252007 - length of arrays is incorrect if a text key is used

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[Kae Verens]

User-Agent:       Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.7) Gecko/20040626 Firefox/0.9.1
Build Identifier: Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.7) Gecko/20040626 Firefox/0.9.1

If a text key is used for an array, then the .length property is incorrect.

For example; arr['test'] is a valid array element, but it will not be counted
for the .length property of an array

Reproducible: Always
Steps to Reproduce:
1. create an array in javascript
2. assign at least one text key
3. read the array's length

Actual Results:  
The number of items does not include the text element.

For example, in this code, the length is reported as 3:

test=new Array();
test[test.length]=1;
test[test.length]=2;
test[test.length]=3;
test['test']=4;
alert(test.length);

strangely, the following reports 3 as well, but that may not be a bug:

test=new Array();
test[2]=1;
alert(test.length);

Expected Results:  
In the example website, the result should be 1. In the two code snippets above,
the values should be 4 and 1 respectively.

Comment 1

[R.J. Keller]

--> JavaScript Engine

Comment 2

[Boris Zbarsky [:bzbarsky]]

The length property is defined to only count numeric properties.  From
http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf (the
EcmaScript spec), section 15.4.5.2:

  length 
    The length property of this Array object is always numerically greater than
    the name of every property whose name is an array index. The length property
    has the attributes { DontEnum, DontDelete }.

Earlier, from section 15.4 (the "Array Objects" introduction):

  A property name P (in the form of a string value) is an array index if and
  only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to
  2^(32)-1.

So "test" is not an array index and is not counted in the length.

Marking invalid; Mozilla's behavior is exactly correct.

Comment 3

[Erik Fabert]

*** Bug 285291 has been marked as a duplicate of this bug. ***

Comment 4

[Brendan Eich [:brendan]]

If you want to know the number of enumerable (generally, user-defined -- all
user-defined properties are enumerable, and most predefined properties are not)
properties, SpiderMonkey supports a __count__ property:

o = {p:1, q:2};
alert(o.__count__);  // alerts 2

/be