Arrow function binds the function to the instance....

//constructor
function Person() {
     this.age = 0

    //now in the constructor here create an interval that attempts to age the person every second
    setInterval(function () {
         //now you might think you can just access 'this' and think it is the person object. 
        //however, that is not true since you are in a new function, this will refer to the global object i belive
       this.age++ //will not work
    }, 1000)

}

//So if you now do
var person = new Person(); //You will likely get an undefined error as 'age' does not exist in the global object

//NOW THERE HAVE BEEN WAYS AROUND THIS BY REASINING 'this' INSIDE THE CONSTRUCTOR
//constructor
function Person() {
     var self = this;
     self.age = 0;

     setInterval(function(){
             //now you can access the 'self' variable in here which correctly refers to this as the Person
             self.age++;
     }, 1000);
}

var person = new Person(); // now should work and the person should age very second

//Now new with ES6 we can even skip this step 
function Person() {
      this.age = 0;

      setInterval(()=>{
               //since we are using arrow syntax, the this value will refer to person
              this.age++;
      },1000);
}