Hi @endel

Okay I figured out how to do. I had this trouble and I am using plain JS, which adds more trouble :p (and I hadn't read the documentation about State, I have to admit)
So, if anyone has the same trouble, here is a solution for plain Javascript :

import { Room } from "colyseus";
// Imports need to be done with require for Schema
const schema = require('@colyseus/schema');
const { defineTypes, Schema } = schema;

export class TestState extends Schema {
  foo = "bar";
  anything = "you want"
}

export class TestRoom extends Room {
  onCreate(options) {
    this.setState(new TestState());
  }
}

// This is the important part
defineTypes(TestState, {
  whatever: "string"
});

Thanks for the help !

@boygiandi Could you paste some code please, I'm having the same trouble but I can't fix it with your method :/