unregister(unregisterToken)

class Thingy {
  static #cleanup = (label) => {
    //               ^^^^^−−−−− held value
    console.error(
      `The "release" method was never called for the object with the label "${label}"`,
    );
  };
  #registry = new FinalizationRegistry(Thingy.#cleanup);

  /**
   * Constructs a `Thingy` instance.
   * Be sure to call `release` when you're done with it.
   *
   * @param label A label for the `Thingy`.
   */
  constructor(label) {
    //                            vvvvv−−−−− held value
    this.#registry.register(this, label, this);
    //          target −−−−−^^^^         ^^^^−−−−− unregister token
  }

  /**
   * Releases resources held by this `Thingy` instance.
   */
  release() {
    this.#registry.unregister(this);
    //                        ^^^^−−−−− unregister token
  }
}

class Thingy {
  static #cleanup = (file) => {
    //               ^^^^−−−−− held value
    console.error(
      `The "release" method was never called for the "Thingy" for the file "${file.name}"`,
    );
  };
  #registry = new FinalizationRegistry(Thingy.#cleanup);
  #file;

  /**
   * Constructs a `Thingy` instance for the given file.
   * Be sure to call `release` when you're done with it.
   *
   * @param filename The name of the file.
   */
  constructor(filename) {
    this.#file = File.open(filename);
    //                            vvvvv−−−−− held value
    this.#registry.register(this, label, this.#file);
    //          target −−−−−^^^^         ^^^^^^^^^^−−−−− unregister token
  }

  /**
   * Releases resources held by this `Thingy` instance.
   */
  release() {
    if (this.#file) {
      this.#registry.unregister(this.#file);
      //                        ^^^^^^^^^^−−−−− unregister token
      File.close(this.#file);
      this.#file = null;
    }
  }
}