Add expression to read the authenticated user unique identifier (#6192)

* This is useful if you want to interface with a third party or in-house solution to store user data.

Extensions/PlayerAuthentication/JsExtension.js

@@ -169,6 +169,23 @@ module.exports = {
       )
       .setFunctionName('gdjs.playerAuthentication.getUsername');
 
+    extension
+      .addStrExpression(
+        'UserID',
+        _('User ID'),
+        _('Get the unique user ID of the authenticated player.'),
+        '',
+        'JsPlatform/Extensions/authentication.svg'
+      )
+      .getCodeExtraInformation()
+      .setIncludeFile(
+        'Extensions/PlayerAuthentication/playerauthenticationcomponents.js'
+      )
+      .addIncludeFile(
+        'Extensions/PlayerAuthentication/playerauthenticationtools.js'
+      )
+      .setFunctionName('gdjs.playerAuthentication.getUserId');
+
     extension
       .addCondition(
         'IsPlayerAuthenticated',

Extensions/PlayerAuthentication/playerauthenticationtools.ts

@@ -171,7 +171,7 @@ namespace gdjs {
       if (!_checkedLocalStorage) {
         readAuthenticatedUserFromLocalStorage();
       }
-      return _userId || null;
+      return _userId || '';
     };
 
     /**