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.defineProperty(o, "foo", {value: 43, enumerable: true}); {foo: 43}  > o {foo: 43}  > o.foo 43  > for (var i in o) { console.log(i); } foo  > Object.keys(o) ['foo']  > Object.defineProperty(o, "foo", {value: 43, enumerable: false }); TypeError: Cannot redefine property: bar 

Q1) Why can't I redefine the property ?

> o.__proto__ {}  > o.prototype undefined 

Q2) Why is the prototype empty ? And why are these 2 values different i.e. {} vs undefined ?

Answer 1

You are unable to redefine the property because Object.defineProperty() defaults to non-configurable properties, from the docs:

configurable

true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object. Defaults to false.

So this defaults to false - you'd need to pass it configurable: true to allow it.