x is not a function … what would you expect Object.create to do with a constructor

Question

For this question, I\'m not expecting a solution to solve something but would like to understand things better ..

Some quote from the specifications:

Answer 1

So I'm wondering what would you expect Object.create to do with a constructor?

I would expect it to follow the spec of course…

I'm thinking about why doesn't it either disallow a constructor to be used as O

Why should it? Every constructor function is an object (§8.6).

… or just create a valid constructor.

The spec says it should create a plain object (as by new Object), whose [[prototype]] is set to O. Plain objects are no functions, they don't have a [[call]] or [[construct]] property. It also will have [[class]] Object, not Function.

x.prototype.constructor===F // true
x instanceof Function // true
typeof x // 'object'

Seems it created an object of a type derives from (sorry for the poor terminology .. ) Function

From F, actually. It does inherit the .prototype property from F (that's why your first test is true), and through F it also inherits from Function.prototype which makes it instanceof Function. Yet, it doesn't have a [[call]] property (it's not callable) so typeof does not yield "function", but just "object".

Answer 2

Unfortunately that won't work. What you have is an object that has F in its prototype chain; the fact that F is a function doesn't make x a function.

Only objects created via a function declaration or a function expression will have "Function" as their [[Class]], and a [[Call]] method, which makes it callable. Those are created according to the steps detailed on section 13.2 of the ECMAScript 5 specification.

The algorithm for Object.create does something different, as you can see on your quote. In your case, x will be a regular object with [[Class]] "Object" and no [[Call]] method. If you try Object.prototype.toString.call(x), you'll get "[object Object]", where "Object" is the [[Class]] of x. x instanceof Function only returns true because the Function constructor is part of the prototype chain of x (via F).

I'm not sure if any of that is going to be changed in ES6, but I suppose it won't.