Subj : Re: Confused about enumerating
To   : netscape.public.mozilla.jseng
From : Brian Genisio <BrianGenisio@yahoo.com>
Date : Thu Feb 19 2004 07:43 am

Brendan Eich wrote:
> 
> That difference does not account for what is going on.  Are you defining 
> the length property on the prototype (via JS_InitClass)?  If so, the 
> for..in code will not find it if there is a length property in the 
> original object (the one being enumerated).  The for..in code suppresses 
> ids of shadowed properties in prototypes.  If the original object's 
> length property names an empty string value, that might explain what you 
> are seeing -- but you have to show a test case and its input and output.
> 
> Code excerpts from your embedding might be helpful too.
> 
> /be


Sure :)  ( I really appreciate the help )

Here is the test code in Javascript.  getCollection() will simply return 
a collection object, much like an array, but it enumerates differently:

/////////////////////////////////
var myColl = getCollection();
for(name in myColl)
   alert (name);
/////////////////////////////////

I am creating an enumeration JSID based off of the Ids of the objects in 
the collection.  For a collection with two elements, I expect the 
following to be printed, using my alert method:

ALERT: length
ALERT: value1ID
ALERT: value2ID

Instead, I see the following:

ALERT: value1ID
ALERT: value2ID

If I change my embedded code to give "Length", instead of "length", I 
get the following:

ALERT: Length
ALERT: value1ID
ALERT: value2ID

If I keep the value to "length", but get rid of "length" as a defined 
property of the object, it comes out the way I expect it.

In my embedded code, I initialize the object like this (cut and pasted 
from many places):

//////////////////////////////////////////////////////////////////////
// Define the properties for the collection
JSPropertySpec m_properties[] =
{
  {"length", JS_COLLECTION_LENGTH_PROP, JSPROP_ENUMERATE },
  { 0 }
};

// Init the class with the other specifications
JS_InitClass(glob_cx, glob_obj, parent_obj, m_classSpec, Constructor, 2,
		m_properties, m_methods, NULL, NULL);

///////////////////////////////////////////////////////////////////////

Then, in my enumeration code, my state variable is an int, representing 
the current index of the collection to be enumerated.  I initialize it 
to -1, so if I get -1, I give the length, and increment the value to 0. 
  If I get between 0 and the length-1 of the collection, I return the 
given object's id if it exists, or the current index.  Here is my code 
for the JSENUMERATE_NEXT request:


/////////////////////////////////////////////////////////////////////////
// In the next state, we look at the current state, return the idp, and 
// update the state.  privColl is the private data in the object
case JSENUMERATE_NEXT:
  // Get the current state
  current_index = (unsigned long*)JSVAL_TO_PRIVATE(*statep);

  // In the case of -1, we will send the length
  if(*current_index == (unsigned long)(-1))
  {
    // For some reason, "length" does not work here.
    JS_ValueToId(cx,
       STRING_TO_JSVAL(JS_InternString(cx, "length")), idp);

    // After this, start enumerating the actual values
    *current_index = 0;
    *statep = PRIVATE_TO_JSVAL(current_index);
  }

  // if the state is a value within range of the collection,
  // return the indexed ID
  else if(*current_index < privColl->length() && *current_index >= 0)
  {	
    // Storage for the name of the property
    const char* name;

    // Get the item
    Item *tmpItem = privColl->item(*current_index);

    // If an ID exists, set the name to the ID.  If not, set it to
    // the current index
    if(!tmpItem->id().empty())
	name = tmpItem->id().c_str();
    else
	name = itoa(*current_index, tmp_str, 10);

    // Translate the name to a JS ID value
    JS_ValueToId(cx, STRING_TO_JSVAL(JS_InternString(cx, name)), idp);

    // Update the current state
    *current_index = *current_index + 1;
    *statep = PRIVATE_TO_JSVAL(current_index);
  }

  // If we get here, then we have exhausted the enumeration.
  // Delete the state, and tell the JS engine we are done with JSVAL_NULL
  else
  {
    delete current_index;
    *statep = JSVAL_NULL;
  }
  break;

///////////////////////////////////////////////////////

Thanks,
Brian

.