What is Function's Data Type : function or object? in JavaScript

Question

The MDN said :

Six data types that are primitives:

• Boolean
• Null
• Undefined
• Number
• String

• Symbol (new in ECMAScript 6)

• and Object

But I confused, the function data type and object data type.

Let's see :

var func = function() { 
    console.log ('Hello World ! ')
};

var obj = { 
    property : something
}  


console.log(typeof(func)); // ===> function
console.log(typeof(obj)); // ===> object 

Is it different function data type and object data type? Why typeof(func) is function? not a object? The document said there are 7 data type (6 primitive, 1 object). function is not include anywhere.

Until now, over 1 year, I think function's data type is object, I heard the function is first class object in JavaScript, so I don't have doubt about function is object but today I think more time, and wondered.

Is it different?

Answer 1

You can logically think of Function as a subclass of Object. It has all the methods of Object plus some more that are specific to a function (such as .bind(), .call(), .apply(), etc...).

Why Javascript decided to make Function report it's own unique type, but not Array (which is a similar derivation from Object) is anyone's guess and probably only known to the original designers of the language. It is extremely useful that Function does report its own type so you can easily check if a property is callable as a function and perhaps that is the main reason why it was done this way.

Here's a demonstration of how a Function object has the methods from an Object:

function f() {}
f.someProperty = "foo";

log(f.hasOwnProperty("someProperty"));
log(f instanceof Object);
log(f instanceof Function);

function log(x) {
    var div = document.createElement("div");
    div.innerHTML = x;
    document.body.appendChild(div);
}