ai.js

define(['cricket/server_player', 'lib/event_emitter'], function(ServerPlayer, EventEmitter) {
  "use strict";

  return function(fielding_ai, bowling_ai, batting_ai) {
    var _this = new ServerPlayer();

  	_this.init = function() {
		_this.typename = "ai";
		EventEmitter.call(_this);

		fielding_ai.call(_this);
	  };   
    _this.init();
    return _this;
  };
});

mixed_in_behaviour.js

define(['cricket/terms'], function(Terms) {
  "use strict";

  var fielding_ai = function(slice) {
    function set_field(field) {
      field.mad_logic();
 
  		this.emit_event('player/over/set_field/complete');
    }

    return function() {
      this.set_field = set_field;

      return this;
    };
  }([].slice);

  return fielding_ai;
});

usage.js

function some_codez(ai, field) {
  ai.set_field(field);
}

//Uncaught TypeError: Cannot call method 'emit_event' of undefined