JavaScript Interval timer

Interval.js

/*
 * A timer that attempts to be more accurate than native setInterval.
 * It calculates an appropriate delay to timeout after the callback is called.
 * It is intended that the Callback function return ASAP this will not handle async operations that may be delayed.
 */
class Interval {
  constructor(cb, ms) {
    this.cb = cb;
    this.ms = ms;
    this.timeout = null;
  }

  start(callNow) {
    // Already initialized.
    if (this.timeout) {
      return;
    }

    var loop = () => {
      this.cb();
      var now = new Date();                  // allow for time passing
      var delay = this.ms - (now % this.ms); // exact ms to next interval
      this.timeout = setTimeout(loop, delay);
    };

    if (callNow) {
      this.cb();
    }

    var now = new Date();                  // allow for time passing
    var delay = this.ms - (now % this.ms); // exact ms to next interval
    this.timeout = setTimeout(loop, delay);
  }

  stop() {
    clearTimeout(this.timeout);
  }
}

Based from the stackoverflow solutions here: https://stackoverflow.com/questions/19088040/how-can-i-run-a-function-at-specific-time-date#comment28224988_19089196