es2015: converting to es2015 with arrow function

Consider I have this:

mongoose.connection.on('error', function (err) {
  if (err) {
    throw err;
  }
});

How do I convert this to ES2015 syntax?

I tried:

export class MongooseConnectionUtil extends MongooseUtil {
  constructor(...args) {
    super(...args);
    this.connection = mongoose.connection;
  }

  on('error', err) => err {
      if (err) {
        throw err;
      } 
  } 

also tried with the ()

  on('error', ()) => err {
    if (err) {
      throw err;
    }
  }