idManagement.ts

const ids = new Set();

export function isIdFree(id: string): boolean {
  return ids.has(id);
}

export function takeId(id: string) {
  ids.add(id);
}

export function freeId(id: string) {
   ids.delete(id);
}

roomIdChange.ts

import {isIdFree, freeId, takeId} from '../idManagement';

class State extends Schema{}
export class SomeRoom extends Room<SomeState> {

  onCreate (options: Partial<Types.RoomOptions>){
    this.setState(new State())
    if (options.roomId && idFree(options.roomId)) {
      this.roomId = options.roomId;
    }
    takeId(this.roomId);
  }
  
  onDispose() {
      freeId(this.roomId);
  }
}

The caveat you need to avoid here is that IDs must be globally unique.