Trying to understand Javascript collection numbers

Question

I found this bit in the Adobe introduction to scripting:
JS collection numbers are static; they don’t shift when you add a new object to the collection. Object
numbering in JS indicates the order in which the objects were added to the collection. Because the
first object you added was assigned the number 0, the next object you add to the collection is number
1; if you add a third object, it is number 2. For example, when you add a document, the document
automatically contains a layer. The layer’s index is [0]. If you add a layer, the new layer’s index is [1]; if
you add a second layer, its index is [2]. If you drag layer [2] to the bottom position in the Layers palette,
it still has index [2].
If I understand this correctly the original layer is always layer[0], however when I try this slightly modified script from later in the guide:
var myDoc = app.activeDocument
var myLayerCount = 4
for(var myCounter = 0; myCounter < myLayerCount; myCounter++)
{var myLayer = myDoc.layers[myCounter]
myLayer.visible = false}
the top 4 layers are set to invisible, while I was expecting it to be the bottom 4.
What am I missing?

Answer 1

If your document has no groups, then doc.layers and doc.artLayers will be the same. But if you groups then doc.layers will give all
art layers in the doc root
the groups you have an doc root
any art layers and sub groups in the root level groups.
I.e. doc.layers will return the full document hierarchy as a flat list, but the order that list is not guaranteed to be something (as far as I know). Most likely order is always the visual top to bottom. I.e. item 0 is always the visually topmost element.
Also note that if you use doc.layers, you need to check if the item is a layer or a group (=folder=layerset). That's why you also have the collections .artLayers and .layerSets that give just layers and groups respectfully.