en.javascript.info/1-js/9-object-inheritance/10-class-inheritance/3-clock-class/source.view/clock.js

function Clock(options) {

  var template = options.template;
  var timer;

  function render() {
    var date = new Date();

    var hours = date.getHours();
    if (hours < 10) hours = '0' + hours;

    var min = date.getMinutes();
    if (min < 10) min = '0' + min;

    var sec = date.getSeconds();
    if (sec < 10) sec = '0' + sec;

    var output = template.replace('h', hours).replace('m', min).replace('s', sec);

    console.log(output);
  }

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

  this.start = function() {
    render();
    timer = setInterval(render, 1000);
  }

}