en.javascript.info/1-js/09-classes/02-class-inheritance/2-clock-class-extended/solution.view/extended-clock.js

class ExtendedClock extends Clock {
  constructor(options) {
    super(options);
    let { precision = 1000 } = options;
    this.precision = precision;
  }

  start() {
    this.render();
    this.timer = setInterval(() => this.render(), this.precision);
  }
};