Difference between Configurable and Writable attributes of an Object

Question

I saw the following regarding javascript, object data property attributes

— 
Configurable: Specifies whether the property can be deleted or changed.

— Enumerable: Specifies whether the property can be returned in a for/in loop.

— Writable: Specifies whether the property can be changed.

Here "Configurable" and "Writable" are representing the same (whether the property can be changed), then why do we need two separate attributes?

Answer 1

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 2

configurable and writable are NOT representing the same thing.

configurable means property descriptor and existence.

writable means property value only.

A property's descriptor contains value, enumerable, configurable and writable.

scenario 1: create property by assignment

'use strict';  // non-strict mode behaves slightly different  var foo = {}; foo.bar = 1;  // operated by CreateDataProperty*  // the above is the same as Object.defineProperty(foo, 'bar', {   value: 1,   configurable: true,   writable: true,   // ... }); 

• CreateDataProperty is an operation defined together with ECMAScript spec.

scenario 2: create property by descriptor

'use strict';  // non-strict mode behaves slightly different  var foo = {}; Object.defineProperty(foo, 'bar', {   value: 1,   // configurable => false   // writable => false });  foo.bar = 2;    // throw TypeError: Cannot assign to read only property  Object.defineProperty(foo, 'bar', {   value: 2   // ... }); // throw TypeError: Cannot redefine property  delete foo.bar; // throw TypeError: Cannot delete property