facing problem while implementing plugin development

[ThisIsFaar]

I was learning plugin development from Joomla docs, I want to add my js file in the head by a plugin that I am working on

I am using onBeforeCompileHead event.

PROBLEM: not able to add my js file by this code, also implement a couple of ways nothing works for me.

Code is below

<?php
defined('_JEXEC') or die;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\CMS\Factory;
$document = Factory::getDocument();
use Joomla\CMS\Uri\Uri;
use Joomla\Event\Event;
use Joomla\Event\SubscriberInterface;


class plgSystemInjectName extends CMSPlugin implements SubscriberInterface
{
    /**
	 * Returns an array of events this subscriber will listen to.
	 *
	 * @return  array
	 */
	public static function getSubscribedEvents(): array
	{
		return [
			'onBeforeCompileHead' => 'myFunctionName',
		];
	}

	/**
	 * Plugin method is the array value in the getSubscribedEvents method
	 * The plugin then modifies the Event object (if it's not immutable)
	 */
	 public function myFunctionName(Event $event)
	 {
		/*
		 * Plugin code goes here.
		 * You can access parameters via $this->params
		 */
        $url = Uri::base().'plugins/system/injectname/js/main.js';
        $document->addScript($url);


		return true;
	}
}

[Fedik]

Never call Factory::getDocument(); as you doing, it should be called not earlier than onAfterDispatch event.
Also it is recomended that you store all asset files (js, css) under /media folder.
Here example how to add media files in to a manifest of your plugin https://docs.joomla.org/Manifest_files#Media_files

Kind of this:

<media folder="media" destination="plg_system_injectname">
	<folder>js</folder>
</media>

For joomla 4 look https://docs.joomla.org/J4.x:Web_Assets how to attach asset to document.
Basically inside onBeforeCompileHead event you do something like:

/** @var Joomla\CMS\WebAsset\WebAssetManager $wa */
$wa = Factory::getApplication()->getDocument()->getWebAssetManager();
$wa->registerAndUseScript('plg_system_injectname', 'plg_system_injectname/main.js', []);

plg_system_injectname/main.js - assuming that the file in /media/plg_system_injectname/js/main.js

[ThisIsFaar]

Never call Factory::getDocument(); as you doing, it should be called not earlier than onAfterDispatch event.
Also it is recomended that you store all asset files (js, css) under /media folder.
Here example how to add media files in to a manifest of your plugin https://docs.joomla.org/Manifest_files#Media_files

Kind of this:

<media folder="media" destination="plg_system_injectname">
	<folder>js</folder>
</media>

For joomla 4 look https://docs.joomla.org/J4.x:Web_Assets how to attach asset to document.
Basically inside onBeforeCompileHead event you do something like:

/** @var Joomla\CMS\WebAsset\WebAssetManager $wa */
$wa = Factory::getApplication()->getDocument()->getWebAssetManager();
$wa->registerAndUseScript('plg_system_injectname', 'plg_system_injectname/main.js', []);

plg_system_injectname/main.js - assuming that the file in /media/plg_system_injectname/js/main.js

forget the previous code,

now I just want to add inline js below is the code that is not working:

<?php
defined('_JEXEC') or die;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\CMS\Factory;

class plgSystemInjectName extends CMSPlugin
{
    public function onAfterDispatch () {
        $document = Factory::getDocument();
        $document->addScriptDeclaration('
            window.event("domready", function() {
                alert("An inline JavaScript Declaration");
            });
        ');
    }
}

[Fedik]

because it should not :)

joomla 3:

$document = Factory::getDocument();
$document->addScriptDeclaration('
   alert("An inline JavaScript Declaration");
');

joomla 4:

$wa = Factory::getApplication()->getDocument()->getWebAssetManager();
$wa->addInlineScript('
  alert("An inline JavaScript Declaration");
');