Body node ID class module is used to add node ID (nid) as a class to <body> tag on node pages.
In Drupal 7 core there was a unique node ID class in the <body> tag.
This is forward-port of that functionality for Drupal 8.

Installation

Install as you would normally install a contributed Drupal module.

https://www.drupal.org/project/body_node_id_class