I have been using this library in my google apps script, particually the Unique function.

So far this has done what I require, if I have two arrays like:

[1,2,3] and [2,3,4], I have been using array1.concat(array2) and the using the unique function, which will return [1,2,3,4].

How can I retrieve unique values which in my example will be [1,4] ?