Laravel 5.8 VM2666:1 Uncaught SyntaxError: Unexpected token s in JSON at position 0

I want the user to register using ajax and when everything goes right I want to return JSON response {'success', 'thank you'} now in order to do that in register controller i overwrite the register method as follow:

 public function register(Request $request)
    {
        $this->validator($request->all())->validate();
        event(new Registered($user = $this->create($request->all())));
        if($request->ajax()){
            return response()->json(["success", "thank you"], '200');
        }
        $this->guard()->login($user);
        return $this->registered($request, $user)
            ?: redirect($this->redirectPath());
    }

but in Javascript/JQuery I get the following:

VM2666:1 Uncaught SyntaxError: Unexpected token s in JSON at position 0
    at JSON.parse (<anonymous>)
    at Object.success ((index):2003)
    at i (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at z (jquery.min.js:4)
    at XMLHttpRequest.<anonymous> (jquery.min.js:4)
success @ (index):2003
i @ jquery.min.js:2
fireWith @ jquery.min.js:2
z @ jquery.min.js:4
(anonymous) @ jquery.min.js:4
load (async)
send @ jquery.min.js:4
ajax @ jquery.min.js:4
(anonymous) @ (index):1997
dispatch @ jquery.min.js:3
r.handle @ jquery.min.js:3

what is wrong? how to fix it please?

via Chebli Mohamed