Solution to this: https://gist.github.com/goldhand/ee3a60bdab87ccf7a52e

javascript-prototype-solution.js

function Car() {  // constructor
  this.driverCount = 0;  // this refers to the newly created object
}

Car.prototype.incrementDriverCount = function() {
  // increments the driverCount of the instance of the new object invoking this method
  this.driverCount++;  // this still refers to the object who is invoking this function
  return this.driverCount;
};

var myCar = new Car();
var myOtherCar = new Car();
var myWeekendCar = new Car();

function increaseCarDriverCount() {
  this.driverCount++;
  return this.driverCount;
}

function Car() {
  this.driverCount = 0;
  this.incrementDriverCount = increaseCarDriverCount;
}

var myCar = new Car();
var myOtherCar = new Car();
var myWeekendCar = new Car();