how "this" keyword behaves

Inside Function declaration.js

function c(){
	return this;
}

c() //prints window

var obj = {
  a:1,
  b:2
}

c.call(document);// prints document
c.call(obj); // prints {a:1,b:2}

function d(){
	return this.a;
}

d()//prints undefined
d.call(obj);//prints 1

Inside Object literal.js

var a = {
  b: this,
  c: function() {
    return this;
  }
}
var ob = {
  a: 1,
  b: 1
}
a.b //prints window
a.c()
  // prints {b:this, c:function(){return this;}} 
  //reference to a, i.e the parent of the method

a.b.call(window) // error:: a.b.call is not a function
a.c.call(ob); // prints {a:1,b:1}


var a = {
  b: {
    d: "abe",
    c: function() {
      return this.d;
    }
  }
}

a.b.c() //prints "abe"

//now

var o = {
  d: "mah"
};
a.b.c.call(o) //prints "mah"

//let n is undefined
a.b.c.call(n) //prints ReferenceError: op is not defined