A DESCRIPTION OF THE PROBLEM :
nashorn> a = "A,B,C,D".split(",")
[object Array]
nashorn> Array.prototype.foo=function(){};
function(){}
nashorn> a
[object Array]
nashorn> for each(x in a) print (x)
A
B
C
D
function(){}

function() {} is not expected to be in the list of "elements" of the array.

nashorn> for (i in a) print( i)
0
1
2
3
foo

nashorn> for (i in a.length) print( i)
nashorn> for (i=0;i<a.length;i++) print( i)
0
1
2
3

nashorn> for (i=0;i<a.length;i++) print( a[i])
A
B
C
D
nashorn>