Could someone please explain how to use the index() method?

Could someone please explain how to use the index() method?  The reference says:

app.activeDocument.pathItems.index(itemKey)

Gets an element from the collection.  itemKey is a string or number.

I presumed that it would be similar to getByName(), but returns an object by an array's index

instead.  This doesn't seem to be the case.  For example, if I want to reference "pathItems[2]" in the example below, I get an error.

So what actually is "itemKey" and how does one use the index() method? 

Thanks in advance. 

var path0 = activeDocument.pathItems.add();  // pathItems[2]
path0.name = "path0";
var path1 = activeDocument.pathItems.add();  // pathItems[1]
path1.name = "path1";
var path2 = activeDocument.pathItems.add();  // pathItems[0]
path2.name = "path2";

alert(activeDocument.pathItems[2].name);  // works, i.e. alerts "path0"

var xxx = activeDocument.pathItems.index(2);  // doesn't work
alert(xxx.name);