How can we prove that javascript functions are objects?

Question

i am using

var foo = function(){}

typeof(foo) - returns function instead of Object

while javascript concept tell that all function in javascript are objects?

Answer 1

First and foremost, there's the fact that the specification explicitly states they're objects, in a lot of places, including here where it defines the term "function":

function

member of the Object type that may be invoked as a subroutine

Empirically, there are lots of ways to prove it, but one of the simplest is to assign a value to a property on it:

var foo = function() { };
foo.myProperty = "my value";
console.log(foo.myProperty); // "my value"

Or use one as a prototype (which is unusual, but possible), which proves it as only objects can be prototypes:

var foo = function() { };
var obj = Object.create(foo);
console.log(Object.getPrototypeOf(obj) === foo); // true

Answer 2

How about this for a test

>  x = function(){}
<- function (){}
>  x.a = "Asd"
<- "Asd"
>  x
<- function (){}
>  x.a
<- "Asd"

Doing the same on an integer will result in undefined assignment

>  x = 1
<- 1
>  x.a = 123
<- 123
>  x
<- 1
>  x.a
<- undefined

Answer 3

See my list of proper ways to test if a value belongs to the Object type.

They detect functions as objects, e.g.

var func = function(){};
Object(func) === func; // true  -->  func is an object

If you want a more formal proof you will need to see the spec, for example in ECMAScript Overview

a function is a callable object

Answer 4

You can prove this very simply:

console.log(Object.getPrototypeOf(Function.prototype));
console.log(Object.prototype === Object.getPrototypeOf(Function.prototype));

// or, being evil
Object.prototype.testFn = () => console.log('Exists on all objects');
({}).testFn();
[].testFn();
(new Number(5)).testFn();
Math.sqrt.testFn();

The first two lines demonstrate that the next step in the prototype chain beyond Function.prototype is Object.prototype.

The other lines show that you can add a property to Object.prototype (seriously don't do this ever) and it exists on all objects. In this case, we test it on an empty object, an empty array, a Number object and on the Math.sqrt function. By adding a property (a function in this case) to Object.prototype, all the others gain the property too.

Answer 5

console.log(Function.prototype instanceof Object);