Arrays in JS vs arrays in Ruby

I've been using Johnson's gem and have discovered some (sort of) bugs, but still have no time to check what the precise problem is.

There seems to be a compatibility problem between arrays from the Ruby land in JS, as their behaviour is different:

require 'rubygems'
require 'johnson'
Js = Johnson::Runtime.new
Js['alert'] = lambda { |x| puts x }

# Let's test JS arrays
Js.evaluate('array = [1,2,3,4]')
Js.evaluate('for (i in array) { alert(i) }')   # 0 1 2 3
Js.evaluate('for (i in array) { alert(array[i]) }')   # 1 2 3 4

# Let's test Ruby arrays acting as JS arrays
Js['array'] = [1,2,3,4]
Js.evaluate('for (i in array) { alert(i) }')   # 2 3 4 5 (should be 0 1 2 3)
Js.evaluate('for (i in array) { alert(array[i]) }')   # 2 3 4 nil (should be 1 2 3 4)

Also, if I add a function to Array.prototype, it is not available in arrays from the Ruby land, so there are incompatibilities between JS & Ruby lands. Similarly, hashes show different behaviours in each land.