promisify repl server

promisifyREPLServer.js

function promisify(server) {
  var _eval = server.eval;
  server.eval = function promisifiedEval(cmd, context, filename, callback) {
    _eval(cmd, context, filename, function (err, result) {
      if (err) {
        callback(err);
        return;
      }

      Promise.resolve(result).then(function (result) {
        callback(null, result);
      }, function (err) {
        callback(err);
      });
    });
  }

  return server;
}

var replServer = promisify(repl.start({
  prompt: '...> '
}));