writing modules using new feature class

Like to share how to write a module containing a class

// modules/stopWatch.js

class stopWatch {
      constructor() {
        var timeStart = 0;
        var timeStop = 0;
      }
    // Method start()
      start() { 
        this.timeStart = Date.now();
        this.timeStop = 0;
      }
    // Method stop()
      stop() { 
        this.timeStop = Date.now();
      }
      // Method duration()
      duration(){
        return this.timeStop-this.timeStart;
      }
}

exports  = stopWatch

and use it.

// projects/test_class_stopWatch.js

stopWatch = new (require('stopWatch'))();

function onInit(){
      stopWatch.start();
      setTimeout( function(){
        stopWatch.stop();
        console.log('duration in sec: ',
                ( stopWatch.duration() / 1000 ).toFixed(2) );
      }, 1000);
}

setTimeout(onInit,1000);

// output
duration in sec: 1.00