Private/readonly class properties with weak maps

index.js

// To keep the data truly private, just don't export these
const privateMaps = {
  secret: new WeakMap(),
  constant: new WeakMap(),
}

export class Test {
  constructor(secret, constant) {
    privateMaps.secret.set(this, secret);
    privateMaps.constant.set(this, constant);
  }

  get constant() {
    return privateMaps.constant.get(this);
  }
}