gistfile1.js

// Chainable API for URIBuilder

var uri = new URLBuilder(logErrorHandler)
  .parse('bad_url_format')
  .addQueryParam('param','value')
  .setPort('443')
  .toString();

function logErrorHandler(err, url_object){
  if('parse_error'===err.type)
    console.log(err.message);
  return url_object;
}

// XXXXXXXXXXXXXX OR XXXXXXXXXXXXXXX

// Chainable API for URIBuilder

var uri = new URLBuilder()
  .parse('bad_url_format')
  .addQueryParam('param','value')
  .setPort('443')
  .toString();

if(uri===null)
  {
  console.log('error')
  }
  
// Maybe the 2 solutions 
// Or throw exception if i suppose the parsed string is never a user input ?
// Not sure it's good to suppose as well.

Alternative:

var uri = new URLBuilder()
  .on('error', logErrorHandler)
  .parse('bad_url_format')
  .addQueryParam('param','value')
  .setPort('443')
  .toString();

No real need for EventEmitter (still, better if you have it) you can implement it manually or call it onError or whatever.

I still prefer exceptions, becasue as soon as input is invalid, the rest of the operations is senseless. Particularly: what will toString() return?

That's something I don't like in moment.js for example: give invalid (or unsufficient) input and you may have outputs like "NaN/NaN/NaN" I would really have preferred an exception here :s