case1.js

calculateAge(1985);

function calculateAge(year) {
	console.log(2016 - year);
	console.log(this); //First 'this'
}

var john = {
	name: 'John',
	yearOfBirth: 1990,
	calculateAge: function() {
		console.log(this); //Second 'this'
		console.log(2016 - this.yearOfBirth);
	}
}
john.calculateAge();
// In this case, the first ‘this’ is the window object, and the second ‘this’ is the john object.

case2.js

var john = {
	name: 'John',
	yearOfBirth: 1990,
	calculateAge: function() {
		console.log(this); //First 'this'
		console.log(2016 - this.yearOfBirth);
		function innerFunction() {
			console.log(this); //Second 'this'
		}
		innerFunction();
	}
}

john.calculateAge();
// In this case, the first ‘this’ is the john object, however, the second object is the window object.
//It’s because the second ‘this’ is inside a regular function call.

case3.js

var john = {
	name: 'John',
	yearOfBirth: 1990,
	calculateAge: function() {
		console.log(this);
		console.log(2016 - this.yearOfBirth);
	}
}

john.calculateAge();

var mike = {
	name: 'Mike',
	yearOfBirth: 1984
}

mike.calculateAge = john.calculateAge;
// In this case, ‘this’ in calling john.calculateAge() is john object,
// and ‘this’ in calling mike.calculateAge() is mike object.