976582 - Custom derived Error class' lineNumber, columnNumber and fileName properties return undefined on Object.getOwnPropertyDescriptor().

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[Frederik Maucksch]

User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/32.0.1700.107 Safari/537.36

Steps to reproduce:

1. Derive a custom Error type from Error using MDN recommended notation
var MyError = function() { return this; }; MyError.prototype = new Error(); MyError.prototype.constructor = MyError; var x = new MyError(); 

or ECMA5 way
var MyError = function() { return this; }; Object.create(Error.prototype, { constructor: { value: MyError }}); var x = new MyError(); 

2. evaluate any of the expressions: Object.getOwnPropertyDescriptor(x, "lineNumber");
Object.getOwnPropertyDescriptor(x, "columnNumber");
Object.getOwnPropertyDescriptor(x, "fileName");


Actual results:

Object.getOwnPropertyDescriptor returns undefined. Object.hasOwnProperty returns true.


Expected results:

Object.getOwnPropertyDescriptor returns a valid property descriptor, because the property is technically not absent as per ecma-262 8.10.

Comment 1

[Boris Zbarsky [:bzbarsky]]

> Object.hasOwnProperty returns true.

You mean false?  I see false returned from Object.hasOwnProperty(x, "fileName"), as expected, since the object has no own property with that name.  Why were you expecting it to have one?

> or ECMA5 way

The code there makes no sense; what's the point of the unused Object.create call?

Comment 2

[Frederik Maucksch]

Of course, I forgot to correctly assign the result of Object.create to the prototype of MyError:

var MyError = function() { return this; }; MyError.prototype = Object.create(Error.prototype, { constructor: { value: MyError }}); var x = new MyError(); 

Sorry about that!

Well, after more inspection, and after further decoupling the code from the working code I was seeing the issue in, I think the problem boils down to a difference between "Error.prototype.message" and e.g. "Error.prototype.lineNumber". I was lead to the conclusion that it had something to do with inheritance, due to the convoluted nature of the working code I had at hand.

Object.getOwnPropertyDescriptor(new Error(), "lineNumber"); // returns propertyDesc should (probably?) return undefined
Object.hasOwnProperty(new Error(), "lineNumber"); // returns false

this is in contrast to the standard defined "message" prototype as said:

Object.getOwnPropertyDescriptor(new Error(), "message"); // returns undefined
Object.hasOwnProperty(new Error(), "message"); // returns false

If that is expected behavior, the bug should be marked invalid. Sorry!

Comment 3

[Boris Zbarsky [:bzbarsky]]

> Object.getOwnPropertyDescriptor(new Error(), "lineNumber"); // returns propertyDesc should (probably?) return undefined

No, this should return a property descriptor per spec.

> Object.hasOwnProperty(new Error(), "lineNumber"); // returns false

hasOwnProperty is not a static method on Object.  It's a method on Object.prototype.  So you want:

  (new Error()).hasOwnProperty("lineNumber"); // returns true

> Object.getOwnPropertyDescriptor(new Error(), "message"); // returns undefined

Correct.  But if you did:

  Object.getOwnPropertyDescriptor(new Error("something"), "message");

you would get a descriptor.  The reason is http://people.mozilla.org/~jorendorff/es6-draft.html#sec-error-message step 6, which only defines the "message" property on the return value of |new Error(something)| if "something" is not undefined.