/* jshint esnext: true */

​

function Period (hours, minutes) {  

  this.hours = hours;

  this.minutes = minutes;

}

Period.prototype.format = () => {  

  console.log(this === window); // => true

  return this.hours + ' hours and ' + this.minutes + ' minutes';

};

var walkPeriod = new Period(2, 30);  

console.log(walkPeriod.format());