gistfile1.js

    var Foo = function(element, options) {
        var basic_manager = new HydraEditor.FieldManager(element, options);
        basic_manager.createNewField = function(stuff) {
              this.updateName(stuff); // Causes: "undefined is not a function"
        }

        return basic_manager;
    }

    Foo.prototype = {
        updateName: function (stuff) {
        }
    }
    
    f.new Foo(this, options)
    f.createNewField('blah')