380182 - undetectable property: Error.prototype.stack

Status: VERIFIED INVALID

(Core :: JavaScript Engine, defect)

Description

[Garrett Smith]

"stack" in Error.prototype; // false
Error.prototype.stack; // undefined

Error.prototype.stack should be detectable without throwing an error.

Comment 1

[:Gavin Sharp [email: [e-mail]]]

What error does it throw? What are the expected and actual results? Can you provide a testcase?

Comment 2

[Garrett Smith]

> What error does it throw
Throws no error.
> What are the expected and actual results?
Expected: 
"stack" in Error.prototype;// should be true, regardless of the value.
I'd normally expect an empty Stack from a "stack" property (or getStack method)

Testcase: Type into Firebug, click run: 
Error.prototype.stack;

    

Comment 3

[:Gavin Sharp [email: [e-mail]]]

Why do you expect Error.prototype.stack to exist?

Comment 4

[Garrett Smith]

stack property is on an error instance. 

https://bugzilla.mozilla.org/show_bug.cgi?id=123177

I expected it would be in the prototype.

Comment 5

[Boris Zbarsky [:bzbarsky]]

The property is just defined on the error object directly when it's thrown:

+    ok = JS_DefineProperty(cx, obj, js_stack_str,
+                           STRING_TO_JSVAL(stack),
+                           NULL, NULL, JSPROP_ENUMERATE);

(from the patch in the cited bug).  So no, it's not on the prototype.  

Comment 6

[Brian Crowder]

Not all prototypes are instances of the class they proto for; this is, I think one of those cases.  I also think that is fine behavior.

Comment 7

[Brendan Eich [:brendan]]

Prototypes are odd, they should appear to be instances of their class (ES3 notably dictates that RegExp.prototype is Object, but browsers don't do that and ES4 is fixing to match the de-facto standard), but if you get picky about their type you can see they must be of a different type from instances.

But that's neither here nor there. The stack property is a property of each thrown Error instance, not of Error.prototype:

js> try { function f(){throw new Error("moo")}; f()}catch(e){E=e}
Error: moo
js> 'stack' in E
true
js> 'stack' in Error.prototype
false
js> for (i in Error.prototype) print(i)
name
message
fileName
lineNumber
js> for (i in Error.prototype) print(Error.prototype[i])
Error


0
js> Error.prototype.constructor
function Error() {
    [native code]
}
js> E.constructor
function Error() {
    [native code]
}

/be