<?php
class SiteSpecificJS extends StudIPPlugin implements SystemPlugin
{
    public function __construct()
    {
        parent::__construct();

        NotificationCenter::addObserver($this, 'injectJS', 'NavigationDidActivateItem', '/course/main/details');
    }

    public function injectJS()
    {
        static $injected = false;
        if ($injected === false) {
            PageLayout::addScript($this->getPluginURL() . '/assets/foo.js');
            $injected = true;
        }
    }
}