Are functions objects or types in Javascript?

Question

In his Eloquent Javascript, Haverbeke claims that (page 16):

"In a JavaScript system, most of this data is neatly separated into things called values. Every value has a type, which determines the kind of role it can play. There are six basic types of values: numbers, strings, Booleans, objects, functions, and undefined values."

But Crockford in Javascript: The Good Parts says:

"The simple types of JavaScript are numbers, strings, booleans (true and false), null, and undefined. All other values are objects. Numbers, strings, and booleans are object-like in that they have methods, but they are immutable. Objects in JavaScript are mutable keyed collections. In JavaScript, arrays are objects, functions are objects, regular expressions are objects, and, of course, objects are objects."

Now, at least under V8 I get this:

> typeof function(){};
'function'
> typeof {};
'object'

I don't understand if object is a type and function is an object or if function and object are both types. I guess I'm missing the distinction between primitive types and other kind of types (composite types?).

Answer 1

They're a type of object.

The typeof is "function":

typeof (function() {}) === "function" // true

The internal [[Class]] is [object Function]:

({}).toString.call(function() {}) === "[object Function]" // true

They're an instance of the Function constructor prototype:

(function(){}) instanceof Function // true

They're an instance of the Object constructor prototype:

(function(){}) instanceof Object // true

Answer 2

You need to be careful when talking about types in javascript. Values have a Type, which may be one of the following:

1. Undefined
2. Null
3. Boolean
4. String
5. Number
6. Object

Perversely, the value returned by the typeof operator is not the Type, it's a string that is the same as the Type for most values, but is different for:

1. Null returns 'object', even though its Type is Null
2. An object that implements [[Call]] returns function, even though its Type is Object
3. Host objects can return anything they like other than one of the restricted values

So the bottom line is that the Type of a function is Object, but typeof someFn returns function.