You need to return the call of the function. array_diff(a, b) should be return array_diff(a, b). Your code recursively recalls the function but there is nothing to return, therefore the default value is "undefined".

I think I'm having trouble with this site. I'm currently getting an error 504 when I submit a solution. Is anyone else having this issue? If so, were you able to work around it some how and submit a solution. Any help would be greatly appreiated. I know that my solution is correct because I am testing it in the chrome console.