在SocialEngine挂钩中不执行JavaScript

I used to employ Javascript alert as a way of debugging my PHP code. But, I just realized that it is not always executed. For example, when you register a hook to be run after the signup process, JavaScript logging will not work in the hook.

First, the hook method is defined in the module manifest.php file:

'hooks' => [ 
    [ 
        'event' => 'onUserCreateAfter', 
        'resource' => 'User_Plugin_Test', 
    ], 
],

Then, onUserCreateAfter must be defined in the User_Plugin_Test class:

 class User_Plugin_Test {

//put your code here

public function onUserCreateAfter($event) {
    $log = Zend_Registry::get('Zend_Log');
    $userId = Engine_Api::_()->user()->getViewer()->getIdentity();
    $log->log('User_Plugin_Test->onUserCreateAfter: User id: ' . $userId, Zend_Log::ERR);
    echo '<script>alert("User_Plugin_Test->onUserCreateAfter");</script>';
}

}

However, the Zend log works. Why JavaScript does not run in the hook method.