Constructor.prototype not in the prototype chain?

Question

related: Confusion about protype chain , primitives and objects

in Firebug console :

a = 12
a.constructor.prototype.isPrototypeOf(a) // prints \'fals         


        

Answer 1

a = 12 creates a primitive number, which is not quite the same as a Number object. Primitives are implicitly cast to objects for purposes of property access.

a = 12; //a is a primitive
b = new Number(12); //b is an object
a.constructor.prototype.isPrototypeOf(a); //false because a is primitive
b.constructor.prototype.isPrototypeOf(b); //true because b is an object

As per the ECMAScript spec:

When the isPrototypeOf method is called with argument V, the following steps are taken:

1. If V is not an object, return false.

primitive numbers are not, strictly speaking, objects.