Drupal - Get node ID in a form submit

For Drupal 8 Add your custom submit handler in form alter:

$form['actions']['submit']['#submit'][] = '_custom_node_form_submit';

And add custom submit handler:

function _custom_node_form_submit($form, \Drupal\Core\Form\FormStateInterface $form_state) {
  echo $form_state->getValue('nid');
}