Wrong result?

In http://www.jsfuck.com/ I type this source A:

[1,2][1]

and select "Eval source" and push "Encode" button.

I get source B (long - I put it at the bottom of this post)

When I paste source A to chrome console I get result: 2 When I paste source B to chrome console I get result: undefined

It look's like it is bug or I do something wrong... - what to do? (without "Eval source" problem is the same)