647484 - Object.defineProperty error message is confusing when writable: false is included in an accessor property descriptor

Status: NEW

(Core :: JavaScript Engine, defect)

Description

[gossi]

User-Agent:       Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0) Gecko/20100101 Firefox/4.0
Build Identifier: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0) Gecko/20100101 Firefox/4.0

This snippet:

"use strict";

var obj = {};

Object.defineProperty(obj, "a", {
writable : false,
set : function () {},
get: function () {return 3}
});

console.log(obj.a);

creates this error:

TypeError: property descriptors must not specify a value or be writable when a getter or setter has been specified

Well, writable is set to false, means it is *not* writable. If one ommits the writable property, it is working fine. Nevertheless, the error should only be shown when the writable is explicitely set to true.

Reproducible: Always

Comment 1

[Boris Zbarsky [:bzbarsky]]

I believe the relevant spec here is ECMA-262 edition 5 section 8.10.5, which defines "ToPropertyDescriptor ( Obj )".

Step 9 there says:

9. If either desc.[[Get]] or desc.[[Set]] are present, then 
  a.  If either desc.[[Value]] or desc.[[Writable]] are present, then throw a
      TypeError exception.

Note that it says to throw if those fields of the descriptor are _present_, no matter what value they're set to.

Or put another way, it doesn't make any sense to set "writable" to either false or true if you have a getter or setter, since either value will be ignored.

So I believe this is invalid.  I do think the error message should be changed to say "property descriptors must not specify a value or set writable when ...."

Comment 2

[Brendan Eich [:brendan]]

Morphing to keep this useful and fix the diagnostic.

/be

Comment 3

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee is inactive on Bugzilla, so the assignee is being reset.