Exposed Object.getOwnPropertyDescriptor()

Original algorithm

The algorithm is specified in E5 Section 15.2.3.3:

1. If Type(O) is not Object throw a TypeError exception.
2. Let name be ToString(P). (Note: this may have a side effect.)
3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
4. Return the result of calling FromPropertyDescriptor(desc) (E5 Section 8.10.4).

FromPropertyDescriptor

The FromPropertyDescriptor() algorithm in E5 Section 8.10.4 is as follows:

1. If Desc is undefined, then return undefined.
2. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
3. If IsDataDescriptor(Desc) is true, then a. Call the [[DefineOwnProperty]] internal method of obj with arguments "value", Property Descriptor {[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false. b. Call the [[DefineOwnProperty]] internal method of obj with arguments "writable", Property Descriptor {[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
4. Else, IsAccessorDescriptor(Desc) must be true, so a. Call the [[DefineOwnProperty]] internal method of obj with arguments "get", Property Descriptor {[[Value]]: Desc.[[Get]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false. b. Call the [[DefineOwnProperty]] internal method of obj with arguments "set", Property Descriptor {[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
5. Call the [[DefineOwnProperty]] internal method of obj with arguments "enumerable", Property Descriptor {[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
6. Call the [[DefineOwnProperty]] internal method of obj with arguments "configurable", Property Descriptor {[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
7. Return obj.

Notes:

• Since all the [[DefineOwnProperty]] calls create new property values, and the property attributes match the defaults for [[Put]], we can simply use [[Put]] instead. The Throw flag does not matter as the [[Put]] operations cannot fail (except for some internal reason, which is thrown unconditionally without regard for Throw anyway).
• The order of settings properties to obj matters since it will affect the enumeration order of obj.

Changing [[DefineOwnProperty]] to [[Put]] and renaming Desc to desc (for compatibility with Object.getOwnPropertyDescriptor() algorithm):

1. If desc is undefined, then return undefined.
2. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
3. If IsDataDescriptor(desc) is true, then a. Call obj.[[Put]] with arguments "value", desc.[[Value]], and false. b. Call obj.[[Put]] with arguments "writable", desc.[[Writable]], and false.
4. Else, IsAccessorDescriptor(Desc) must be true, so a. Call obj.[[Put]] with arguments "get", desc.[[Get]], and false. (Note: desc.[[Get]] may be undefined.) b. Call obj.[[Put]] with arguments "set", desc.[[Set]], and false. (Note: desc.[[Set]] may be undefined.)
5. Call obj.[[Put]] with arguments "enumerable", desc.[[Enumerable]], and false.
6. Call obj.[[Put]] with arguments "configurable", desc.[[Configurable]], and false.
7. Return obj.

Inlining FromPropertyDescriptor

1. If Type(O) is not Object throw a TypeError exception.
2. Let name be ToString(P). (Note: this may have a side effect.)
3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
4. If desc is undefined, then return undefined.
5. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
6. If IsDataDescriptor(desc) is true, then a. Call obj.[[Put]] with arguments "value", desc.[[Value]], and false. b. Call obj.[[Put]] with arguments "writable", desc.[[Writable]], and false.
7. Else, IsAccessorDescriptor(Desc) must be true, so a. Call obj.[[Put]] with arguments "get", desc.[[Get]], and false. (Note: desc.[[Get]] may be undefined.) b. Call obj.[[Put]] with arguments "set", desc.[[Set]], and false. (Note: desc.[[Set]] may be undefined.)
8. Call obj.[[Put]] with arguments "enumerable", desc.[[Enumerable]], and false.
9. Call obj.[[Put]] with arguments "configurable", desc.[[Configurable]], and false.
10. Return obj.