Where has basePath gone in Drupal 8?

Javascript code in Drupal 7 used drupal_add_js() to attach Javascript code to a webpage.  The Javascript code then used Drupal.settings.basePath to prepend the path to the Drupal installation.  For Drupal installed in the root directory, Drupal.settings.basePath is just the string '/' but if Drupal is installed in a subdirectory then this is necessary for the Javascript code to continue to work.  The php part is

drupal_add_js(drupal_get_path('module', 'mymodule') . '/mymodule.js');

and the Javascript part is