I got an error when i want to validate an unique username using Boostrap FormValidation remote.

Below are the error that i received in the browser console:

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.

Uncaught TypeError: b.success is not a function

Below are my code

$('#user_form').formValidation({
    framework: 'bootstrap',
    fields: {
        username: {
            validators: {
                notEmpty: {
                    message: 'Username is required'
                },
                remote: {
                    message: 'This Username has already been taken',
                    url: '/check_unique',
                    data: {
                        type: 'username'
                    },
                    type: 'POST'
                }
            }
        },

Web.php

Route::post('/check_unique', 'UserController@unique');     

Controller

public function unique()
{
$user= User::where('username', Input::get('username'))->count();

if($user> 0) {
    $isAvailable = FALSE;
} else {
    $isAvailable = TRUE;
}

echo json_encode(
        array(
            'valid' => $isAvailable
        ));

}