gistfile1.js

function WidgetConfig() {}
WidgetConfig.prototype.__fields__ = [];
WidgetConfig.addFields = function (newFields) {
  var Super = this;
  function Child() {
    return Super.apply(this, arguments);
  }
  _.extend(Child, Super);
  Child.prototype = Object.create(Super.prototype);
  Child.prototype.__fields__ = Super.prototype.__fields__.concat(newFields)

  return Child;
}

Ok you really want to instantiate with:

var PollingHighchartWidgetConfig = CustomWidgetConfig
.addFields([HIGHCHART_DATA_FEED_URL, API_KEY_FIELD, API_TOKEN_FIELD]);

Why is it better than a addFields in the constructor? Also I don't get the point of doing a kind of constructor of new class like this??

Ok I think that you want to build a Class which is more abstract with that implementation. Is that right?

It means we can construct our classes once and instantiate them many times.

Your addFields() is like an extend method. But then if I need to tweak a result for example for Highchart should I return like this:

var PollingHighchartWidgetConfig = CustomWidgetConfig
.addFields([HIGHCHART_DATA_FEED_URL, API_KEY_FIELD, API_TOKEN_FIELD]);

PollingHighChartWidgetConfig.prototype.changeUrl = function() {
      var idx = _.findIndex(this.fields, { name: 'url' });
      this.fields[idx] = HIGHCHART_FORMAT_FIELD;
}

// Instantiation
var config = PollingHighChartWidgetConfig();
config.changeUrl();

?

Just start a new inheritance chain.

var CustomWidgetConfig = WidgetConfig
.addFields([FORMAT_FIELD, DATA_FEED_URL]);
var PollingCustomWidgetConfig = CustomWidgetConfig
.addFields(POLLING_FIELDS);

var HighchartWidgetConfig = WidgetConfig
.addFields([HIGHCHART_DATA_FEED_URL]);
var PollingHighchartWidgetConfig = HighChartWidgetConfig
.addFields(POLLING_FIELDS)

// Etc...