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

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

Object.create

> 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 ?

{}

undefined

__proto__

3 Answers
3

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

Object.defineProperty()

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.

configurable: true

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

Object.defineProperty()

configurable

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

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 }


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

o.prototype

undefined

prototype

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

function

new

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:

[[Prototype]]

__proto__

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


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

[[Prototype]]

Object.getPrototypeOf()

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


Only when both writable and configurable are false, "Cannot redefine property" will happen.

If writable or configurable is true, the error will disappear.

"use strict"

var obj = {};

Object.defineProperty(obj, "name",
{
value: "Fundebug",
writable: false,
configurable: false
})

Object.defineProperty(obj, "name",
{
value: "云麒"
}) // “Uncaught TypeError: Cannot redefine property: name”


Therefore, jdphenix and Jonathan are not exactly correct.

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