Hello world react app in admin

.gitignore

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+node_modules
+build

includes/admin-menu.php

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+<?php
+
+namespace TodoApp;
+
+add_action('admin_menu', __NAMESPACE__ . '\\menu');
+
+function menu() {
+  add_menu_page(
+    'Todo App',
+    'Todo App',
+    'manage_options',
+    'todoapp',
+    __NAMESPACE__ . '\\admin_page',
+  );
+}
+
+function admin_page() {
+  echo '<div id="todo-app"></div>';
+}

includes/admin-scripts.php

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+<?php
+
+namespace TodoApp;
+
+add_action('admin_enqueue_scripts', __NAMESPACE__ . '\\scripts');
+
+function scripts() {
+  $screen = get_current_screen();
+
+  if ($screen->id !== 'toplevel_page_todoapp') {
+    return;
+  }
+
+  $admin_deps = require_once TODOAPP_PLUGIN_DIR_PATH . '/build/index.asset.php';
+
+  wp_enqueue_script(
+    'todoapp',
+    TODOAPP_PLUGIN_DIR_URL . 'build/index.js',
+    $admin_deps['dependencies'],
+    $admin_deps['version'],
+    true
+  );
+}