Javascript - Every function an object?

Question

In Javascript every function is an object.

function a() {
 this.x = function() { console.log("x"); }
}

Here "a" is a function, which is an object. right?

var b = Object.create(a);
b.x(); //does not work!!

The above code would work if we wrote -

var b = Object.create(new a())

So does that mean only the instance of a function is an object? not the function?

• Frédéric Hamidi: A function is an object, but inside the function this is not that object. It is the context set by the caller.
• Grundy: if you check - a variable does not have x property :-) so and inherited b also does not have it
• nnnnnn: Further to Frédéric's comment, not only is this not a reference to a, in the code that didn't work you never actually call the function so the code inside it hasn't been run.
• Grundy: seems you a bit confused what do Object.create function. this not call passed function as constructor, for new object, but just create new object and set prototype.

Answer 1

They are not instance of functions you are messing with the new keyword the new keywords does many things:

• A brand new object is created
• The new object is [[prototype]]-linked
• The new object is set as the 'this' binding for the function call
• Unless the function returns its own object the newly invoked call function will return the new object

In your first example you are creating an object from function a which does not have any x property.

In the last you are creating and object of the object returned by the new a() invocation which has a x property because it is asigned in the execution of the a function

• GottZ: this is a pretty accurate explaination.

Answer 2

function a() {
 this.x = function() { console.log("x"); }
}
var b = new a();
b.x();

A function is like a class. You can create instances or objects using the new keyword.

• nnnnnn: I think it's clear from the question that the OP already knows that new creates an instance.
• Felix A J: just wanted to specify the function is more like a class than an object
• nnnnnn: A function isn't like an object, it is an object.
• TaoPR: There is no class in ECMAScript 5 even it acts partially like.

Answer 3

What this is depends on how you use your a function. If you don't run it as a constructor function (you don't use a new keyword), this points to the outer scope. Try this:

function a() {
 this.x = function() { console.log("x"); }
}
a();
console.log(window.x());

On the other hand, when you write new a(), you are running it as a constructor function, in which case this points to the instance of newly created object. Try now this example:

function a() {
 this.x = function() { console.log("x"); }
}
var b = new a(); // mind the "new" keyword here
console.log(b.x());
console.log(window.x);

And as a simplest proof that every function is an object:

function c() {};
c instanceof Object; // ;-)

Answer 4

Every function is a class not an object, every function value is an object. A class is a set of objects and a function is a set of function values, therefore every function is a class and in the spirit of pure functional programming, every function value is an object.