Your code is mutating the input, and you're printing it after it was mutated, print it before doing anything else.

There are a few problems:

• There is a test with input = null, and you are trying to access .length on it, which crashes the program. You can console log input on codewars too.
• You cannot compare arrays like you're trying. Even [] === [] is false.
• Not a big deal, but using .unshift is very slow. There are better methods out there.