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Javascript property assignment question

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Hi,
  I am trying to understand assignment in javascript. Assume ECMAScript 5.
My Question is why does step 4 in the note below work. I would expect it to fail.

1. //Create an Object...
var buffer = Object.create(Object.prototype);

2. //Add a property
Object.defineProperty(buffer, "__data__", {value:[],writable:false,enumerable:false,configurable:false});

Note in setp 2 the property is completely closed to any modification.
In particular...

3.var buffer_copy = Object.create(buffer);

//I would expect the line below to fail.. Since in Step 2 __data__ is made //writable false. BUT It works and the assignment does take place.
//As per JavaScript the Definitive Guide pg 122: If  o inherits a read-only //property named  x, for example, then the assignment is not allowed.
4.buffer_copy.__data__ = []; 

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Javascript property assignment question karl.lopes@gmail.com - 2011-11-23 22:06 -0800
  Re: Javascript property assignment question Richard Cornford <Richard@litotes.demon.co.uk> - 2011-11-24 04:10 -0800

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