Question

If I write this

var o = Object.create(null)
alert(o instanceof Object) // this is false

How come this ends up being true

function o() {

}
o.prototype = null
alert(new o() instanceof Object) // this is true

Shouldn’t manually setting the prototype to null cause it to inherit from nothing as Object.create does. Thanks in advance 🙂

Answer 1

Briefly, if a constructor’s prototype isn’t an Object, then instances are given Object.prototype as their [[prototype]].

The detail is in ECMA-262, §13.2.2 [[Construct]]:

When the [[Construct]] internal method
for a Function object F is called with
a possibly empty list of arguments,
the following steps are taken:

1. Let obj be a newly created native ECMAScript object.
2. Set all the internal methods of obj as specified in 8.12.
3. Set the [[Class]] internal property of obj to “Object“.
4. Set the [[Extensible]] internal property of obj to true.
5. Let proto be the value of calling the [[Get]] internal property of F
   with argument “prototype“.
6. If Type(proto) is Object, set the [[Prototype]] internal property of obj
   to proto.
7. If Type(proto) is not Object, set the [[Prototype]] internal property of
   obj to the standard built-in Object
   prototype object as described in
   15.2.4.
8. Let result be the result of calling the [[Call]] internal property
   of F, providing obj as the this value
   and providing the argument list passed
   into [[Construct]] as args.
9. If Type(result) is Object then return result.
10. Return obj.

Noting that in items 6 and 7, null is Type null (ECMA-262 §8.2), it is not the same as typeof null, which is object.