264469 - error or feature in operator 'in' ?

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[dmitry rusanov]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.0; rv:1.7.3) Gecko/20040913 Firefox/0.10
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; rv:1.7.3) Gecko/20040913 Firefox/0.10

if execute below script , message box "true - undefined - undefined"

operator 'in' find variable 'init' defined in function, this is error or feature?

var obj = new Object();

obj.func = function()
{	var init = 'this init value';
};

alert(('init' in obj.func) + ' - ' + obj.func['init'] + ' - ' + obj.func.init);



Reproducible: Always
Steps to Reproduce:
1.
2.
3.

Comment 1

[Stefan Borggraefe]

*** Bug 264470 has been marked as a duplicate of this bug. ***

Comment 2

[Uri Bernstein]

This is the correct behaviour.
Function variables are considered properties of the Function object (see
http://iis1.cps.unizar.es/Oreilly/web/jscript/ch06_03.html, look for "Function
arguments and variables are properties").
The "in" operator therefore correctly determines that "init" is a property of
the obj.func Function object.

Comment 3

[dmitry rusanov]

I see your documentation
I draw attention on string "When the function returns, JavaScript deletes these
properties."  if property delete why operator 'in' correctly determines that
property exist? 

Function arguments and variables are properties

In all versions of JavaScript, global variables are actually properties of some
top-level object. In client-side JavaScript, as we'll see, this top-level object
is the browser window or frame that contains the JavaScript code. This raises
the obvious question: if global variables are properties of an object, what are
local function variables? It would make sense that they, too, are properties of
some object. The only obvious object is the function (or Function) itself. The
following code demonstrates:

function f(x)
{
   var y = 3;         // a local variable
   return f.x + f.y;  // refer to the argument and variable as properties    
}

If we invoke the function, we see that function arguments and local variables
really can be accessed as properties of the function itself:

result = f(2);        // returns 5

However, if we try to read these properties ourselves, we will be unable to:

typeof f.x      // yields "undefined"
typeof f.y      // yields "undefined"

What this means is that, like the arguments[] array and the caller property, the
local variable and argument properties are only accessible while the function is
running. When the function returns, JavaScript deletes these properties. 

Comment 4

[Brendan Eich [:brendan]]

SpiderMonkey, and its progenitor, the "Mocha" runtime I wrote in a few weeks in
May 1995, both have reflected local variable and formal parameter properties in
function objects.  This is an ECMA extension, arguably allowed by Section 16.

I do not want to remove this extension right now, although if you make a
sufficiently persuasive case, I could be persuaded.  I've made the case myself
at the code level: see the XXX comments around
http://lxr.mozilla.org/aviarybranch/source/js/src/jsinterp.c#459.  But the time
is not right to change this early design decision.

/be