FULL PRODUCT VERSION :
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b17, mixed mode)

A DESCRIPTION OF THE PROBLEM :
For Java objects being used from JavaScript, accessing an undefined property using a variable returns 'null' whereas it should returned 'undefined'.

jjs> var point = new java.awt.Point(2,3)
jjs> point['x']
2
jjs> var propertyName = 'x'
jjs> point[propertyName]
2
jjs> propertyName = 'foo'
foo
jjs> point[propertyName] === undefined
false
jjs> point[propertyName]
null
jjs> point['foo'] === undefined
true
jjs>

JavaScript objects behave correctly.

jjs> var point = {x:2, y:3}
jjs> var propertyName = 'foo'
jjs> point[propertyName] === undefined
true
jjs>


REPRODUCIBILITY :
This bug can be reproduced always.