gistfile1.txt

router.post('/login', function(req, res) {
  var client = xmlrpc.createClient({ host: 'https://wordpress.url/xmlrpc.php', port: 80, path: '/'})
    client.methodCall('external.login', [req.body.username, req.body.password], function (error, value) {
      if(value) {
        req.session.user_id = value;
        res.send('success');
      } else {
        res.send('Error connecting to server');
      }
    })
});

gistfile2.txt

// LOGIN VIA NODE
add_filter('xmlrpc_methods', 'api_login_method' );

function api_login_method( $methods )
{
    $methods['external.login'] = 'external_check_login';
    return $methods;
}

function external_check_login( $args )
{
    $username = $args[0];
    $password = $args[1];

    $user = wp_authenticate( $username, $password );

    if( is_wp_error( $user ) )
    {
        return false;
    }
    return $user->id;
}