I don't understand writable and configurable property attributes of Objects

Question

I don't understand the Writable and Configurable attributes of Objects. For example, in the MDN for Object.prototype, there is a table where I can clearly see that Configurable, Writable and Enumerable Property Attributes of Object.prototype are locked.

However, I can write and extend Object.prototype, for example with the following code:

// Example 1
Object.prototype.testing=999;
console.log(Object.testing); // 999

// Example 2
var o = {};
console.log(o.testing); // 999

Answer 1

What the MDN is referring to is the property prototype of Object itself. You cannot overwrite Object.prototype itself. If you try and make Object.prototype undefined, that will fail:

Object.prototype = 1;
console.log(Object.prototype); // [object Object]

If you try this in strict mode, you will get a TypeError upon attempting to assign to a non-writable property:

'use strict';
Object.prototype = 1; // TypeError: Cannot assign to read only property 'prototype' of function Object() { [native code] }

You can write to an object's own properties without changing what the object's reference is, and those have separate attributes. For example, see this:

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'toString');

console.log(descriptor.writable); // true
console.log(descriptor.enumerable); // false
console.log(descriptor.configurable); // true

There is a separate [[Extensible]] internal property that prevents the creation of new properties on an object -- this is set to false if you call Object.preventExtensions, Object.seal or Object.freeze.

Note that it's not a good idea to call Object.freeze on something like Object.prototype, as really weird things can happen:

Object.freeze(Object.prototype);
var building = {};
building.name = 'Alcove Apartments';
building.constructor = 'Meriton Apartments Pty Ltd';
console.log(building.constructor); // function Object() { [native code] } 

Just like the previous example, it will also throw a TypeError in strict mode.

Basically, even though it would be a property on the object itself, it uses the attributes from the prototype chain to check whether or not it can assign the property. This has been considered as a mistake in the language by some people, however others consider this behaviour to be by design.

Answer 2

From: http://ejohn.org/blog/ecmascript-5-objects-and-properties/

Writable: If false, the value of the property can not be changed.

Configurable: If false, any attempts to delete the property or change its attributes (Writable, Configurable, or Enumerable) will fail.

Enumerable: If true, the property will be iterated over when a user does for (var prop in obj){} (or similar).

Answer 3

The Writable, Enumerable and Configurable attributes in MDN appear to be about the Object.prototype object itself, not its properties.

So, what that means is that you can't replace Object.prototype with a different object, but you are allowed to add properties to it.

So, what that means is if you do this:

Object.prototype = {foo: "whatever"};   // doesn't work - is just ignored
var j = {};
console.log(j.foo);   // undefined

Then, the first line of code won't do anything.

Answer 4

I can clearly see that Configurable, Writable and Enumerable Property Attributes of Object.prototype are locked. However, I can write Object.prototype.

No. The writability only concerns the prototype property of the Object object:

Object.prototype = {}; // Error: Invalid assignment (in strict mode)
                       // simply fails in lax mode

And I can extend Object.prototype

Yes. You can extend the Object.prototype object (regardless how you refer to it); that's a different attribute (of the object, not of a single property):

var proto = Object.getPrototypeOf({});
proto.testing1 = 9999; // works
Object.preventExtensions(proto);
proto.testing2 = 9999; // Error: Invalid assignment (in strict mode)