How to extend a class if declared this way

For a certain reason, I have to declare my classes this way:

const core = {
    character: class {
        constructor(arg) {}
    }
}

Works pretty, but I don’t know how to extends the class ‘character’. I’m trying the way below, but it doesn’t work:

const core = {
  character: class {
    constructor(arg) {}
  },
  enemy: class extends core.character {
    constructor(arg) {}
  }
}