Why can't I redefine a property in a Javascript object?

Question

I am creating a object using Object.create and I want to add properties to it.

> var o = Object.create({});
undefined

> Object.defineProp         


        

Answer 1

1. Properties defined through Object.defineProperty() are, by default, non-configurable.

   To allow them to be redefined, or reconfigured, they have to be defined with this attribute set to true.

   var o = Object.create({});
   
   Object.defineProperty(o, "foo", {
       value: 42,
       enumerable: true,
       configurable: true
   });
   
   console.log(o); // { foo: 42 }
   
   Object.defineProperty(o, "foo", {
       value: 45,
       enumerable: true,
       configurable: true
   });
   
   console.log(o); // { foo: 45 }
   

   ---

2. o.prototype is undefined because objects don't typically have prototype properties.

   Such properties are found on constructor functions for new instances to inherit from, roughly equivalent to:

   function Foo() {}
   
   //  ... = new Foo();
   var bar = Object.create(Foo.prototype);
   Foo.call(bar);
   

   Object are, however, aware of their prototype objects. They're referenced through an internal [[Prototype]] property, which __proto__ is/was an unofficial getter/setter of:

   console.log(o.__proto__); // {}
   

   The standardized way to read the [[Prototype]] is with Object.getPrototypeOf():

   console.log(Object.getPrototypeOf(o)); // {}