I'm developing a card game using Phaser, and I ran into some really weird behaviour.
I'm using built in Phaser EventEmitter object for handling some card effects.

EventEmitter example:

const eventEmitter = new Phaser.Events.EventEmitter();
eventEmitter.on('event-name', (...args) => {
  ...
});
eventEmitter.emit('event-name', arg1, arg2);

When I pass an object as an argument to the eventHandler function, and when I try to read any property from that object using dot notation, the result is undefined, when I try to call a method from that object, it throws an exception "obj.func is not a function", but when I log that object in the console, it prints the object with all the properties and methods.

I can find a workaround for this problem, but I want to understand why this is happening.

What's going on here?

Code:

import Phaser from "phaser";

enum Events {
  TEST = "TEST",
}

class EventsHandler {
  private eventEmitter = new Phaser.Events.EventEmitter();

  public on(event: Events, fn: (...args: any[]) => void) {
    this.eventEmitter.on(event, fn);
  }

  public off(event: Events, fn: (...args: any[]) => void) {
    this.eventEmitter.off(event, fn);
  }

  public emitEvent(event: Events, ...args: any[]) {
    this.eventEmitter.emit(event, args);
  }
}

export class Game extends Phaser.Scene {
  eventsHandler: EventsHandler = new EventsHandler();

  constructor() {
    super("Game");
  }

  create() {
    const testEventListener = (...args: any[]) => {
      console.log(args[0]);
      //prints object with all the properties and methods

      console.log(args[0].testProperty);
      //prints undefined

      console.log(args[0].sayHello());
      //throws an exception "args[0].sayHello is not a function"
    };
    this.eventsHandler.on(Events.TEST, testEventListener);

    const testObj = {
      sayHello: () => console.log("hello"),
      testProperty: "hello",
    };
    this.eventsHandler.emitEvent(Events.TEST, testObj);
  }
}

Exception:

Uncaught TypeError: args[0].sayHello is not a function
    at EventEmitter2.testEventListener (Game.ts:34:27)
    at EventEmitter2.emit (phaser.js:198:35)
    at EventsHandler.emitEvent (Game.ts:19:23)
    at Game.create (Game.ts:42:24)