Object Scope Help and [return] Keyword Definition Needed

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// From "Passing Objects into Functions" > 28/33 > Objects I > CodeAcademy.com
 
// QUESTIONS:
// 1. Per line 17, I understand that the I am passing the [ageDifference] object's properties as parameters into the [Person] constructor. Does an object always have access to an object constructor's properties when the [this] keyword is used, along with the object using the same properties as the object constructor?

// 2. Per line 17, what does the [return] keyword do? When should the [return] be used?
 

function Person (name, age) {
    this.name = name;
    this.age = age;
}

// We can make a function which takes persons as arguments
// This one computes the difference in ages between two people
var ageDifference = function(person1, person2) {
    return person1.age - person2.age;
}

var alice = new Person("Alice", 30);
var billy = new Person("Billy", 25);

// get the difference in age between alice and billy using our function
var diff = ageDifference(alice, billy);