checkout-experiment.php

<?php
/*
Plugin Name: Checkout Experiment
Description: An experimental WordPress.com checkout inside wp-admin
Author: Automattic
Version: 1.0
Author URI: http://automattic.com
*/

class CheckoutExperiment {
	public static function init(): void {
		$checkout = new self();
		add_action( 'admin_menu', [ $checkout, 'add_menu' ] );
		add_action( 'admin_enqueue_scripts', [ $checkout, 'add_scripts' ] );
	}

	public function add_menu(): void {
		add_menu_page( 'Checkout Experiment', 'Checkout Experiment', 'manage_options', 'checkout-experiment', [ $this, 'render_checkout' ] );
	}

	public function add_scripts( string $hook_suffix ): void {
		if ( $hook_suffix !== 'toplevel_page_checkout-experiment' ) {
			return;
		}
		$version = '5';
		wp_enqueue_script( 'react' );
		wp_enqueue_script( 'react-dom' );
		wp_register_script( 'checkout-experiment', plugin_dir_url( __FILE__ ) . 'checkout-experiment-app/dist/bundle.js', [ 'react', 'react-dom' ], $version, true );
		wp_localize_script( 'checkout-experiment', 'checkout_data', [ 'blog_id' => get_current_blog_id() ] );
		wp_enqueue_script( 'checkout-experiment' );
	}

	public function render_checkout(): void {
		echo '<div id="checkout-experiment-app">js goes here</div>';
	}
}

add_action( 'init', array( 'CheckoutExperiment', 'init' ) );