Why can't I use this.arguments, while arguments work?

Question

function MyArray() {
    var self = this.arguments; //<---
    this.toString = function() {
        return self;
    };

}

var c = new MyArray(2, 3, 1, '232');
console.log(c.toString());

undefined

function MyArray() {
    var self = arguments; //<---
    this.toString = function() {
        return self;
    };

}

var c = new MyArray(2, 3, 1, '232');
console.log(c.toString());

[2,3,1,'232']

So, why is that so? What is the difference between this.arguments and arguments ?

Answer 1

There is no this.arguments . However arguments is an array-like object which always exists inside functions and refers to the arguments given in the call.

Answer 2

In JS, this represents "calling context", which can be set a number of different ways. It has really nothing to do with variable scope. They're entirely separate concepts. That's why arguments is available as a variable, but not a property of this .

The value of this varies based on how a function is called.

---

Called as a method

For example, if I do...

my_obj.myMethod();

...then this will refer to my_obj because the function myMethod was called from the context of my_obj .

---

Called independently as a function

but if I do...

var m = my_obj.myMethod;
m();

Even though my_obj and myMethod are the same code as the first example, the value of this will usually change to the global object.

---

Called using .call or .apply

There are several other ways this can be set as well.

m.call(my_other_obj, "more", "args");

m.apply(my_other_obj, ["more", "args"]);

Using .call or .apply , the first argument passed becomes the value of this . The difference between the two is how the rest of the arguments are passed.

---

Created using .bind

There's also .bind() which takes arguments in the same manner as .call() , but creates and returns a new function with the this value and any other arguments permanently set.

---

Called using the new operator

The final way is when you call a function as a constructor. To do this, you use the new operator.

var new_obj = new m;

var new_obj = new obj.m;

In both cases, the value of this will be a new object being constructed. YOu should never use new unless the function called has been set up to be used as a constructor.