Q:

Opencart 3.0 - how to add in-line javascript code from catalog controller with event system?

How to add inline js code from controller ?
admin module controller 
$this->model_setting_event
     ->addEvent("one_two_three","catalog/view/common/header/before","extension/module/oone_two_three/inject_abc_javascript");

and in catalog module controller 
public function inject_abc_javascript(&$route, &$data){
    $code = $this->config->get("module_one_two_three_code");
    $data["scripts"] = "<script>OneTwo.push(function() {
  OneTwo.init({
      appId: $code,
  });});</script>";
}

Is there any way to add inline javascript in OpenCart header?

A:

Yes, you can do it.
First of all you have an error/typo in addEvent parameters, change oone_two_three to one_two_three.
Then you need a third parameter for your function:
public function inject_abc_javascript($route, &$data = false, &$output = false){
    // Before closing tag
    $hook = '</head>';
    $js = '<script>alert("Hello from one_two_three module!");</script></head>';
    $output = str_replace($hook, $js, $output);
}

