58230 - Length of associative arrays is reported as zero (0)

Status: VERIFIED INVALID

(Core :: JavaScript Engine, defect, P3)

Description

[kens]

var x = new Array();
x["foo"] = "bar";
x["bar"] = "baz";
x.length

This script produces: "0"

Shouldn't associative arrays have a length attribute which reports the number 
of keys contained in the array's hash table?

Comment 1

[Phil Schwartau]

From the ECMA3 spec: 


15.4 Array Objects

Array objects give special treatment to a certain class of property names. 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.

Every Array object has a length property whose value is always a nonnegative 
integer less than 2 32 . The value of the length property is numerically 
greater than the name of every property whose name is an array index;
whenever a property of an Array object is created or changed, other properties 
are adjusted as necessary to maintain this invariant. Specifically, whenever a 
property is added whose name is an array index, the length property is changed, 
if necessary, to be one more than the numeric value of that array index; and 
whenever the length property is changed, every property whose name is an array 
index whose value is not smaller than the new length is automatically deleted. 

This constraint applies only to properties of the Array object itself and is 
unaffected by length or array index properties that may be inherited from its 
prototype.

Comment 2

[Phil Schwartau]

Have to mark this one as invalid. According to the spec, the length property
of an Array object counts the number of properties whose name P is an 
"array index" as defined by the spec. This means in particular,


                   P == ToString(ToUint32(P))    


In short, P must be numerical. So, for example, if we do this: 


var x = new Array();
x['0'] = 'bar';
x[1] = 'baz';
x['a'] = 'foo';
x.length;


the output will be 2, not 3, because both '0' and 1 are 
ECMA "array indices", but 'a' is not.

Comment 3

[kens]

Okay, this is all cool. So the only way to find the number of keys in 
an "associative" array in JavaScript is by iterating over its contents and 
maintaining a count? e.g.:

var counter = 0;
for(x in a) {
  counter++;
}

Forgive me for asking, but is there a better way?

Comment 4

[rogerl (gone)]

Verifying Phil's analysis.

I'm afraid I can think of no better way of counting the properties of a generic 
object - and the code snippet you show runs afoul of properties in the 
prototype, too. You need :
for (x in a) if (a.hasOwnProperty(x)) counter++;

bleagh!

Comment 5

[Phil Schwartau]

See bug 57048 for more info on this -