';
import { action, makeAutoObservable } from 'mobx';

class Counter {
  value = 0;
  intervalId?: NodeJS.Timer;

  constructor() {
    makeAutoObservable(this);
  }

  start() {
    this.intervalId = setInterval(
      action(() => {
        this.value++;
      }),
      1000
    );
  }

  stop() {
    if (this.intervalId) {
      clearInterval(this.intervalId);
    }
  }
}

describe('TimeTrackerStore', () => {
  it('increments time', () => {
    jest.useFakeTimers();

    const counter = new Counter();
    expect(counter.value).toBe(0);

    counter.start();

    jest.advanceTimersByTime(2500);

    expect(counter.value).toBe(2);

    jest.advanceTimersByTime(3000);

    expect(counter.value).toBe(5);

    counter.stop();

    jest.advanceTimersByTime(3000);

    expect(counter.value).toBe(5);
  });
});