Question

this is one of most mystery feature in JavaScript, after assigning the object method to other variable, the binding (this keyword) is lost

var john = {
  name: 'John',
  greet: function(person) {
    alert("Hi " + person + ", my name is " + this.name);
  }
};

john.greet("Mark"); // Hi Mark, my name is John

var fx = john.greet;  
fx("Mark"); // Hi Mark, my name is 

my question is:

1) what is happening behind the assignment? var fx = john.greet;
is this copy by value or copy by reference?
fx and john.greet point to two diferent function, right?

2) since fx is a global method, the scope chain contains only global object. what is the value of this property in Variable object?

Answer 1

1) fx and john.greet are referring to the same function object, the assignment operation for objects, works by reference.

For primitive values, like String, Number, Boolean undefined or null, a copy of the value will be made.

2) The this value refers to the global object.

The this value is not a property of the Variable Object and it has nothing to do with the scope chain, is a special reserved word, and it is determined implicitly when a function is called (you can also set it explicitly via call or apply).

JavaScript internally handles a Reference type, which consists of two components, the base object and the property name, when a function is invoked, the this value is determined implicitly by getting the base object (by the internal GetValue operation).

And finally, the last case where this is set implicitly is when you invoke a function with the new operator, the this keyword will refer to a newly created object.

So in brief, here is how this works implicitly:

1- When a function is called as a method (the function is invoked as member of an object):

obj.method(); // 'this' inside method will refer to obj

2- A normal function call:

myFunction(); // 'this' inside the function will refer to the Global object
// or 
(function () {})();

3- When the new operator is used:

var obj = new MyObj(); // 'this' will refer to a newly created object.