Player.hidePlayer() expects a Plugin as it's first argument, the other version without is deprecated.
I would like to follow the best practice of course. How am I supposed to extend PluginBase from Javascript?

I tried doing the following:

const PluginBase = core.type('org.bukkit.plugin.PluginBase');
class TestPlugin extends PluginBase {}
const TEST_PLUGIN = new TestPlugin();

...

// Later in my event handler:
targetPlayer.hidePlayer(TEST_PLUGIN, player);

But this seems to cause the following error:
[19:12:21 WARN]: TypeError: protoParent is neither Object nor Null

The reason why onEnable doesn't fire is probably because the plugin isn't being loaded by a plugin manager, so it just never runs

Hey, thank you for your help. I have figured out some similar solution. I just noticed two issues:

1. onEnable doesn't seem to run at all. (At least console.log doesn't seem to work in that state)
2. You need to implement getDescription

This is the code I used:

const PluginBase = core.type('org.bukkit.plugin.PluginBase'),
const PluginDescriptionFile = core.type('org.bukkit.plugin.PluginDescriptionFile');

const pluginDescriptionFile = new PluginDescriptionFile('Test Plugin', '1.0', 'TestPlugin');

const TestPlugin = new PluginBaseAdapter({
    getDescription: () => {
        return pluginDescriptionFile;
    },
    onEnable: () => {
        console.log("Test");
    },
    isEnabled: () => {
        return true;
    }
});

But yeah, core.plugin is prolly the better and easier solution.

Notes for other people who might stumble across this:
https://www.graalvm.org/latest/reference-manual/js/JavaInteroperability/
https://hub.spigotmc.org/javadocs/spigot/org/bukkit/plugin/PluginBase.html

if you don't need a specific plugin you can just use core.plugin, but if you want to make your own instance of PluginBase you can use Java.extend

Example:

const PluginBase = Java.type("org.bukkit.plugin.PluginBase")
const TestPlugin = Java.extend(PluginBase, {
    onEnable: ()=>{
        console.log("New Plugin enabled")
    },
    isEnabled:()=>{
        return true
    }
})
const plugin = new TestPlugin()
const player = core.type("org.bukkit.Bukkit").getPlayerExact("Losin6450_2nd")
player.hidePlayer(plugin, player)

But i wouldn't recommend using Java.extend to make an instance of PluginBase as you could just use core.plugin