The concat method joins two or more Array objects producing one new one. The original Array objects are unaffected by this but, if one copy of a string or number is altered, it is not reflected in the other, whereas a change to an object reference can be seen in both copies.
Cats.concat(trees, cars)
This example adds the elements of the array 'cars' onto the array 'trees' on to the 'Cats' array.